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1 | ftrace - Function Tracer |
2 | ======================== | |
3 | ||
4 | Copyright 2008 Red Hat Inc. | |
a41eebab SR |
5 | Author: Steven Rostedt <srostedt@redhat.com> |
6 | License: The GNU Free Documentation License, Version 1.2 | |
a97762a7 | 7 | (dual licensed under the GPL v2) |
f2d9c740 SR |
8 | Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, |
9 | John Kacur, and David Teigland. | |
42ec632e | 10 | Written for: 2.6.28-rc2 |
8d016091 | 11 | Updated for: 3.10 |
eb6d42ea SR |
12 | |
13 | Introduction | |
14 | ------------ | |
15 | ||
16 | Ftrace is an internal tracer designed to help out developers and | |
17 | designers of systems to find what is going on inside the kernel. | |
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18 | It can be used for debugging or analyzing latencies and |
19 | performance issues that take place outside of user-space. | |
eb6d42ea | 20 | |
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21 | Although ftrace is typically considered the function tracer, it |
22 | is really a frame work of several assorted tracing utilities. | |
23 | There's latency tracing to examine what occurs between interrupts | |
24 | disabled and enabled, as well as for preemption and from a time | |
25 | a task is woken to the task is actually scheduled in. | |
26 | ||
27 | One of the most common uses of ftrace is the event tracing. | |
28 | Through out the kernel is hundreds of static event points that | |
29 | can be enabled via the debugfs file system to see what is | |
30 | going on in certain parts of the kernel. | |
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31 | |
32 | ||
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33 | Implementation Details |
34 | ---------------------- | |
35 | ||
36 | See ftrace-design.txt for details for arch porters and such. | |
37 | ||
38 | ||
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39 | The File System |
40 | --------------- | |
41 | ||
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42 | Ftrace uses the debugfs file system to hold the control files as |
43 | well as the files to display output. | |
eb6d42ea | 44 | |
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45 | When debugfs is configured into the kernel (which selecting any ftrace |
46 | option will do) the directory /sys/kernel/debug will be created. To mount | |
47 | this directory, you can add to your /etc/fstab file: | |
48 | ||
49 | debugfs /sys/kernel/debug debugfs defaults 0 0 | |
50 | ||
51 | Or you can mount it at run time with: | |
52 | ||
53 | mount -t debugfs nodev /sys/kernel/debug | |
eb6d42ea | 54 | |
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55 | For quicker access to that directory you may want to make a soft link to |
56 | it: | |
eb6d42ea | 57 | |
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58 | ln -s /sys/kernel/debug /debug |
59 | ||
60 | Any selected ftrace option will also create a directory called tracing | |
61 | within the debugfs. The rest of the document will assume that you are in | |
62 | the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate | |
63 | on the files within that directory and not distract from the content with | |
64 | the extended "/sys/kernel/debug/tracing" path name. | |
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65 | |
66 | That's it! (assuming that you have ftrace configured into your kernel) | |
67 | ||
8d016091 | 68 | After mounting debugfs, you can see a directory called |
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69 | "tracing". This directory contains the control and output files |
70 | of ftrace. Here is a list of some of the key files: | |
71 | ||
72 | ||
73 | Note: all time values are in microseconds. | |
74 | ||
5752674e IM |
75 | current_tracer: |
76 | ||
77 | This is used to set or display the current tracer | |
78 | that is configured. | |
79 | ||
80 | available_tracers: | |
81 | ||
82 | This holds the different types of tracers that | |
83 | have been compiled into the kernel. The | |
84 | tracers listed here can be configured by | |
85 | echoing their name into current_tracer. | |
86 | ||
6752ab4a | 87 | tracing_on: |
5752674e | 88 | |
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89 | This sets or displays whether writing to the trace |
90 | ring buffer is enabled. Echo 0 into this file to disable | |
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91 | the tracer or 1 to enable it. Note, this only disables |
92 | writing to the ring buffer, the tracing overhead may | |
93 | still be occurring. | |
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94 | |
95 | trace: | |
96 | ||
97 | This file holds the output of the trace in a human | |
98 | readable format (described below). | |
99 | ||
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100 | trace_pipe: |
101 | ||
102 | The output is the same as the "trace" file but this | |
103 | file is meant to be streamed with live tracing. | |
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104 | Reads from this file will block until new data is |
105 | retrieved. Unlike the "trace" file, this file is a | |
106 | consumer. This means reading from this file causes | |
107 | sequential reads to display more current data. Once | |
108 | data is read from this file, it is consumed, and | |
109 | will not be read again with a sequential read. The | |
110 | "trace" file is static, and if the tracer is not | |
111 | adding more data,they will display the same | |
112 | information every time they are read. | |
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113 | |
114 | trace_options: | |
115 | ||
116 | This file lets the user control the amount of data | |
117 | that is displayed in one of the above output | |
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118 | files. Options also exist to modify how a tracer |
119 | or events work (stack traces, timestamps, etc). | |
120 | ||
121 | options: | |
122 | ||
123 | This is a directory that has a file for every available | |
124 | trace option (also in trace_options). Options may also be set | |
125 | or cleared by writing a "1" or "0" respectively into the | |
126 | corresponding file with the option name. | |
5752674e | 127 | |
42b40b3d | 128 | tracing_max_latency: |
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129 | |
130 | Some of the tracers record the max latency. | |
131 | For example, the time interrupts are disabled. | |
132 | This time is saved in this file. The max trace | |
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133 | will also be stored, and displayed by "trace". |
134 | A new max trace will only be recorded if the | |
135 | latency is greater than the value in this | |
136 | file. (in microseconds) | |
5752674e | 137 | |
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138 | tracing_thresh: |
139 | ||
140 | Some latency tracers will record a trace whenever the | |
141 | latency is greater than the number in this file. | |
142 | Only active when the file contains a number greater than 0. | |
143 | (in microseconds) | |
144 | ||
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145 | buffer_size_kb: |
146 | ||
147 | This sets or displays the number of kilobytes each CPU | |
8d016091 | 148 | buffer holds. By default, the trace buffers are the same size |
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149 | for each CPU. The displayed number is the size of the |
150 | CPU buffer and not total size of all buffers. The | |
151 | trace buffers are allocated in pages (blocks of memory | |
152 | that the kernel uses for allocation, usually 4 KB in size). | |
153 | If the last page allocated has room for more bytes | |
154 | than requested, the rest of the page will be used, | |
155 | making the actual allocation bigger than requested. | |
156 | ( Note, the size may not be a multiple of the page size | |
8d016091 | 157 | due to buffer management meta-data. ) |
5752674e | 158 | |
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159 | buffer_total_size_kb: |
160 | ||
161 | This displays the total combined size of all the trace buffers. | |
162 | ||
163 | free_buffer: | |
164 | ||
165 | If a process is performing the tracing, and the ring buffer | |
166 | should be shrunk "freed" when the process is finished, even | |
167 | if it were to be killed by a signal, this file can be used | |
168 | for that purpose. On close of this file, the ring buffer will | |
169 | be resized to its minimum size. Having a process that is tracing | |
170 | also open this file, when the process exits its file descriptor | |
171 | for this file will be closed, and in doing so, the ring buffer | |
172 | will be "freed". | |
173 | ||
174 | It may also stop tracing if disable_on_free option is set. | |
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175 | |
176 | tracing_cpumask: | |
177 | ||
178 | This is a mask that lets the user only trace | |
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179 | on specified CPUs. The format is a hex string |
180 | representing the CPUs. | |
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181 | |
182 | set_ftrace_filter: | |
183 | ||
184 | When dynamic ftrace is configured in (see the | |
185 | section below "dynamic ftrace"), the code is dynamically | |
186 | modified (code text rewrite) to disable calling of the | |
187 | function profiler (mcount). This lets tracing be configured | |
188 | in with practically no overhead in performance. This also | |
189 | has a side effect of enabling or disabling specific functions | |
190 | to be traced. Echoing names of functions into this file | |
191 | will limit the trace to only those functions. | |
192 | ||
07271aa4 CD |
193 | This interface also allows for commands to be used. See the |
194 | "Filter commands" section for more details. | |
195 | ||
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196 | set_ftrace_notrace: |
197 | ||
198 | This has an effect opposite to that of | |
199 | set_ftrace_filter. Any function that is added here will not | |
200 | be traced. If a function exists in both set_ftrace_filter | |
201 | and set_ftrace_notrace, the function will _not_ be traced. | |
202 | ||
203 | set_ftrace_pid: | |
204 | ||
205 | Have the function tracer only trace a single thread. | |
206 | ||
207 | set_graph_function: | |
208 | ||
209 | Set a "trigger" function where tracing should start | |
210 | with the function graph tracer (See the section | |
211 | "dynamic ftrace" for more details). | |
212 | ||
213 | available_filter_functions: | |
214 | ||
215 | This lists the functions that ftrace | |
216 | has processed and can trace. These are the function | |
217 | names that you can pass to "set_ftrace_filter" or | |
218 | "set_ftrace_notrace". (See the section "dynamic ftrace" | |
219 | below for more details.) | |
eb6d42ea | 220 | |
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221 | enabled_functions: |
222 | ||
223 | This file is more for debugging ftrace, but can also be useful | |
224 | in seeing if any function has a callback attached to it. | |
225 | Not only does the trace infrastructure use ftrace function | |
226 | trace utility, but other subsystems might too. This file | |
227 | displays all functions that have a callback attached to them | |
228 | as well as the number of callbacks that have been attached. | |
229 | Note, a callback may also call multiple functions which will | |
230 | not be listed in this count. | |
231 | ||
232 | If the callback registered to be traced by a function with | |
233 | the "save regs" attribute (thus even more overhead), a 'R' | |
234 | will be displayed on the same line as the function that | |
235 | is returning registers. | |
236 | ||
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237 | If the callback registered to be traced by a function with |
238 | the "ip modify" attribute (thus the regs->ip can be changed), | |
239 | an 'I' will be displayed on the same line as the function that | |
240 | can be overridden. | |
241 | ||
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242 | function_profile_enabled: |
243 | ||
244 | When set it will enable all functions with either the function | |
245 | tracer, or if enabled, the function graph tracer. It will | |
246 | keep a histogram of the number of functions that were called | |
247 | and if run with the function graph tracer, it will also keep | |
248 | track of the time spent in those functions. The histogram | |
249 | content can be displayed in the files: | |
250 | ||
251 | trace_stats/function<cpu> ( function0, function1, etc). | |
252 | ||
253 | trace_stats: | |
254 | ||
255 | A directory that holds different tracing stats. | |
256 | ||
257 | kprobe_events: | |
258 | ||
259 | Enable dynamic trace points. See kprobetrace.txt. | |
260 | ||
261 | kprobe_profile: | |
262 | ||
263 | Dynamic trace points stats. See kprobetrace.txt. | |
264 | ||
265 | max_graph_depth: | |
266 | ||
267 | Used with the function graph tracer. This is the max depth | |
268 | it will trace into a function. Setting this to a value of | |
269 | one will show only the first kernel function that is called | |
270 | from user space. | |
271 | ||
272 | printk_formats: | |
273 | ||
274 | This is for tools that read the raw format files. If an event in | |
275 | the ring buffer references a string (currently only trace_printk() | |
276 | does this), only a pointer to the string is recorded into the buffer | |
277 | and not the string itself. This prevents tools from knowing what | |
278 | that string was. This file displays the string and address for | |
279 | the string allowing tools to map the pointers to what the | |
280 | strings were. | |
281 | ||
282 | saved_cmdlines: | |
283 | ||
284 | Only the pid of the task is recorded in a trace event unless | |
285 | the event specifically saves the task comm as well. Ftrace | |
286 | makes a cache of pid mappings to comms to try to display | |
287 | comms for events. If a pid for a comm is not listed, then | |
288 | "<...>" is displayed in the output. | |
289 | ||
290 | snapshot: | |
291 | ||
292 | This displays the "snapshot" buffer and also lets the user | |
293 | take a snapshot of the current running trace. | |
294 | See the "Snapshot" section below for more details. | |
295 | ||
296 | stack_max_size: | |
297 | ||
298 | When the stack tracer is activated, this will display the | |
299 | maximum stack size it has encountered. | |
300 | See the "Stack Trace" section below. | |
301 | ||
302 | stack_trace: | |
303 | ||
304 | This displays the stack back trace of the largest stack | |
305 | that was encountered when the stack tracer is activated. | |
306 | See the "Stack Trace" section below. | |
307 | ||
308 | stack_trace_filter: | |
309 | ||
310 | This is similar to "set_ftrace_filter" but it limits what | |
311 | functions the stack tracer will check. | |
312 | ||
313 | trace_clock: | |
314 | ||
315 | Whenever an event is recorded into the ring buffer, a | |
316 | "timestamp" is added. This stamp comes from a specified | |
317 | clock. By default, ftrace uses the "local" clock. This | |
318 | clock is very fast and strictly per cpu, but on some | |
319 | systems it may not be monotonic with respect to other | |
320 | CPUs. In other words, the local clocks may not be in sync | |
321 | with local clocks on other CPUs. | |
322 | ||
323 | Usual clocks for tracing: | |
324 | ||
325 | # cat trace_clock | |
326 | [local] global counter x86-tsc | |
327 | ||
328 | local: Default clock, but may not be in sync across CPUs | |
329 | ||
330 | global: This clock is in sync with all CPUs but may | |
331 | be a bit slower than the local clock. | |
332 | ||
333 | counter: This is not a clock at all, but literally an atomic | |
334 | counter. It counts up one by one, but is in sync | |
335 | with all CPUs. This is useful when you need to | |
336 | know exactly the order events occurred with respect to | |
337 | each other on different CPUs. | |
338 | ||
339 | uptime: This uses the jiffies counter and the time stamp | |
340 | is relative to the time since boot up. | |
341 | ||
342 | perf: This makes ftrace use the same clock that perf uses. | |
343 | Eventually perf will be able to read ftrace buffers | |
344 | and this will help out in interleaving the data. | |
345 | ||
346 | x86-tsc: Architectures may define their own clocks. For | |
347 | example, x86 uses its own TSC cycle clock here. | |
348 | ||
349 | To set a clock, simply echo the clock name into this file. | |
350 | ||
351 | echo global > trace_clock | |
352 | ||
353 | trace_marker: | |
354 | ||
355 | This is a very useful file for synchronizing user space | |
356 | with events happening in the kernel. Writing strings into | |
357 | this file will be written into the ftrace buffer. | |
358 | ||
359 | It is useful in applications to open this file at the start | |
360 | of the application and just reference the file descriptor | |
361 | for the file. | |
362 | ||
363 | void trace_write(const char *fmt, ...) | |
364 | { | |
365 | va_list ap; | |
366 | char buf[256]; | |
367 | int n; | |
368 | ||
369 | if (trace_fd < 0) | |
370 | return; | |
371 | ||
372 | va_start(ap, fmt); | |
373 | n = vsnprintf(buf, 256, fmt, ap); | |
374 | va_end(ap); | |
375 | ||
376 | write(trace_fd, buf, n); | |
377 | } | |
378 | ||
379 | start: | |
380 | ||
381 | trace_fd = open("trace_marker", WR_ONLY); | |
382 | ||
383 | uprobe_events: | |
384 | ||
385 | Add dynamic tracepoints in programs. | |
386 | See uprobetracer.txt | |
387 | ||
388 | uprobe_profile: | |
389 | ||
390 | Uprobe statistics. See uprobetrace.txt | |
391 | ||
392 | instances: | |
393 | ||
394 | This is a way to make multiple trace buffers where different | |
395 | events can be recorded in different buffers. | |
396 | See "Instances" section below. | |
397 | ||
398 | events: | |
399 | ||
400 | This is the trace event directory. It holds event tracepoints | |
401 | (also known as static tracepoints) that have been compiled | |
402 | into the kernel. It shows what event tracepoints exist | |
403 | and how they are grouped by system. There are "enable" | |
404 | files at various levels that can enable the tracepoints | |
405 | when a "1" is written to them. | |
406 | ||
407 | See events.txt for more information. | |
408 | ||
409 | per_cpu: | |
410 | ||
411 | This is a directory that contains the trace per_cpu information. | |
412 | ||
413 | per_cpu/cpu0/buffer_size_kb: | |
414 | ||
415 | The ftrace buffer is defined per_cpu. That is, there's a separate | |
416 | buffer for each CPU to allow writes to be done atomically, | |
417 | and free from cache bouncing. These buffers may have different | |
418 | size buffers. This file is similar to the buffer_size_kb | |
419 | file, but it only displays or sets the buffer size for the | |
420 | specific CPU. (here cpu0). | |
421 | ||
422 | per_cpu/cpu0/trace: | |
423 | ||
424 | This is similar to the "trace" file, but it will only display | |
425 | the data specific for the CPU. If written to, it only clears | |
426 | the specific CPU buffer. | |
427 | ||
428 | per_cpu/cpu0/trace_pipe | |
429 | ||
430 | This is similar to the "trace_pipe" file, and is a consuming | |
431 | read, but it will only display (and consume) the data specific | |
432 | for the CPU. | |
433 | ||
434 | per_cpu/cpu0/trace_pipe_raw | |
435 | ||
436 | For tools that can parse the ftrace ring buffer binary format, | |
437 | the trace_pipe_raw file can be used to extract the data | |
438 | from the ring buffer directly. With the use of the splice() | |
439 | system call, the buffer data can be quickly transferred to | |
440 | a file or to the network where a server is collecting the | |
441 | data. | |
442 | ||
443 | Like trace_pipe, this is a consuming reader, where multiple | |
444 | reads will always produce different data. | |
445 | ||
446 | per_cpu/cpu0/snapshot: | |
447 | ||
448 | This is similar to the main "snapshot" file, but will only | |
449 | snapshot the current CPU (if supported). It only displays | |
450 | the content of the snapshot for a given CPU, and if | |
451 | written to, only clears this CPU buffer. | |
452 | ||
453 | per_cpu/cpu0/snapshot_raw: | |
454 | ||
455 | Similar to the trace_pipe_raw, but will read the binary format | |
456 | from the snapshot buffer for the given CPU. | |
457 | ||
458 | per_cpu/cpu0/stats: | |
459 | ||
460 | This displays certain stats about the ring buffer: | |
461 | ||
462 | entries: The number of events that are still in the buffer. | |
463 | ||
464 | overrun: The number of lost events due to overwriting when | |
465 | the buffer was full. | |
466 | ||
467 | commit overrun: Should always be zero. | |
468 | This gets set if so many events happened within a nested | |
469 | event (ring buffer is re-entrant), that it fills the | |
470 | buffer and starts dropping events. | |
471 | ||
472 | bytes: Bytes actually read (not overwritten). | |
473 | ||
474 | oldest event ts: The oldest timestamp in the buffer | |
475 | ||
476 | now ts: The current timestamp | |
477 | ||
478 | dropped events: Events lost due to overwrite option being off. | |
479 | ||
480 | read events: The number of events read. | |
eb6d42ea SR |
481 | |
482 | The Tracers | |
483 | ----------- | |
484 | ||
f2d9c740 | 485 | Here is the list of current tracers that may be configured. |
eb6d42ea | 486 | |
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487 | "function" |
488 | ||
489 | Function call tracer to trace all kernel functions. | |
490 | ||
bc5c6c04 | 491 | "function_graph" |
5752674e IM |
492 | |
493 | Similar to the function tracer except that the | |
494 | function tracer probes the functions on their entry | |
495 | whereas the function graph tracer traces on both entry | |
496 | and exit of the functions. It then provides the ability | |
497 | to draw a graph of function calls similar to C code | |
498 | source. | |
499 | ||
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500 | "irqsoff" |
501 | ||
502 | Traces the areas that disable interrupts and saves | |
503 | the trace with the longest max latency. | |
504 | See tracing_max_latency. When a new max is recorded, | |
505 | it replaces the old trace. It is best to view this | |
4a88d44a | 506 | trace with the latency-format option enabled. |
eb6d42ea | 507 | |
5752674e | 508 | "preemptoff" |
985ec20a | 509 | |
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510 | Similar to irqsoff but traces and records the amount of |
511 | time for which preemption is disabled. | |
eb6d42ea | 512 | |
5752674e | 513 | "preemptirqsoff" |
eb6d42ea | 514 | |
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515 | Similar to irqsoff and preemptoff, but traces and |
516 | records the largest time for which irqs and/or preemption | |
517 | is disabled. | |
eb6d42ea | 518 | |
5752674e | 519 | "wakeup" |
eb6d42ea | 520 | |
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521 | Traces and records the max latency that it takes for |
522 | the highest priority task to get scheduled after | |
523 | it has been woken up. | |
cdfb0d30 GL |
524 | Traces all tasks as an average developer would expect. |
525 | ||
526 | "wakeup_rt" | |
527 | ||
528 | Traces and records the max latency that it takes for just | |
529 | RT tasks (as the current "wakeup" does). This is useful | |
530 | for those interested in wake up timings of RT tasks. | |
eb6d42ea | 531 | |
5752674e IM |
532 | "nop" |
533 | ||
534 | This is the "trace nothing" tracer. To remove all | |
535 | tracers from tracing simply echo "nop" into | |
536 | current_tracer. | |
e2ea5399 | 537 | |
eb6d42ea SR |
538 | |
539 | Examples of using the tracer | |
540 | ---------------------------- | |
541 | ||
5752674e IM |
542 | Here are typical examples of using the tracers when controlling |
543 | them only with the debugfs interface (without using any | |
544 | user-land utilities). | |
eb6d42ea SR |
545 | |
546 | Output format: | |
547 | -------------- | |
548 | ||
f2d9c740 | 549 | Here is an example of the output format of the file "trace" |
eb6d42ea SR |
550 | |
551 | -------- | |
9b803c0f | 552 | # tracer: function |
eb6d42ea | 553 | # |
8d016091 SRRH |
554 | # entries-in-buffer/entries-written: 140080/250280 #P:4 |
555 | # | |
556 | # _-----=> irqs-off | |
557 | # / _----=> need-resched | |
558 | # | / _---=> hardirq/softirq | |
559 | # || / _--=> preempt-depth | |
560 | # ||| / delay | |
561 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
562 | # | | | |||| | | | |
563 | bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath | |
564 | bash-1977 [000] .... 17284.993653: __close_fd <-sys_close | |
565 | bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd | |
566 | sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify | |
567 | bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock | |
568 | bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd | |
569 | bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock | |
570 | bash-1977 [000] .... 17284.993657: filp_close <-__close_fd | |
571 | bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close | |
572 | sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath | |
eb6d42ea SR |
573 | -------- |
574 | ||
5752674e | 575 | A header is printed with the tracer name that is represented by |
8d016091 SRRH |
576 | the trace. In this case the tracer is "function". Then it shows the |
577 | number of events in the buffer as well as the total number of entries | |
578 | that were written. The difference is the number of entries that were | |
579 | lost due to the buffer filling up (250280 - 140080 = 110200 events | |
580 | lost). | |
581 | ||
582 | The header explains the content of the events. Task name "bash", the task | |
583 | PID "1977", the CPU that it was running on "000", the latency format | |
584 | (explained below), the timestamp in <secs>.<usecs> format, the | |
585 | function name that was traced "sys_close" and the parent function that | |
586 | called this function "system_call_fastpath". The timestamp is the time | |
587 | at which the function was entered. | |
eb6d42ea | 588 | |
eb6d42ea SR |
589 | Latency trace format |
590 | -------------------- | |
591 | ||
8d016091 SRRH |
592 | When the latency-format option is enabled or when one of the latency |
593 | tracers is set, the trace file gives somewhat more information to see | |
594 | why a latency happened. Here is a typical trace. | |
eb6d42ea SR |
595 | |
596 | # tracer: irqsoff | |
597 | # | |
8d016091 SRRH |
598 | # irqsoff latency trace v1.1.5 on 3.8.0-test+ |
599 | # -------------------------------------------------------------------- | |
600 | # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
601 | # ----------------- | |
602 | # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0) | |
603 | # ----------------- | |
604 | # => started at: __lock_task_sighand | |
605 | # => ended at: _raw_spin_unlock_irqrestore | |
606 | # | |
607 | # | |
608 | # _------=> CPU# | |
609 | # / _-----=> irqs-off | |
610 | # | / _----=> need-resched | |
611 | # || / _---=> hardirq/softirq | |
612 | # ||| / _--=> preempt-depth | |
613 | # |||| / delay | |
614 | # cmd pid ||||| time | caller | |
615 | # \ / ||||| \ | / | |
616 | ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand | |
617 | ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore | |
618 | ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore | |
619 | ps-6143 2d..1 306us : <stack trace> | |
620 | => trace_hardirqs_on_caller | |
621 | => trace_hardirqs_on | |
622 | => _raw_spin_unlock_irqrestore | |
623 | => do_task_stat | |
624 | => proc_tgid_stat | |
625 | => proc_single_show | |
626 | => seq_read | |
627 | => vfs_read | |
628 | => sys_read | |
629 | => system_call_fastpath | |
eb6d42ea SR |
630 | |
631 | ||
5752674e | 632 | This shows that the current tracer is "irqsoff" tracing the time |
8d016091 SRRH |
633 | for which interrupts were disabled. It gives the trace version (which |
634 | never changes) and the version of the kernel upon which this was executed on | |
635 | (3.10). Then it displays the max latency in microseconds (259 us). The number | |
636 | of trace entries displayed and the total number (both are four: #4/4). | |
637 | VP, KP, SP, and HP are always zero and are reserved for later use. | |
638 | #P is the number of online CPUs (#P:4). | |
eb6d42ea | 639 | |
5752674e | 640 | The task is the process that was running when the latency |
8d016091 | 641 | occurred. (ps pid: 6143). |
eb6d42ea | 642 | |
5752674e IM |
643 | The start and stop (the functions in which the interrupts were |
644 | disabled and enabled respectively) that caused the latencies: | |
eb6d42ea | 645 | |
8d016091 SRRH |
646 | __lock_task_sighand is where the interrupts were disabled. |
647 | _raw_spin_unlock_irqrestore is where they were enabled again. | |
eb6d42ea SR |
648 | |
649 | The next lines after the header are the trace itself. The header | |
650 | explains which is which. | |
651 | ||
652 | cmd: The name of the process in the trace. | |
653 | ||
654 | pid: The PID of that process. | |
655 | ||
f2d9c740 | 656 | CPU#: The CPU which the process was running on. |
eb6d42ea SR |
657 | |
658 | irqs-off: 'd' interrupts are disabled. '.' otherwise. | |
9244489a SR |
659 | Note: If the architecture does not support a way to |
660 | read the irq flags variable, an 'X' will always | |
661 | be printed here. | |
eb6d42ea | 662 | |
e5137b50 PZ |
663 | need-resched: |
664 | 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set, | |
665 | 'n' only TIF_NEED_RESCHED is set, | |
666 | 'p' only PREEMPT_NEED_RESCHED is set, | |
667 | '.' otherwise. | |
eb6d42ea SR |
668 | |
669 | hardirq/softirq: | |
f2d9c740 | 670 | 'H' - hard irq occurred inside a softirq. |
eb6d42ea SR |
671 | 'h' - hard irq is running |
672 | 's' - soft irq is running | |
673 | '.' - normal context. | |
674 | ||
675 | preempt-depth: The level of preempt_disabled | |
676 | ||
677 | The above is mostly meaningful for kernel developers. | |
678 | ||
4a88d44a AT |
679 | time: When the latency-format option is enabled, the trace file |
680 | output includes a timestamp relative to the start of the | |
681 | trace. This differs from the output when latency-format | |
682 | is disabled, which includes an absolute timestamp. | |
eb6d42ea SR |
683 | |
684 | delay: This is just to help catch your eye a bit better. And | |
5752674e IM |
685 | needs to be fixed to be only relative to the same CPU. |
686 | The marks are determined by the difference between this | |
687 | current trace and the next trace. | |
688 | '!' - greater than preempt_mark_thresh (default 100) | |
689 | '+' - greater than 1 microsecond | |
690 | ' ' - less than or equal to 1 microsecond. | |
eb6d42ea SR |
691 | |
692 | The rest is the same as the 'trace' file. | |
693 | ||
8d016091 SRRH |
694 | Note, the latency tracers will usually end with a back trace |
695 | to easily find where the latency occurred. | |
eb6d42ea | 696 | |
ee6bce52 SR |
697 | trace_options |
698 | ------------- | |
eb6d42ea | 699 | |
8d016091 SRRH |
700 | The trace_options file (or the options directory) is used to control |
701 | what gets printed in the trace output, or manipulate the tracers. | |
702 | To see what is available, simply cat the file: | |
eb6d42ea | 703 | |
156f5a78 | 704 | cat trace_options |
8d016091 SRRH |
705 | print-parent |
706 | nosym-offset | |
707 | nosym-addr | |
708 | noverbose | |
709 | noraw | |
710 | nohex | |
711 | nobin | |
712 | noblock | |
713 | nostacktrace | |
714 | trace_printk | |
715 | noftrace_preempt | |
716 | nobranch | |
717 | annotate | |
718 | nouserstacktrace | |
719 | nosym-userobj | |
720 | noprintk-msg-only | |
721 | context-info | |
722 | latency-format | |
723 | sleep-time | |
724 | graph-time | |
725 | record-cmd | |
726 | overwrite | |
727 | nodisable_on_free | |
728 | irq-info | |
729 | markers | |
730 | function-trace | |
eb6d42ea | 731 | |
5752674e IM |
732 | To disable one of the options, echo in the option prepended with |
733 | "no". | |
eb6d42ea | 734 | |
156f5a78 | 735 | echo noprint-parent > trace_options |
eb6d42ea SR |
736 | |
737 | To enable an option, leave off the "no". | |
738 | ||
156f5a78 | 739 | echo sym-offset > trace_options |
eb6d42ea SR |
740 | |
741 | Here are the available options: | |
742 | ||
5752674e IM |
743 | print-parent - On function traces, display the calling (parent) |
744 | function as well as the function being traced. | |
eb6d42ea SR |
745 | |
746 | print-parent: | |
0ab943bf | 747 | bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul |
eb6d42ea SR |
748 | |
749 | noprint-parent: | |
750 | bash-4000 [01] 1477.606694: simple_strtoul | |
751 | ||
752 | ||
5752674e IM |
753 | sym-offset - Display not only the function name, but also the |
754 | offset in the function. For example, instead of | |
755 | seeing just "ktime_get", you will see | |
756 | "ktime_get+0xb/0x20". | |
eb6d42ea SR |
757 | |
758 | sym-offset: | |
759 | bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 | |
760 | ||
5752674e IM |
761 | sym-addr - this will also display the function address as well |
762 | as the function name. | |
eb6d42ea SR |
763 | |
764 | sym-addr: | |
765 | bash-4000 [01] 1477.606694: simple_strtoul <c0339346> | |
766 | ||
4a88d44a AT |
767 | verbose - This deals with the trace file when the |
768 | latency-format option is enabled. | |
eb6d42ea SR |
769 | |
770 | bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ | |
0ab943bf | 771 | (+0.000ms): simple_strtoul (kstrtoul) |
eb6d42ea | 772 | |
5752674e IM |
773 | raw - This will display raw numbers. This option is best for |
774 | use with user applications that can translate the raw | |
775 | numbers better than having it done in the kernel. | |
eb6d42ea | 776 | |
5752674e IM |
777 | hex - Similar to raw, but the numbers will be in a hexadecimal |
778 | format. | |
eb6d42ea SR |
779 | |
780 | bin - This will print out the formats in raw binary. | |
781 | ||
8d016091 | 782 | block - When set, reading trace_pipe will not block when polled. |
eb6d42ea | 783 | |
5752674e IM |
784 | stacktrace - This is one of the options that changes the trace |
785 | itself. When a trace is recorded, so is the stack | |
786 | of functions. This allows for back traces of | |
787 | trace sites. | |
eb6d42ea | 788 | |
8d016091 SRRH |
789 | trace_printk - Can disable trace_printk() from writing into the buffer. |
790 | ||
791 | branch - Enable branch tracing with the tracer. | |
792 | ||
793 | annotate - It is sometimes confusing when the CPU buffers are full | |
794 | and one CPU buffer had a lot of events recently, thus | |
795 | a shorter time frame, were another CPU may have only had | |
796 | a few events, which lets it have older events. When | |
797 | the trace is reported, it shows the oldest events first, | |
798 | and it may look like only one CPU ran (the one with the | |
799 | oldest events). When the annotate option is set, it will | |
800 | display when a new CPU buffer started: | |
801 | ||
802 | <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on | |
803 | <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on | |
804 | <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore | |
805 | ##### CPU 2 buffer started #### | |
806 | <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle | |
807 | <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog | |
808 | <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock | |
809 | ||
5752674e IM |
810 | userstacktrace - This option changes the trace. It records a |
811 | stacktrace of the current userspace thread. | |
02b67518 | 812 | |
5752674e IM |
813 | sym-userobj - when user stacktrace are enabled, look up which |
814 | object the address belongs to, and print a | |
815 | relative address. This is especially useful when | |
816 | ASLR is on, otherwise you don't get a chance to | |
817 | resolve the address to object/file/line after | |
818 | the app is no longer running | |
b54d3de9 | 819 | |
5752674e | 820 | The lookup is performed when you read |
4a88d44a | 821 | trace,trace_pipe. Example: |
b54d3de9 TE |
822 | |
823 | a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 | |
824 | x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] | |
825 | ||
8d016091 SRRH |
826 | |
827 | printk-msg-only - When set, trace_printk()s will only show the format | |
828 | and not their parameters (if trace_bprintk() or | |
829 | trace_bputs() was used to save the trace_printk()). | |
830 | ||
831 | context-info - Show only the event data. Hides the comm, PID, | |
832 | timestamp, CPU, and other useful data. | |
eb6d42ea | 833 | |
4a88d44a AT |
834 | latency-format - This option changes the trace. When |
835 | it is enabled, the trace displays | |
836 | additional information about the | |
837 | latencies, as described in "Latency | |
838 | trace format". | |
eb6d42ea | 839 | |
8d016091 SRRH |
840 | sleep-time - When running function graph tracer, to include |
841 | the time a task schedules out in its function. | |
842 | When enabled, it will account time the task has been | |
843 | scheduled out as part of the function call. | |
844 | ||
845 | graph-time - When running function graph tracer, to include the | |
846 | time to call nested functions. When this is not set, | |
847 | the time reported for the function will only include | |
848 | the time the function itself executed for, not the time | |
849 | for functions that it called. | |
850 | ||
851 | record-cmd - When any event or tracer is enabled, a hook is enabled | |
852 | in the sched_switch trace point to fill comm cache | |
853 | with mapped pids and comms. But this may cause some | |
854 | overhead, and if you only care about pids, and not the | |
855 | name of the task, disabling this option can lower the | |
856 | impact of tracing. | |
857 | ||
750912fa DS |
858 | overwrite - This controls what happens when the trace buffer is |
859 | full. If "1" (default), the oldest events are | |
860 | discarded and overwritten. If "0", then the newest | |
861 | events are discarded. | |
8d016091 | 862 | (see per_cpu/cpu0/stats for overrun and dropped) |
750912fa | 863 | |
8d016091 SRRH |
864 | disable_on_free - When the free_buffer is closed, tracing will |
865 | stop (tracing_on set to 0). | |
eb6d42ea | 866 | |
8d016091 SRRH |
867 | irq-info - Shows the interrupt, preempt count, need resched data. |
868 | When disabled, the trace looks like: | |
eb6d42ea | 869 | |
8d016091 SRRH |
870 | # tracer: function |
871 | # | |
872 | # entries-in-buffer/entries-written: 144405/9452052 #P:4 | |
873 | # | |
874 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
875 | # | | | | | | |
876 | <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up | |
877 | <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89 | |
878 | <idle>-0 [002] 23636.756055: enqueue_task <-activate_task | |
eb6d42ea | 879 | |
eb6d42ea | 880 | |
8d016091 SRRH |
881 | markers - When set, the trace_marker is writable (only by root). |
882 | When disabled, the trace_marker will error with EINVAL | |
883 | on write. | |
884 | ||
885 | ||
886 | function-trace - The latency tracers will enable function tracing | |
887 | if this option is enabled (default it is). When | |
888 | it is disabled, the latency tracers do not trace | |
889 | functions. This keeps the overhead of the tracer down | |
890 | when performing latency tests. | |
eb6d42ea | 891 | |
8d016091 SRRH |
892 | Note: Some tracers have their own options. They only appear |
893 | when the tracer is active. | |
eb6d42ea | 894 | |
eb6d42ea SR |
895 | |
896 | ||
897 | irqsoff | |
898 | ------- | |
899 | ||
900 | When interrupts are disabled, the CPU can not react to any other | |
901 | external event (besides NMIs and SMIs). This prevents the timer | |
5752674e IM |
902 | interrupt from triggering or the mouse interrupt from letting |
903 | the kernel know of a new mouse event. The result is a latency | |
904 | with the reaction time. | |
eb6d42ea | 905 | |
5752674e IM |
906 | The irqsoff tracer tracks the time for which interrupts are |
907 | disabled. When a new maximum latency is hit, the tracer saves | |
908 | the trace leading up to that latency point so that every time a | |
909 | new maximum is reached, the old saved trace is discarded and the | |
910 | new trace is saved. | |
eb6d42ea | 911 | |
5752674e IM |
912 | To reset the maximum, echo 0 into tracing_max_latency. Here is |
913 | an example: | |
eb6d42ea | 914 | |
8d016091 | 915 | # echo 0 > options/function-trace |
156f5a78 | 916 | # echo irqsoff > current_tracer |
6752ab4a | 917 | # echo 1 > tracing_on |
8d016091 | 918 | # echo 0 > tracing_max_latency |
eb6d42ea SR |
919 | # ls -ltr |
920 | [...] | |
6752ab4a | 921 | # echo 0 > tracing_on |
4a88d44a | 922 | # cat trace |
eb6d42ea SR |
923 | # tracer: irqsoff |
924 | # | |
8d016091 SRRH |
925 | # irqsoff latency trace v1.1.5 on 3.8.0-test+ |
926 | # -------------------------------------------------------------------- | |
927 | # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
928 | # ----------------- | |
929 | # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0) | |
930 | # ----------------- | |
931 | # => started at: run_timer_softirq | |
932 | # => ended at: run_timer_softirq | |
933 | # | |
934 | # | |
935 | # _------=> CPU# | |
936 | # / _-----=> irqs-off | |
937 | # | / _----=> need-resched | |
938 | # || / _---=> hardirq/softirq | |
939 | # ||| / _--=> preempt-depth | |
940 | # |||| / delay | |
941 | # cmd pid ||||| time | caller | |
942 | # \ / ||||| \ | / | |
943 | <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq | |
944 | <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq | |
945 | <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq | |
946 | <idle>-0 0dNs3 25us : <stack trace> | |
947 | => _raw_spin_unlock_irq | |
948 | => run_timer_softirq | |
949 | => __do_softirq | |
950 | => call_softirq | |
951 | => do_softirq | |
952 | => irq_exit | |
953 | => smp_apic_timer_interrupt | |
954 | => apic_timer_interrupt | |
955 | => rcu_idle_exit | |
956 | => cpu_idle | |
957 | => rest_init | |
958 | => start_kernel | |
959 | => x86_64_start_reservations | |
960 | => x86_64_start_kernel | |
961 | ||
962 | Here we see that that we had a latency of 16 microseconds (which is | |
963 | very good). The _raw_spin_lock_irq in run_timer_softirq disabled | |
964 | interrupts. The difference between the 16 and the displayed | |
965 | timestamp 25us occurred because the clock was incremented | |
5752674e IM |
966 | between the time of recording the max latency and the time of |
967 | recording the function that had that latency. | |
eb6d42ea | 968 | |
8d016091 SRRH |
969 | Note the above example had function-trace not set. If we set |
970 | function-trace, we get a much larger output: | |
971 | ||
972 | with echo 1 > options/function-trace | |
eb6d42ea SR |
973 | |
974 | # tracer: irqsoff | |
975 | # | |
8d016091 SRRH |
976 | # irqsoff latency trace v1.1.5 on 3.8.0-test+ |
977 | # -------------------------------------------------------------------- | |
978 | # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
979 | # ----------------- | |
980 | # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0) | |
981 | # ----------------- | |
982 | # => started at: ata_scsi_queuecmd | |
983 | # => ended at: ata_scsi_queuecmd | |
984 | # | |
985 | # | |
986 | # _------=> CPU# | |
987 | # / _-----=> irqs-off | |
988 | # | / _----=> need-resched | |
989 | # || / _---=> hardirq/softirq | |
990 | # ||| / _--=> preempt-depth | |
991 | # |||| / delay | |
992 | # cmd pid ||||| time | caller | |
993 | # \ / ||||| \ | / | |
994 | bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd | |
995 | bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave | |
996 | bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd | |
997 | bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev | |
998 | bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev | |
999 | bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd | |
1000 | bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd | |
1001 | bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd | |
1002 | bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat | |
eb6d42ea | 1003 | [...] |
8d016091 SRRH |
1004 | bash-2042 3d..1 67us : delay_tsc <-__delay |
1005 | bash-2042 3d..1 67us : add_preempt_count <-delay_tsc | |
1006 | bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc | |
1007 | bash-2042 3d..1 67us : add_preempt_count <-delay_tsc | |
1008 | bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc | |
1009 | bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue | |
1010 | bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd | |
1011 | bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd | |
1012 | bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd | |
1013 | bash-2042 3d..1 120us : <stack trace> | |
1014 | => _raw_spin_unlock_irqrestore | |
1015 | => ata_scsi_queuecmd | |
1016 | => scsi_dispatch_cmd | |
1017 | => scsi_request_fn | |
1018 | => __blk_run_queue_uncond | |
1019 | => __blk_run_queue | |
1020 | => blk_queue_bio | |
1021 | => generic_make_request | |
1022 | => submit_bio | |
1023 | => submit_bh | |
1024 | => __ext3_get_inode_loc | |
1025 | => ext3_iget | |
1026 | => ext3_lookup | |
1027 | => lookup_real | |
1028 | => __lookup_hash | |
1029 | => walk_component | |
1030 | => lookup_last | |
1031 | => path_lookupat | |
1032 | => filename_lookup | |
1033 | => user_path_at_empty | |
1034 | => user_path_at | |
1035 | => vfs_fstatat | |
1036 | => vfs_stat | |
1037 | => sys_newstat | |
1038 | => system_call_fastpath | |
1039 | ||
1040 | ||
1041 | Here we traced a 71 microsecond latency. But we also see all the | |
5752674e IM |
1042 | functions that were called during that time. Note that by |
1043 | enabling function tracing, we incur an added overhead. This | |
1044 | overhead may extend the latency times. But nevertheless, this | |
1045 | trace has provided some very helpful debugging information. | |
eb6d42ea SR |
1046 | |
1047 | ||
1048 | preemptoff | |
1049 | ---------- | |
1050 | ||
5752674e IM |
1051 | When preemption is disabled, we may be able to receive |
1052 | interrupts but the task cannot be preempted and a higher | |
1053 | priority task must wait for preemption to be enabled again | |
1054 | before it can preempt a lower priority task. | |
eb6d42ea | 1055 | |
a41eebab | 1056 | The preemptoff tracer traces the places that disable preemption. |
5752674e IM |
1057 | Like the irqsoff tracer, it records the maximum latency for |
1058 | which preemption was disabled. The control of preemptoff tracer | |
1059 | is much like the irqsoff tracer. | |
eb6d42ea | 1060 | |
8d016091 | 1061 | # echo 0 > options/function-trace |
156f5a78 | 1062 | # echo preemptoff > current_tracer |
6752ab4a | 1063 | # echo 1 > tracing_on |
8d016091 | 1064 | # echo 0 > tracing_max_latency |
eb6d42ea SR |
1065 | # ls -ltr |
1066 | [...] | |
6752ab4a | 1067 | # echo 0 > tracing_on |
4a88d44a | 1068 | # cat trace |
eb6d42ea SR |
1069 | # tracer: preemptoff |
1070 | # | |
8d016091 SRRH |
1071 | # preemptoff latency trace v1.1.5 on 3.8.0-test+ |
1072 | # -------------------------------------------------------------------- | |
1073 | # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1074 | # ----------------- | |
1075 | # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0) | |
1076 | # ----------------- | |
1077 | # => started at: do_IRQ | |
1078 | # => ended at: do_IRQ | |
1079 | # | |
1080 | # | |
1081 | # _------=> CPU# | |
1082 | # / _-----=> irqs-off | |
1083 | # | / _----=> need-resched | |
1084 | # || / _---=> hardirq/softirq | |
1085 | # ||| / _--=> preempt-depth | |
1086 | # |||| / delay | |
1087 | # cmd pid ||||| time | caller | |
1088 | # \ / ||||| \ | / | |
1089 | sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ | |
1090 | sshd-1991 1d..1 46us : irq_exit <-do_IRQ | |
1091 | sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ | |
1092 | sshd-1991 1d..1 52us : <stack trace> | |
1093 | => sub_preempt_count | |
1094 | => irq_exit | |
1095 | => do_IRQ | |
1096 | => ret_from_intr | |
eb6d42ea SR |
1097 | |
1098 | ||
5752674e | 1099 | This has some more changes. Preemption was disabled when an |
8d016091 SRRH |
1100 | interrupt came in (notice the 'h'), and was enabled on exit. |
1101 | But we also see that interrupts have been disabled when entering | |
1102 | the preempt off section and leaving it (the 'd'). We do not know if | |
1103 | interrupts were enabled in the mean time or shortly after this | |
1104 | was over. | |
eb6d42ea SR |
1105 | |
1106 | # tracer: preemptoff | |
1107 | # | |
8d016091 SRRH |
1108 | # preemptoff latency trace v1.1.5 on 3.8.0-test+ |
1109 | # -------------------------------------------------------------------- | |
1110 | # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1111 | # ----------------- | |
1112 | # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0) | |
1113 | # ----------------- | |
1114 | # => started at: wake_up_new_task | |
1115 | # => ended at: task_rq_unlock | |
1116 | # | |
1117 | # | |
1118 | # _------=> CPU# | |
1119 | # / _-----=> irqs-off | |
1120 | # | / _----=> need-resched | |
1121 | # || / _---=> hardirq/softirq | |
1122 | # ||| / _--=> preempt-depth | |
1123 | # |||| / delay | |
1124 | # cmd pid ||||| time | caller | |
1125 | # \ / ||||| \ | / | |
1126 | bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task | |
1127 | bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq | |
1128 | bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair | |
1129 | bash-1994 1d..1 1us : source_load <-select_task_rq_fair | |
1130 | bash-1994 1d..1 1us : source_load <-select_task_rq_fair | |
eb6d42ea | 1131 | [...] |
8d016091 SRRH |
1132 | bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt |
1133 | bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter | |
1134 | bash-1994 1d..1 13us : add_preempt_count <-irq_enter | |
1135 | bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt | |
1136 | bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt | |
1137 | bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt | |
1138 | bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock | |
1139 | bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt | |
eb6d42ea | 1140 | [...] |
8d016091 SRRH |
1141 | bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event |
1142 | bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt | |
1143 | bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit | |
1144 | bash-1994 1d..2 36us : do_softirq <-irq_exit | |
1145 | bash-1994 1d..2 36us : __do_softirq <-call_softirq | |
1146 | bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq | |
1147 | bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq | |
1148 | bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq | |
1149 | bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock | |
1150 | bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq | |
eb6d42ea | 1151 | [...] |
8d016091 SRRH |
1152 | bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks |
1153 | bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq | |
1154 | bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable | |
1155 | bash-1994 1dN.2 82us : idle_cpu <-irq_exit | |
1156 | bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit | |
1157 | bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit | |
1158 | bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock | |
1159 | bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock | |
1160 | bash-1994 1.N.1 104us : <stack trace> | |
1161 | => sub_preempt_count | |
1162 | => _raw_spin_unlock_irqrestore | |
1163 | => task_rq_unlock | |
1164 | => wake_up_new_task | |
1165 | => do_fork | |
1166 | => sys_clone | |
1167 | => stub_clone | |
eb6d42ea SR |
1168 | |
1169 | ||
5752674e | 1170 | The above is an example of the preemptoff trace with |
8d016091 | 1171 | function-trace set. Here we see that interrupts were not disabled |
5752674e IM |
1172 | the entire time. The irq_enter code lets us know that we entered |
1173 | an interrupt 'h'. Before that, the functions being traced still | |
1174 | show that it is not in an interrupt, but we can see from the | |
1175 | functions themselves that this is not the case. | |
eb6d42ea | 1176 | |
eb6d42ea SR |
1177 | preemptirqsoff |
1178 | -------------- | |
1179 | ||
5752674e IM |
1180 | Knowing the locations that have interrupts disabled or |
1181 | preemption disabled for the longest times is helpful. But | |
1182 | sometimes we would like to know when either preemption and/or | |
1183 | interrupts are disabled. | |
eb6d42ea | 1184 | |
f2d9c740 | 1185 | Consider the following code: |
eb6d42ea SR |
1186 | |
1187 | local_irq_disable(); | |
1188 | call_function_with_irqs_off(); | |
1189 | preempt_disable(); | |
1190 | call_function_with_irqs_and_preemption_off(); | |
1191 | local_irq_enable(); | |
1192 | call_function_with_preemption_off(); | |
1193 | preempt_enable(); | |
1194 | ||
1195 | The irqsoff tracer will record the total length of | |
1196 | call_function_with_irqs_off() and | |
1197 | call_function_with_irqs_and_preemption_off(). | |
1198 | ||
1199 | The preemptoff tracer will record the total length of | |
1200 | call_function_with_irqs_and_preemption_off() and | |
1201 | call_function_with_preemption_off(). | |
1202 | ||
5752674e IM |
1203 | But neither will trace the time that interrupts and/or |
1204 | preemption is disabled. This total time is the time that we can | |
1205 | not schedule. To record this time, use the preemptirqsoff | |
1206 | tracer. | |
eb6d42ea | 1207 | |
5752674e IM |
1208 | Again, using this trace is much like the irqsoff and preemptoff |
1209 | tracers. | |
eb6d42ea | 1210 | |
8d016091 | 1211 | # echo 0 > options/function-trace |
156f5a78 | 1212 | # echo preemptirqsoff > current_tracer |
6752ab4a | 1213 | # echo 1 > tracing_on |
8d016091 | 1214 | # echo 0 > tracing_max_latency |
eb6d42ea SR |
1215 | # ls -ltr |
1216 | [...] | |
6752ab4a | 1217 | # echo 0 > tracing_on |
4a88d44a | 1218 | # cat trace |
eb6d42ea SR |
1219 | # tracer: preemptirqsoff |
1220 | # | |
8d016091 SRRH |
1221 | # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ |
1222 | # -------------------------------------------------------------------- | |
1223 | # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1224 | # ----------------- | |
1225 | # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0) | |
1226 | # ----------------- | |
1227 | # => started at: ata_scsi_queuecmd | |
1228 | # => ended at: ata_scsi_queuecmd | |
1229 | # | |
1230 | # | |
1231 | # _------=> CPU# | |
1232 | # / _-----=> irqs-off | |
1233 | # | / _----=> need-resched | |
1234 | # || / _---=> hardirq/softirq | |
1235 | # ||| / _--=> preempt-depth | |
1236 | # |||| / delay | |
1237 | # cmd pid ||||| time | caller | |
1238 | # \ / ||||| \ | / | |
1239 | ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd | |
1240 | ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd | |
1241 | ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd | |
1242 | ls-2230 3...1 111us : <stack trace> | |
1243 | => sub_preempt_count | |
1244 | => _raw_spin_unlock_irqrestore | |
1245 | => ata_scsi_queuecmd | |
1246 | => scsi_dispatch_cmd | |
1247 | => scsi_request_fn | |
1248 | => __blk_run_queue_uncond | |
1249 | => __blk_run_queue | |
1250 | => blk_queue_bio | |
1251 | => generic_make_request | |
1252 | => submit_bio | |
1253 | => submit_bh | |
1254 | => ext3_bread | |
1255 | => ext3_dir_bread | |
1256 | => htree_dirblock_to_tree | |
1257 | => ext3_htree_fill_tree | |
1258 | => ext3_readdir | |
1259 | => vfs_readdir | |
1260 | => sys_getdents | |
1261 | => system_call_fastpath | |
eb6d42ea | 1262 | |
eb6d42ea SR |
1263 | |
1264 | The trace_hardirqs_off_thunk is called from assembly on x86 when | |
5752674e IM |
1265 | interrupts are disabled in the assembly code. Without the |
1266 | function tracing, we do not know if interrupts were enabled | |
1267 | within the preemption points. We do see that it started with | |
1268 | preemption enabled. | |
eb6d42ea | 1269 | |
8d016091 | 1270 | Here is a trace with function-trace set: |
eb6d42ea SR |
1271 | |
1272 | # tracer: preemptirqsoff | |
1273 | # | |
8d016091 SRRH |
1274 | # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ |
1275 | # -------------------------------------------------------------------- | |
1276 | # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1277 | # ----------------- | |
1278 | # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0) | |
1279 | # ----------------- | |
1280 | # => started at: schedule | |
1281 | # => ended at: mutex_unlock | |
1282 | # | |
1283 | # | |
1284 | # _------=> CPU# | |
1285 | # / _-----=> irqs-off | |
1286 | # | / _----=> need-resched | |
1287 | # || / _---=> hardirq/softirq | |
1288 | # ||| / _--=> preempt-depth | |
1289 | # |||| / delay | |
1290 | # cmd pid ||||| time | caller | |
1291 | # \ / ||||| \ | / | |
1292 | kworker/-59 3...1 0us : __schedule <-schedule | |
1293 | kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch | |
1294 | kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq | |
1295 | kworker/-59 3d..2 1us : deactivate_task <-__schedule | |
1296 | kworker/-59 3d..2 1us : dequeue_task <-deactivate_task | |
1297 | kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task | |
1298 | kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task | |
1299 | kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair | |
1300 | kworker/-59 3d..2 2us : update_min_vruntime <-update_curr | |
1301 | kworker/-59 3d..2 3us : cpuacct_charge <-update_curr | |
1302 | kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge | |
1303 | kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge | |
1304 | kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair | |
1305 | kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair | |
1306 | kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair | |
1307 | kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair | |
1308 | kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair | |
1309 | kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair | |
1310 | kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule | |
1311 | kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping | |
1312 | kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule | |
1313 | kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task | |
1314 | kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair | |
1315 | kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair | |
1316 | kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity | |
1317 | ls-2269 3d..2 7us : finish_task_switch <-__schedule | |
1318 | ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch | |
1319 | ls-2269 3d..2 8us : do_IRQ <-ret_from_intr | |
1320 | ls-2269 3d..2 8us : irq_enter <-do_IRQ | |
1321 | ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter | |
1322 | ls-2269 3d..2 9us : add_preempt_count <-irq_enter | |
1323 | ls-2269 3d.h2 9us : exit_idle <-do_IRQ | |
eb6d42ea | 1324 | [...] |
8d016091 SRRH |
1325 | ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock |
1326 | ls-2269 3d.h2 20us : irq_exit <-do_IRQ | |
1327 | ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit | |
1328 | ls-2269 3d..3 21us : do_softirq <-irq_exit | |
1329 | ls-2269 3d..3 21us : __do_softirq <-call_softirq | |
1330 | ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq | |
1331 | ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip | |
1332 | ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip | |
1333 | ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr | |
1334 | ls-2269 3d.s5 31us : irq_enter <-do_IRQ | |
1335 | ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter | |
eb6d42ea | 1336 | [...] |
8d016091 SRRH |
1337 | ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter |
1338 | ls-2269 3d.s5 32us : add_preempt_count <-irq_enter | |
1339 | ls-2269 3d.H5 32us : exit_idle <-do_IRQ | |
1340 | ls-2269 3d.H5 32us : handle_irq <-do_IRQ | |
1341 | ls-2269 3d.H5 32us : irq_to_desc <-handle_irq | |
1342 | ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq | |
eb6d42ea | 1343 | [...] |
8d016091 SRRH |
1344 | ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll |
1345 | ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action | |
1346 | ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq | |
1347 | ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable | |
1348 | ls-2269 3d..3 159us : idle_cpu <-irq_exit | |
1349 | ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit | |
1350 | ls-2269 3d..3 160us : sub_preempt_count <-irq_exit | |
1351 | ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock | |
1352 | ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock | |
1353 | ls-2269 3d... 186us : <stack trace> | |
1354 | => __mutex_unlock_slowpath | |
1355 | => mutex_unlock | |
1356 | => process_output | |
1357 | => n_tty_write | |
1358 | => tty_write | |
1359 | => vfs_write | |
1360 | => sys_write | |
1361 | => system_call_fastpath | |
1362 | ||
1363 | This is an interesting trace. It started with kworker running and | |
1364 | scheduling out and ls taking over. But as soon as ls released the | |
1365 | rq lock and enabled interrupts (but not preemption) an interrupt | |
1366 | triggered. When the interrupt finished, it started running softirqs. | |
1367 | But while the softirq was running, another interrupt triggered. | |
1368 | When an interrupt is running inside a softirq, the annotation is 'H'. | |
eb6d42ea SR |
1369 | |
1370 | ||
1371 | wakeup | |
1372 | ------ | |
1373 | ||
8d016091 SRRH |
1374 | One common case that people are interested in tracing is the |
1375 | time it takes for a task that is woken to actually wake up. | |
1376 | Now for non Real-Time tasks, this can be arbitrary. But tracing | |
1377 | it none the less can be interesting. | |
1378 | ||
1379 | Without function tracing: | |
1380 | ||
1381 | # echo 0 > options/function-trace | |
1382 | # echo wakeup > current_tracer | |
1383 | # echo 1 > tracing_on | |
1384 | # echo 0 > tracing_max_latency | |
1385 | # chrt -f 5 sleep 1 | |
1386 | # echo 0 > tracing_on | |
1387 | # cat trace | |
1388 | # tracer: wakeup | |
1389 | # | |
1390 | # wakeup latency trace v1.1.5 on 3.8.0-test+ | |
1391 | # -------------------------------------------------------------------- | |
1392 | # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1393 | # ----------------- | |
1394 | # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0) | |
1395 | # ----------------- | |
1396 | # | |
1397 | # _------=> CPU# | |
1398 | # / _-----=> irqs-off | |
1399 | # | / _----=> need-resched | |
1400 | # || / _---=> hardirq/softirq | |
1401 | # ||| / _--=> preempt-depth | |
1402 | # |||| / delay | |
1403 | # cmd pid ||||| time | caller | |
1404 | # \ / ||||| \ | / | |
1405 | <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H | |
1406 | <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up | |
1407 | <idle>-0 3d..3 15us : __schedule <-schedule | |
1408 | <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H | |
1409 | ||
1410 | The tracer only traces the highest priority task in the system | |
1411 | to avoid tracing the normal circumstances. Here we see that | |
1412 | the kworker with a nice priority of -20 (not very nice), took | |
1413 | just 15 microseconds from the time it woke up, to the time it | |
1414 | ran. | |
1415 | ||
1416 | Non Real-Time tasks are not that interesting. A more interesting | |
1417 | trace is to concentrate only on Real-Time tasks. | |
1418 | ||
1419 | wakeup_rt | |
1420 | --------- | |
1421 | ||
5752674e IM |
1422 | In a Real-Time environment it is very important to know the |
1423 | wakeup time it takes for the highest priority task that is woken | |
1424 | up to the time that it executes. This is also known as "schedule | |
1425 | latency". I stress the point that this is about RT tasks. It is | |
1426 | also important to know the scheduling latency of non-RT tasks, | |
1427 | but the average schedule latency is better for non-RT tasks. | |
1428 | Tools like LatencyTop are more appropriate for such | |
1429 | measurements. | |
eb6d42ea | 1430 | |
a41eebab | 1431 | Real-Time environments are interested in the worst case latency. |
5752674e IM |
1432 | That is the longest latency it takes for something to happen, |
1433 | and not the average. We can have a very fast scheduler that may | |
1434 | only have a large latency once in a while, but that would not | |
8d016091 | 1435 | work well with Real-Time tasks. The wakeup_rt tracer was designed |
5752674e IM |
1436 | to record the worst case wakeups of RT tasks. Non-RT tasks are |
1437 | not recorded because the tracer only records one worst case and | |
1438 | tracing non-RT tasks that are unpredictable will overwrite the | |
8d016091 SRRH |
1439 | worst case latency of RT tasks (just run the normal wakeup |
1440 | tracer for a while to see that effect). | |
5752674e IM |
1441 | |
1442 | Since this tracer only deals with RT tasks, we will run this | |
1443 | slightly differently than we did with the previous tracers. | |
1444 | Instead of performing an 'ls', we will run 'sleep 1' under | |
1445 | 'chrt' which changes the priority of the task. | |
eb6d42ea | 1446 | |
8d016091 SRRH |
1447 | # echo 0 > options/function-trace |
1448 | # echo wakeup_rt > current_tracer | |
6752ab4a | 1449 | # echo 1 > tracing_on |
8d016091 | 1450 | # echo 0 > tracing_max_latency |
eb6d42ea | 1451 | # chrt -f 5 sleep 1 |
6752ab4a | 1452 | # echo 0 > tracing_on |
4a88d44a | 1453 | # cat trace |
eb6d42ea SR |
1454 | # tracer: wakeup |
1455 | # | |
8d016091 SRRH |
1456 | # tracer: wakeup_rt |
1457 | # | |
1458 | # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ | |
1459 | # -------------------------------------------------------------------- | |
1460 | # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1461 | # ----------------- | |
1462 | # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5) | |
1463 | # ----------------- | |
1464 | # | |
1465 | # _------=> CPU# | |
1466 | # / _-----=> irqs-off | |
1467 | # | / _----=> need-resched | |
1468 | # || / _---=> hardirq/softirq | |
1469 | # ||| / _--=> preempt-depth | |
1470 | # |||| / delay | |
1471 | # cmd pid ||||| time | caller | |
1472 | # \ / ||||| \ | / | |
1473 | <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep | |
1474 | <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up | |
1475 | <idle>-0 3d..3 5us : __schedule <-schedule | |
1476 | <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep | |
1477 | ||
1478 | ||
1479 | Running this on an idle system, we see that it only took 5 microseconds | |
1480 | to perform the task switch. Note, since the trace point in the schedule | |
1481 | is before the actual "switch", we stop the tracing when the recorded task | |
1482 | is about to schedule in. This may change if we add a new marker at the | |
1483 | end of the scheduler. | |
1484 | ||
1485 | Notice that the recorded task is 'sleep' with the PID of 2389 | |
5752674e IM |
1486 | and it has an rt_prio of 5. This priority is user-space priority |
1487 | and not the internal kernel priority. The policy is 1 for | |
1488 | SCHED_FIFO and 2 for SCHED_RR. | |
eb6d42ea | 1489 | |
8d016091 | 1490 | Note, that the trace data shows the internal priority (99 - rtprio). |
eb6d42ea | 1491 | |
8d016091 SRRH |
1492 | <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep |
1493 | ||
1494 | The 0:120:R means idle was running with a nice priority of 0 (120 - 20) | |
1495 | and in the running state 'R'. The sleep task was scheduled in with | |
1496 | 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94) | |
1497 | and it too is in the running state. | |
1498 | ||
1499 | Doing the same with chrt -r 5 and function-trace set. | |
1500 | ||
1501 | echo 1 > options/function-trace | |
1502 | ||
1503 | # tracer: wakeup_rt | |
eb6d42ea | 1504 | # |
8d016091 SRRH |
1505 | # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ |
1506 | # -------------------------------------------------------------------- | |
1507 | # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1508 | # ----------------- | |
1509 | # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5) | |
1510 | # ----------------- | |
1511 | # | |
1512 | # _------=> CPU# | |
1513 | # / _-----=> irqs-off | |
1514 | # | / _----=> need-resched | |
1515 | # || / _---=> hardirq/softirq | |
1516 | # ||| / _--=> preempt-depth | |
1517 | # |||| / delay | |
1518 | # cmd pid ||||| time | caller | |
1519 | # \ / ||||| \ | / | |
1520 | <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep | |
1521 | <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up | |
1522 | <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup | |
8875125e | 1523 | <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr |
8d016091 SRRH |
1524 | <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup |
1525 | <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up | |
1526 | <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock | |
1527 | <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up | |
1528 | <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up | |
1529 | <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore | |
1530 | <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer | |
1531 | <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock | |
1532 | <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt | |
1533 | <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock | |
1534 | <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt | |
1535 | <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event | |
1536 | <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event | |
1537 | <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event | |
1538 | <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt | |
1539 | <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit | |
1540 | <idle>-0 3dN.2 9us : idle_cpu <-irq_exit | |
1541 | <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit | |
1542 | <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit | |
1543 | <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit | |
1544 | <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle | |
1545 | <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit | |
1546 | <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle | |
1547 | <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit | |
1548 | <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit | |
1549 | <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit | |
1550 | <idle>-0 3dN.1 13us : update_cpu_load_nohz <-tick_nohz_idle_exit | |
1551 | <idle>-0 3dN.1 13us : _raw_spin_lock <-update_cpu_load_nohz | |
1552 | <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock | |
1553 | <idle>-0 3dN.2 13us : __update_cpu_load <-update_cpu_load_nohz | |
1554 | <idle>-0 3dN.2 14us : sched_avg_update <-__update_cpu_load | |
1555 | <idle>-0 3dN.2 14us : _raw_spin_unlock <-update_cpu_load_nohz | |
1556 | <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock | |
1557 | <idle>-0 3dN.1 15us : calc_load_exit_idle <-tick_nohz_idle_exit | |
1558 | <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit | |
1559 | <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit | |
1560 | <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel | |
1561 | <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel | |
1562 | <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 | |
1563 | <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave | |
1564 | <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16 | |
1565 | <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer | |
1566 | <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram | |
1567 | <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event | |
1568 | <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event | |
1569 | <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event | |
1570 | <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel | |
1571 | <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore | |
1572 | <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit | |
1573 | <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward | |
1574 | <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward | |
1575 | <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 | |
1576 | <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns | |
1577 | <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns | |
1578 | <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 | |
1579 | <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave | |
1580 | <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns | |
1581 | <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns | |
1582 | <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns | |
1583 | <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event | |
1584 | <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event | |
1585 | <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event | |
1586 | <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns | |
1587 | <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore | |
1588 | <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit | |
1589 | <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks | |
1590 | <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle | |
1591 | <idle>-0 3.N.. 25us : schedule <-cpu_idle | |
1592 | <idle>-0 3.N.. 25us : __schedule <-preempt_schedule | |
1593 | <idle>-0 3.N.. 26us : add_preempt_count <-__schedule | |
1594 | <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule | |
1595 | <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch | |
1596 | <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch | |
1597 | <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule | |
1598 | <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq | |
1599 | <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule | |
1600 | <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task | |
1601 | <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task | |
1602 | <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt | |
1603 | <idle>-0 3d..3 29us : __schedule <-preempt_schedule | |
1604 | <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep | |
1605 | ||
1606 | This isn't that big of a trace, even with function tracing enabled, | |
1607 | so I included the entire trace. | |
1608 | ||
1609 | The interrupt went off while when the system was idle. Somewhere | |
1610 | before task_woken_rt() was called, the NEED_RESCHED flag was set, | |
1611 | this is indicated by the first occurrence of the 'N' flag. | |
1612 | ||
1613 | Latency tracing and events | |
1614 | -------------------------- | |
1615 | As function tracing can induce a much larger latency, but without | |
1616 | seeing what happens within the latency it is hard to know what | |
1617 | caused it. There is a middle ground, and that is with enabling | |
1618 | events. | |
1619 | ||
1620 | # echo 0 > options/function-trace | |
1621 | # echo wakeup_rt > current_tracer | |
1622 | # echo 1 > events/enable | |
1623 | # echo 1 > tracing_on | |
1624 | # echo 0 > tracing_max_latency | |
1625 | # chrt -f 5 sleep 1 | |
1626 | # echo 0 > tracing_on | |
1627 | # cat trace | |
1628 | # tracer: wakeup_rt | |
1629 | # | |
1630 | # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ | |
1631 | # -------------------------------------------------------------------- | |
1632 | # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) | |
1633 | # ----------------- | |
1634 | # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5) | |
1635 | # ----------------- | |
1636 | # | |
1637 | # _------=> CPU# | |
1638 | # / _-----=> irqs-off | |
1639 | # | / _----=> need-resched | |
1640 | # || / _---=> hardirq/softirq | |
1641 | # ||| / _--=> preempt-depth | |
1642 | # |||| / delay | |
1643 | # cmd pid ||||| time | caller | |
1644 | # \ / ||||| \ | / | |
1645 | <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep | |
1646 | <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up | |
1647 | <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002 | |
1648 | <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8 | |
1649 | <idle>-0 2.N.2 2us : power_end: cpu_id=2 | |
1650 | <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2 | |
1651 | <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0 | |
1652 | <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000 | |
1653 | <idle>-0 2.N.2 5us : rcu_utilization: Start context switch | |
1654 | <idle>-0 2.N.2 5us : rcu_utilization: End context switch | |
1655 | <idle>-0 2d..3 6us : __schedule <-schedule | |
1656 | <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep | |
1657 | ||
eb6d42ea | 1658 | |
9b803c0f SR |
1659 | function |
1660 | -------- | |
eb6d42ea | 1661 | |
9b803c0f | 1662 | This tracer is the function tracer. Enabling the function tracer |
5752674e IM |
1663 | can be done from the debug file system. Make sure the |
1664 | ftrace_enabled is set; otherwise this tracer is a nop. | |
8d016091 | 1665 | See the "ftrace_enabled" section below. |
eb6d42ea SR |
1666 | |
1667 | # sysctl kernel.ftrace_enabled=1 | |
156f5a78 | 1668 | # echo function > current_tracer |
6752ab4a | 1669 | # echo 1 > tracing_on |
eb6d42ea | 1670 | # usleep 1 |
6752ab4a | 1671 | # echo 0 > tracing_on |
156f5a78 | 1672 | # cat trace |
9b803c0f | 1673 | # tracer: function |
eb6d42ea | 1674 | # |
8d016091 SRRH |
1675 | # entries-in-buffer/entries-written: 24799/24799 #P:4 |
1676 | # | |
1677 | # _-----=> irqs-off | |
1678 | # / _----=> need-resched | |
1679 | # | / _---=> hardirq/softirq | |
1680 | # || / _--=> preempt-depth | |
1681 | # ||| / delay | |
1682 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
1683 | # | | | |||| | | | |
1684 | bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write | |
1685 | bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock | |
1686 | bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify | |
1687 | bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify | |
1688 | bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify | |
1689 | bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock | |
1690 | bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock | |
1691 | bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify | |
eb6d42ea SR |
1692 | [...] |
1693 | ||
1694 | ||
5752674e IM |
1695 | Note: function tracer uses ring buffers to store the above |
1696 | entries. The newest data may overwrite the oldest data. | |
1697 | Sometimes using echo to stop the trace is not sufficient because | |
1698 | the tracing could have overwritten the data that you wanted to | |
1699 | record. For this reason, it is sometimes better to disable | |
1700 | tracing directly from a program. This allows you to stop the | |
1701 | tracing at the point that you hit the part that you are | |
1702 | interested in. To disable the tracing directly from a C program, | |
1703 | something like following code snippet can be used: | |
eb6d42ea SR |
1704 | |
1705 | int trace_fd; | |
1706 | [...] | |
1707 | int main(int argc, char *argv[]) { | |
1708 | [...] | |
6752ab4a | 1709 | trace_fd = open(tracing_file("tracing_on"), O_WRONLY); |
eb6d42ea SR |
1710 | [...] |
1711 | if (condition_hit()) { | |
f2d9c740 | 1712 | write(trace_fd, "0", 1); |
eb6d42ea SR |
1713 | } |
1714 | [...] | |
1715 | } | |
1716 | ||
df4fc315 SR |
1717 | |
1718 | Single thread tracing | |
1719 | --------------------- | |
1720 | ||
156f5a78 | 1721 | By writing into set_ftrace_pid you can trace a |
df4fc315 SR |
1722 | single thread. For example: |
1723 | ||
156f5a78 | 1724 | # cat set_ftrace_pid |
df4fc315 | 1725 | no pid |
156f5a78 GL |
1726 | # echo 3111 > set_ftrace_pid |
1727 | # cat set_ftrace_pid | |
df4fc315 | 1728 | 3111 |
156f5a78 GL |
1729 | # echo function > current_tracer |
1730 | # cat trace | head | |
df4fc315 SR |
1731 | # tracer: function |
1732 | # | |
1733 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1734 | # | | | | | | |
1735 | yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return | |
1736 | yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range | |
1737 | yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel | |
1738 | yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel | |
1739 | yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll | |
1740 | yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll | |
156f5a78 GL |
1741 | # echo -1 > set_ftrace_pid |
1742 | # cat trace |head | |
df4fc315 SR |
1743 | # tracer: function |
1744 | # | |
1745 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1746 | # | | | | | | |
1747 | ##### CPU 3 buffer started #### | |
1748 | yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait | |
1749 | yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry | |
1750 | yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry | |
1751 | yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit | |
1752 | yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit | |
1753 | ||
1754 | If you want to trace a function when executing, you could use | |
1755 | something like this simple program: | |
1756 | ||
1757 | #include <stdio.h> | |
1758 | #include <stdlib.h> | |
1759 | #include <sys/types.h> | |
1760 | #include <sys/stat.h> | |
1761 | #include <fcntl.h> | |
1762 | #include <unistd.h> | |
67b394f7 | 1763 | #include <string.h> |
df4fc315 | 1764 | |
156f5a78 GL |
1765 | #define _STR(x) #x |
1766 | #define STR(x) _STR(x) | |
1767 | #define MAX_PATH 256 | |
1768 | ||
1769 | const char *find_debugfs(void) | |
1770 | { | |
1771 | static char debugfs[MAX_PATH+1]; | |
1772 | static int debugfs_found; | |
1773 | char type[100]; | |
1774 | FILE *fp; | |
1775 | ||
1776 | if (debugfs_found) | |
1777 | return debugfs; | |
1778 | ||
1779 | if ((fp = fopen("/proc/mounts","r")) == NULL) { | |
1780 | perror("/proc/mounts"); | |
1781 | return NULL; | |
1782 | } | |
1783 | ||
1784 | while (fscanf(fp, "%*s %" | |
1785 | STR(MAX_PATH) | |
1786 | "s %99s %*s %*d %*d\n", | |
1787 | debugfs, type) == 2) { | |
1788 | if (strcmp(type, "debugfs") == 0) | |
1789 | break; | |
1790 | } | |
1791 | fclose(fp); | |
1792 | ||
1793 | if (strcmp(type, "debugfs") != 0) { | |
1794 | fprintf(stderr, "debugfs not mounted"); | |
1795 | return NULL; | |
1796 | } | |
1797 | ||
67b394f7 | 1798 | strcat(debugfs, "/tracing/"); |
156f5a78 GL |
1799 | debugfs_found = 1; |
1800 | ||
1801 | return debugfs; | |
1802 | } | |
1803 | ||
1804 | const char *tracing_file(const char *file_name) | |
1805 | { | |
1806 | static char trace_file[MAX_PATH+1]; | |
1807 | snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name); | |
1808 | return trace_file; | |
1809 | } | |
1810 | ||
df4fc315 SR |
1811 | int main (int argc, char **argv) |
1812 | { | |
1813 | if (argc < 1) | |
1814 | exit(-1); | |
1815 | ||
1816 | if (fork() > 0) { | |
1817 | int fd, ffd; | |
1818 | char line[64]; | |
1819 | int s; | |
1820 | ||
156f5a78 | 1821 | ffd = open(tracing_file("current_tracer"), O_WRONLY); |
df4fc315 SR |
1822 | if (ffd < 0) |
1823 | exit(-1); | |
1824 | write(ffd, "nop", 3); | |
1825 | ||
156f5a78 | 1826 | fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); |
df4fc315 SR |
1827 | s = sprintf(line, "%d\n", getpid()); |
1828 | write(fd, line, s); | |
1829 | ||
1830 | write(ffd, "function", 8); | |
1831 | ||
1832 | close(fd); | |
1833 | close(ffd); | |
1834 | ||
1835 | execvp(argv[1], argv+1); | |
1836 | } | |
1837 | ||
1838 | return 0; | |
1839 | } | |
1840 | ||
8d016091 | 1841 | Or this simple script! |
e2ea5399 | 1842 | |
8d016091 SRRH |
1843 | ------ |
1844 | #!/bin/bash | |
1845 | ||
1846 | debugfs=`sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts` | |
1847 | echo nop > $debugfs/tracing/current_tracer | |
1848 | echo 0 > $debugfs/tracing/tracing_on | |
1849 | echo $$ > $debugfs/tracing/set_ftrace_pid | |
1850 | echo function > $debugfs/tracing/current_tracer | |
1851 | echo 1 > $debugfs/tracing/tracing_on | |
1852 | exec "$@" | |
1853 | ------ | |
e2ea5399 MM |
1854 | |
1855 | ||
985ec20a FW |
1856 | function graph tracer |
1857 | --------------------------- | |
1858 | ||
5752674e IM |
1859 | This tracer is similar to the function tracer except that it |
1860 | probes a function on its entry and its exit. This is done by | |
1861 | using a dynamically allocated stack of return addresses in each | |
1862 | task_struct. On function entry the tracer overwrites the return | |
1863 | address of each function traced to set a custom probe. Thus the | |
1864 | original return address is stored on the stack of return address | |
1865 | in the task_struct. | |
985ec20a | 1866 | |
5752674e IM |
1867 | Probing on both ends of a function leads to special features |
1868 | such as: | |
985ec20a | 1869 | |
5752674e IM |
1870 | - measure of a function's time execution |
1871 | - having a reliable call stack to draw function calls graph | |
985ec20a FW |
1872 | |
1873 | This tracer is useful in several situations: | |
1874 | ||
5752674e IM |
1875 | - you want to find the reason of a strange kernel behavior and |
1876 | need to see what happens in detail on any areas (or specific | |
1877 | ones). | |
1878 | ||
1879 | - you are experiencing weird latencies but it's difficult to | |
1880 | find its origin. | |
1881 | ||
1882 | - you want to find quickly which path is taken by a specific | |
1883 | function | |
1884 | ||
1885 | - you just want to peek inside a working kernel and want to see | |
1886 | what happens there. | |
985ec20a FW |
1887 | |
1888 | # tracer: function_graph | |
1889 | # | |
1890 | # CPU DURATION FUNCTION CALLS | |
1891 | # | | | | | | | | |
1892 | ||
1893 | 0) | sys_open() { | |
1894 | 0) | do_sys_open() { | |
1895 | 0) | getname() { | |
1896 | 0) | kmem_cache_alloc() { | |
1897 | 0) 1.382 us | __might_sleep(); | |
1898 | 0) 2.478 us | } | |
1899 | 0) | strncpy_from_user() { | |
1900 | 0) | might_fault() { | |
1901 | 0) 1.389 us | __might_sleep(); | |
1902 | 0) 2.553 us | } | |
1903 | 0) 3.807 us | } | |
1904 | 0) 7.876 us | } | |
1905 | 0) | alloc_fd() { | |
1906 | 0) 0.668 us | _spin_lock(); | |
1907 | 0) 0.570 us | expand_files(); | |
1908 | 0) 0.586 us | _spin_unlock(); | |
1909 | ||
1910 | ||
5752674e IM |
1911 | There are several columns that can be dynamically |
1912 | enabled/disabled. You can use every combination of options you | |
1913 | want, depending on your needs. | |
985ec20a | 1914 | |
5752674e IM |
1915 | - The cpu number on which the function executed is default |
1916 | enabled. It is sometimes better to only trace one cpu (see | |
1917 | tracing_cpu_mask file) or you might sometimes see unordered | |
1918 | function calls while cpu tracing switch. | |
985ec20a | 1919 | |
156f5a78 GL |
1920 | hide: echo nofuncgraph-cpu > trace_options |
1921 | show: echo funcgraph-cpu > trace_options | |
985ec20a | 1922 | |
5752674e IM |
1923 | - The duration (function's time of execution) is displayed on |
1924 | the closing bracket line of a function or on the same line | |
1925 | than the current function in case of a leaf one. It is default | |
1926 | enabled. | |
985ec20a | 1927 | |
156f5a78 GL |
1928 | hide: echo nofuncgraph-duration > trace_options |
1929 | show: echo funcgraph-duration > trace_options | |
985ec20a | 1930 | |
5752674e IM |
1931 | - The overhead field precedes the duration field in case of |
1932 | reached duration thresholds. | |
985ec20a | 1933 | |
156f5a78 GL |
1934 | hide: echo nofuncgraph-overhead > trace_options |
1935 | show: echo funcgraph-overhead > trace_options | |
985ec20a FW |
1936 | depends on: funcgraph-duration |
1937 | ||
1938 | ie: | |
1939 | ||
1940 | 0) | up_write() { | |
1941 | 0) 0.646 us | _spin_lock_irqsave(); | |
1942 | 0) 0.684 us | _spin_unlock_irqrestore(); | |
1943 | 0) 3.123 us | } | |
1944 | 0) 0.548 us | fput(); | |
1945 | 0) + 58.628 us | } | |
1946 | ||
1947 | [...] | |
1948 | ||
1949 | 0) | putname() { | |
1950 | 0) | kmem_cache_free() { | |
1951 | 0) 0.518 us | __phys_addr(); | |
1952 | 0) 1.757 us | } | |
1953 | 0) 2.861 us | } | |
1954 | 0) ! 115.305 us | } | |
1955 | 0) ! 116.402 us | } | |
1956 | ||
1957 | + means that the function exceeded 10 usecs. | |
1958 | ! means that the function exceeded 100 usecs. | |
1959 | ||
1960 | ||
5752674e IM |
1961 | - The task/pid field displays the thread cmdline and pid which |
1962 | executed the function. It is default disabled. | |
985ec20a | 1963 | |
156f5a78 GL |
1964 | hide: echo nofuncgraph-proc > trace_options |
1965 | show: echo funcgraph-proc > trace_options | |
985ec20a FW |
1966 | |
1967 | ie: | |
1968 | ||
1969 | # tracer: function_graph | |
1970 | # | |
1971 | # CPU TASK/PID DURATION FUNCTION CALLS | |
1972 | # | | | | | | | | | | |
1973 | 0) sh-4802 | | d_free() { | |
1974 | 0) sh-4802 | | call_rcu() { | |
1975 | 0) sh-4802 | | __call_rcu() { | |
1976 | 0) sh-4802 | 0.616 us | rcu_process_gp_end(); | |
1977 | 0) sh-4802 | 0.586 us | check_for_new_grace_period(); | |
1978 | 0) sh-4802 | 2.899 us | } | |
1979 | 0) sh-4802 | 4.040 us | } | |
1980 | 0) sh-4802 | 5.151 us | } | |
1981 | 0) sh-4802 | + 49.370 us | } | |
1982 | ||
1983 | ||
5752674e IM |
1984 | - The absolute time field is an absolute timestamp given by the |
1985 | system clock since it started. A snapshot of this time is | |
1986 | given on each entry/exit of functions | |
985ec20a | 1987 | |
156f5a78 GL |
1988 | hide: echo nofuncgraph-abstime > trace_options |
1989 | show: echo funcgraph-abstime > trace_options | |
985ec20a FW |
1990 | |
1991 | ie: | |
1992 | ||
1993 | # | |
1994 | # TIME CPU DURATION FUNCTION CALLS | |
1995 | # | | | | | | | | | |
1996 | 360.774522 | 1) 0.541 us | } | |
1997 | 360.774522 | 1) 4.663 us | } | |
1998 | 360.774523 | 1) 0.541 us | __wake_up_bit(); | |
1999 | 360.774524 | 1) 6.796 us | } | |
2000 | 360.774524 | 1) 7.952 us | } | |
2001 | 360.774525 | 1) 9.063 us | } | |
2002 | 360.774525 | 1) 0.615 us | journal_mark_dirty(); | |
2003 | 360.774527 | 1) 0.578 us | __brelse(); | |
2004 | 360.774528 | 1) | reiserfs_prepare_for_journal() { | |
2005 | 360.774528 | 1) | unlock_buffer() { | |
2006 | 360.774529 | 1) | wake_up_bit() { | |
2007 | 360.774529 | 1) | bit_waitqueue() { | |
2008 | 360.774530 | 1) 0.594 us | __phys_addr(); | |
2009 | ||
2010 | ||
607e3a29 RE |
2011 | The function name is always displayed after the closing bracket |
2012 | for a function if the start of that function is not in the | |
2013 | trace buffer. | |
2014 | ||
2015 | Display of the function name after the closing bracket may be | |
2016 | enabled for functions whose start is in the trace buffer, | |
2017 | allowing easier searching with grep for function durations. | |
2018 | It is default disabled. | |
2019 | ||
2020 | hide: echo nofuncgraph-tail > trace_options | |
2021 | show: echo funcgraph-tail > trace_options | |
2022 | ||
2023 | Example with nofuncgraph-tail (default): | |
2024 | 0) | putname() { | |
2025 | 0) | kmem_cache_free() { | |
2026 | 0) 0.518 us | __phys_addr(); | |
2027 | 0) 1.757 us | } | |
2028 | 0) 2.861 us | } | |
2029 | ||
2030 | Example with funcgraph-tail: | |
2031 | 0) | putname() { | |
2032 | 0) | kmem_cache_free() { | |
2033 | 0) 0.518 us | __phys_addr(); | |
2034 | 0) 1.757 us | } /* kmem_cache_free() */ | |
2035 | 0) 2.861 us | } /* putname() */ | |
2036 | ||
5752674e | 2037 | You can put some comments on specific functions by using |
5e1607a0 | 2038 | trace_printk() For example, if you want to put a comment inside |
5752674e | 2039 | the __might_sleep() function, you just have to include |
5e1607a0 | 2040 | <linux/ftrace.h> and call trace_printk() inside __might_sleep() |
985ec20a | 2041 | |
5e1607a0 | 2042 | trace_printk("I'm a comment!\n") |
985ec20a FW |
2043 | |
2044 | will produce: | |
2045 | ||
2046 | 1) | __might_sleep() { | |
2047 | 1) | /* I'm a comment! */ | |
2048 | 1) 1.449 us | } | |
2049 | ||
2050 | ||
5752674e IM |
2051 | You might find other useful features for this tracer in the |
2052 | following "dynamic ftrace" section such as tracing only specific | |
2053 | functions or tasks. | |
985ec20a | 2054 | |
eb6d42ea SR |
2055 | dynamic ftrace |
2056 | -------------- | |
2057 | ||
f2d9c740 | 2058 | If CONFIG_DYNAMIC_FTRACE is set, the system will run with |
eb6d42ea SR |
2059 | virtually no overhead when function tracing is disabled. The way |
2060 | this works is the mcount function call (placed at the start of | |
5752674e IM |
2061 | every kernel function, produced by the -pg switch in gcc), |
2062 | starts of pointing to a simple return. (Enabling FTRACE will | |
2063 | include the -pg switch in the compiling of the kernel.) | |
eb6d42ea | 2064 | |
9b803c0f | 2065 | At compile time every C file object is run through the |
8d016091 SRRH |
2066 | recordmcount program (located in the scripts directory). This |
2067 | program will parse the ELF headers in the C object to find all | |
2068 | the locations in the .text section that call mcount. (Note, only | |
2069 | white listed .text sections are processed, since processing other | |
2070 | sections like .init.text may cause races due to those sections | |
2071 | being freed unexpectedly). | |
9b803c0f | 2072 | |
5752674e IM |
2073 | A new section called "__mcount_loc" is created that holds |
2074 | references to all the mcount call sites in the .text section. | |
8d016091 SRRH |
2075 | The recordmcount program re-links this section back into the |
2076 | original object. The final linking stage of the kernel will add all these | |
2077 | references into a single table. | |
9b803c0f SR |
2078 | |
2079 | On boot up, before SMP is initialized, the dynamic ftrace code | |
5752674e IM |
2080 | scans this table and updates all the locations into nops. It |
2081 | also records the locations, which are added to the | |
2082 | available_filter_functions list. Modules are processed as they | |
2083 | are loaded and before they are executed. When a module is | |
2084 | unloaded, it also removes its functions from the ftrace function | |
2085 | list. This is automatic in the module unload code, and the | |
2086 | module author does not need to worry about it. | |
2087 | ||
8d016091 SRRH |
2088 | When tracing is enabled, the process of modifying the function |
2089 | tracepoints is dependent on architecture. The old method is to use | |
2090 | kstop_machine to prevent races with the CPUs executing code being | |
2091 | modified (which can cause the CPU to do undesirable things, especially | |
2092 | if the modified code crosses cache (or page) boundaries), and the nops are | |
5752674e IM |
2093 | patched back to calls. But this time, they do not call mcount |
2094 | (which is just a function stub). They now call into the ftrace | |
2095 | infrastructure. | |
eb6d42ea | 2096 | |
8d016091 SRRH |
2097 | The new method of modifying the function tracepoints is to place |
2098 | a breakpoint at the location to be modified, sync all CPUs, modify | |
2099 | the rest of the instruction not covered by the breakpoint. Sync | |
2100 | all CPUs again, and then remove the breakpoint with the finished | |
2101 | version to the ftrace call site. | |
2102 | ||
2103 | Some archs do not even need to monkey around with the synchronization, | |
2104 | and can just slap the new code on top of the old without any | |
2105 | problems with other CPUs executing it at the same time. | |
2106 | ||
eb6d42ea | 2107 | One special side-effect to the recording of the functions being |
f2d9c740 | 2108 | traced is that we can now selectively choose which functions we |
5752674e IM |
2109 | wish to trace and which ones we want the mcount calls to remain |
2110 | as nops. | |
eb6d42ea | 2111 | |
5752674e IM |
2112 | Two files are used, one for enabling and one for disabling the |
2113 | tracing of specified functions. They are: | |
eb6d42ea SR |
2114 | |
2115 | set_ftrace_filter | |
2116 | ||
2117 | and | |
2118 | ||
2119 | set_ftrace_notrace | |
2120 | ||
5752674e IM |
2121 | A list of available functions that you can add to these files is |
2122 | listed in: | |
eb6d42ea SR |
2123 | |
2124 | available_filter_functions | |
2125 | ||
156f5a78 | 2126 | # cat available_filter_functions |
eb6d42ea SR |
2127 | put_prev_task_idle |
2128 | kmem_cache_create | |
2129 | pick_next_task_rt | |
2130 | get_online_cpus | |
2131 | pick_next_task_fair | |
2132 | mutex_lock | |
2133 | [...] | |
2134 | ||
f2d9c740 | 2135 | If I am only interested in sys_nanosleep and hrtimer_interrupt: |
eb6d42ea | 2136 | |
8d016091 | 2137 | # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter |
6993b1bb | 2138 | # echo function > current_tracer |
6752ab4a | 2139 | # echo 1 > tracing_on |
eb6d42ea | 2140 | # usleep 1 |
6752ab4a | 2141 | # echo 0 > tracing_on |
156f5a78 | 2142 | # cat trace |
8d016091 SRRH |
2143 | # tracer: function |
2144 | # | |
2145 | # entries-in-buffer/entries-written: 5/5 #P:4 | |
eb6d42ea | 2146 | # |
8d016091 SRRH |
2147 | # _-----=> irqs-off |
2148 | # / _----=> need-resched | |
2149 | # | / _---=> hardirq/softirq | |
2150 | # || / _--=> preempt-depth | |
2151 | # ||| / delay | |
2152 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
2153 | # | | | |||| | | | |
2154 | usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath | |
2155 | <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt | |
2156 | usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt | |
2157 | <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt | |
2158 | <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt | |
eb6d42ea | 2159 | |
f2d9c740 | 2160 | To see which functions are being traced, you can cat the file: |
eb6d42ea | 2161 | |
156f5a78 | 2162 | # cat set_ftrace_filter |
eb6d42ea SR |
2163 | hrtimer_interrupt |
2164 | sys_nanosleep | |
2165 | ||
2166 | ||
5752674e IM |
2167 | Perhaps this is not enough. The filters also allow simple wild |
2168 | cards. Only the following are currently available | |
eb6d42ea | 2169 | |
a41eebab | 2170 | <match>* - will match functions that begin with <match> |
eb6d42ea SR |
2171 | *<match> - will match functions that end with <match> |
2172 | *<match>* - will match functions that have <match> in it | |
2173 | ||
f2d9c740 | 2174 | These are the only wild cards which are supported. |
eb6d42ea SR |
2175 | |
2176 | <match>*<match> will not work. | |
2177 | ||
5752674e IM |
2178 | Note: It is better to use quotes to enclose the wild cards, |
2179 | otherwise the shell may expand the parameters into names | |
2180 | of files in the local directory. | |
c072c249 | 2181 | |
156f5a78 | 2182 | # echo 'hrtimer_*' > set_ftrace_filter |
eb6d42ea SR |
2183 | |
2184 | Produces: | |
2185 | ||
8d016091 | 2186 | # tracer: function |
eb6d42ea | 2187 | # |
8d016091 SRRH |
2188 | # entries-in-buffer/entries-written: 897/897 #P:4 |
2189 | # | |
2190 | # _-----=> irqs-off | |
2191 | # / _----=> need-resched | |
2192 | # | / _---=> hardirq/softirq | |
2193 | # || / _--=> preempt-depth | |
2194 | # ||| / delay | |
2195 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
2196 | # | | | |||| | | | |
2197 | <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit | |
2198 | <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel | |
2199 | <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer | |
2200 | <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit | |
2201 | <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 | |
2202 | <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt | |
2203 | <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter | |
2204 | <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem | |
eb6d42ea SR |
2205 | |
2206 | Notice that we lost the sys_nanosleep. | |
2207 | ||
156f5a78 | 2208 | # cat set_ftrace_filter |
eb6d42ea SR |
2209 | hrtimer_run_queues |
2210 | hrtimer_run_pending | |
2211 | hrtimer_init | |
2212 | hrtimer_cancel | |
2213 | hrtimer_try_to_cancel | |
2214 | hrtimer_forward | |
2215 | hrtimer_start | |
2216 | hrtimer_reprogram | |
2217 | hrtimer_force_reprogram | |
2218 | hrtimer_get_next_event | |
2219 | hrtimer_interrupt | |
2220 | hrtimer_nanosleep | |
2221 | hrtimer_wakeup | |
2222 | hrtimer_get_remaining | |
2223 | hrtimer_get_res | |
2224 | hrtimer_init_sleeper | |
2225 | ||
2226 | ||
2227 | This is because the '>' and '>>' act just like they do in bash. | |
2228 | To rewrite the filters, use '>' | |
2229 | To append to the filters, use '>>' | |
2230 | ||
5752674e IM |
2231 | To clear out a filter so that all functions will be recorded |
2232 | again: | |
eb6d42ea | 2233 | |
156f5a78 GL |
2234 | # echo > set_ftrace_filter |
2235 | # cat set_ftrace_filter | |
eb6d42ea SR |
2236 | # |
2237 | ||
2238 | Again, now we want to append. | |
2239 | ||
156f5a78 GL |
2240 | # echo sys_nanosleep > set_ftrace_filter |
2241 | # cat set_ftrace_filter | |
eb6d42ea | 2242 | sys_nanosleep |
156f5a78 GL |
2243 | # echo 'hrtimer_*' >> set_ftrace_filter |
2244 | # cat set_ftrace_filter | |
eb6d42ea SR |
2245 | hrtimer_run_queues |
2246 | hrtimer_run_pending | |
2247 | hrtimer_init | |
2248 | hrtimer_cancel | |
2249 | hrtimer_try_to_cancel | |
2250 | hrtimer_forward | |
2251 | hrtimer_start | |
2252 | hrtimer_reprogram | |
2253 | hrtimer_force_reprogram | |
2254 | hrtimer_get_next_event | |
2255 | hrtimer_interrupt | |
2256 | sys_nanosleep | |
2257 | hrtimer_nanosleep | |
2258 | hrtimer_wakeup | |
2259 | hrtimer_get_remaining | |
2260 | hrtimer_get_res | |
2261 | hrtimer_init_sleeper | |
2262 | ||
2263 | ||
5752674e IM |
2264 | The set_ftrace_notrace prevents those functions from being |
2265 | traced. | |
eb6d42ea | 2266 | |
156f5a78 | 2267 | # echo '*preempt*' '*lock*' > set_ftrace_notrace |
eb6d42ea SR |
2268 | |
2269 | Produces: | |
2270 | ||
8d016091 SRRH |
2271 | # tracer: function |
2272 | # | |
2273 | # entries-in-buffer/entries-written: 39608/39608 #P:4 | |
eb6d42ea | 2274 | # |
8d016091 SRRH |
2275 | # _-----=> irqs-off |
2276 | # / _----=> need-resched | |
2277 | # | / _---=> hardirq/softirq | |
2278 | # || / _--=> preempt-depth | |
2279 | # ||| / delay | |
2280 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
2281 | # | | | |||| | | | |
2282 | bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open | |
2283 | bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last | |
2284 | bash-1994 [000] .... 4342.324897: ima_file_check <-do_last | |
2285 | bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check | |
2286 | bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement | |
2287 | bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action | |
2288 | bash-1994 [000] .... 4342.324899: do_truncate <-do_last | |
2289 | bash-1994 [000] .... 4342.324899: should_remove_suid <-do_truncate | |
2290 | bash-1994 [000] .... 4342.324899: notify_change <-do_truncate | |
2291 | bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change | |
2292 | bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time | |
2293 | bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time | |
eb6d42ea SR |
2294 | |
2295 | We can see that there's no more lock or preempt tracing. | |
2296 | ||
985ec20a | 2297 | |
5752674e IM |
2298 | Dynamic ftrace with the function graph tracer |
2299 | --------------------------------------------- | |
985ec20a | 2300 | |
5752674e IM |
2301 | Although what has been explained above concerns both the |
2302 | function tracer and the function-graph-tracer, there are some | |
2303 | special features only available in the function-graph tracer. | |
985ec20a | 2304 | |
5752674e IM |
2305 | If you want to trace only one function and all of its children, |
2306 | you just have to echo its name into set_graph_function: | |
985ec20a | 2307 | |
5752674e | 2308 | echo __do_fault > set_graph_function |
985ec20a | 2309 | |
5752674e IM |
2310 | will produce the following "expanded" trace of the __do_fault() |
2311 | function: | |
985ec20a FW |
2312 | |
2313 | 0) | __do_fault() { | |
2314 | 0) | filemap_fault() { | |
2315 | 0) | find_lock_page() { | |
2316 | 0) 0.804 us | find_get_page(); | |
2317 | 0) | __might_sleep() { | |
2318 | 0) 1.329 us | } | |
2319 | 0) 3.904 us | } | |
2320 | 0) 4.979 us | } | |
2321 | 0) 0.653 us | _spin_lock(); | |
2322 | 0) 0.578 us | page_add_file_rmap(); | |
2323 | 0) 0.525 us | native_set_pte_at(); | |
2324 | 0) 0.585 us | _spin_unlock(); | |
2325 | 0) | unlock_page() { | |
2326 | 0) 0.541 us | page_waitqueue(); | |
2327 | 0) 0.639 us | __wake_up_bit(); | |
2328 | 0) 2.786 us | } | |
2329 | 0) + 14.237 us | } | |
2330 | 0) | __do_fault() { | |
2331 | 0) | filemap_fault() { | |
2332 | 0) | find_lock_page() { | |
2333 | 0) 0.698 us | find_get_page(); | |
2334 | 0) | __might_sleep() { | |
2335 | 0) 1.412 us | } | |
2336 | 0) 3.950 us | } | |
2337 | 0) 5.098 us | } | |
2338 | 0) 0.631 us | _spin_lock(); | |
2339 | 0) 0.571 us | page_add_file_rmap(); | |
2340 | 0) 0.526 us | native_set_pte_at(); | |
2341 | 0) 0.586 us | _spin_unlock(); | |
2342 | 0) | unlock_page() { | |
2343 | 0) 0.533 us | page_waitqueue(); | |
2344 | 0) 0.638 us | __wake_up_bit(); | |
2345 | 0) 2.793 us | } | |
2346 | 0) + 14.012 us | } | |
2347 | ||
5752674e | 2348 | You can also expand several functions at once: |
985ec20a | 2349 | |
5752674e IM |
2350 | echo sys_open > set_graph_function |
2351 | echo sys_close >> set_graph_function | |
985ec20a | 2352 | |
5752674e IM |
2353 | Now if you want to go back to trace all functions you can clear |
2354 | this special filter via: | |
985ec20a | 2355 | |
5752674e | 2356 | echo > set_graph_function |
985ec20a FW |
2357 | |
2358 | ||
8d016091 SRRH |
2359 | ftrace_enabled |
2360 | -------------- | |
2361 | ||
2362 | Note, the proc sysctl ftrace_enable is a big on/off switch for the | |
2363 | function tracer. By default it is enabled (when function tracing is | |
2364 | enabled in the kernel). If it is disabled, all function tracing is | |
2365 | disabled. This includes not only the function tracers for ftrace, but | |
2366 | also for any other uses (perf, kprobes, stack tracing, profiling, etc). | |
2367 | ||
2368 | Please disable this with care. | |
2369 | ||
2370 | This can be disable (and enabled) with: | |
2371 | ||
2372 | sysctl kernel.ftrace_enabled=0 | |
2373 | sysctl kernel.ftrace_enabled=1 | |
2374 | ||
2375 | or | |
2376 | ||
2377 | echo 0 > /proc/sys/kernel/ftrace_enabled | |
2378 | echo 1 > /proc/sys/kernel/ftrace_enabled | |
2379 | ||
2380 | ||
07271aa4 CD |
2381 | Filter commands |
2382 | --------------- | |
2383 | ||
2384 | A few commands are supported by the set_ftrace_filter interface. | |
2385 | Trace commands have the following format: | |
2386 | ||
2387 | <function>:<command>:<parameter> | |
2388 | ||
2389 | The following commands are supported: | |
2390 | ||
2391 | - mod | |
2392 | This command enables function filtering per module. The | |
2393 | parameter defines the module. For example, if only the write* | |
2394 | functions in the ext3 module are desired, run: | |
2395 | ||
2396 | echo 'write*:mod:ext3' > set_ftrace_filter | |
2397 | ||
2398 | This command interacts with the filter in the same way as | |
2399 | filtering based on function names. Thus, adding more functions | |
2400 | in a different module is accomplished by appending (>>) to the | |
2401 | filter file. Remove specific module functions by prepending | |
2402 | '!': | |
2403 | ||
2404 | echo '!writeback*:mod:ext3' >> set_ftrace_filter | |
2405 | ||
2406 | - traceon/traceoff | |
2407 | These commands turn tracing on and off when the specified | |
2408 | functions are hit. The parameter determines how many times the | |
2409 | tracing system is turned on and off. If unspecified, there is | |
2410 | no limit. For example, to disable tracing when a schedule bug | |
2411 | is hit the first 5 times, run: | |
2412 | ||
2413 | echo '__schedule_bug:traceoff:5' > set_ftrace_filter | |
2414 | ||
8d016091 SRRH |
2415 | To always disable tracing when __schedule_bug is hit: |
2416 | ||
2417 | echo '__schedule_bug:traceoff' > set_ftrace_filter | |
2418 | ||
07271aa4 CD |
2419 | These commands are cumulative whether or not they are appended |
2420 | to set_ftrace_filter. To remove a command, prepend it by '!' | |
2421 | and drop the parameter: | |
2422 | ||
8d016091 SRRH |
2423 | echo '!__schedule_bug:traceoff:0' > set_ftrace_filter |
2424 | ||
2425 | The above removes the traceoff command for __schedule_bug | |
2426 | that have a counter. To remove commands without counters: | |
2427 | ||
07271aa4 CD |
2428 | echo '!__schedule_bug:traceoff' > set_ftrace_filter |
2429 | ||
8d016091 SRRH |
2430 | - snapshot |
2431 | Will cause a snapshot to be triggered when the function is hit. | |
2432 | ||
2433 | echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter | |
2434 | ||
2435 | To only snapshot once: | |
2436 | ||
2437 | echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter | |
2438 | ||
2439 | To remove the above commands: | |
2440 | ||
2441 | echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter | |
2442 | echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter | |
2443 | ||
2444 | - enable_event/disable_event | |
2445 | These commands can enable or disable a trace event. Note, because | |
2446 | function tracing callbacks are very sensitive, when these commands | |
2447 | are registered, the trace point is activated, but disabled in | |
2448 | a "soft" mode. That is, the tracepoint will be called, but | |
2449 | just will not be traced. The event tracepoint stays in this mode | |
2450 | as long as there's a command that triggers it. | |
2451 | ||
2452 | echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \ | |
2453 | set_ftrace_filter | |
2454 | ||
2455 | The format is: | |
2456 | ||
2457 | <function>:enable_event:<system>:<event>[:count] | |
2458 | <function>:disable_event:<system>:<event>[:count] | |
2459 | ||
2460 | To remove the events commands: | |
2461 | ||
2462 | ||
2463 | echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \ | |
2464 | set_ftrace_filter | |
2465 | echo '!schedule:disable_event:sched:sched_switch' > \ | |
2466 | set_ftrace_filter | |
07271aa4 | 2467 | |
ad71d889 SRRH |
2468 | - dump |
2469 | When the function is hit, it will dump the contents of the ftrace | |
2470 | ring buffer to the console. This is useful if you need to debug | |
2471 | something, and want to dump the trace when a certain function | |
2472 | is hit. Perhaps its a function that is called before a tripple | |
2473 | fault happens and does not allow you to get a regular dump. | |
2474 | ||
90e3c03c SRRH |
2475 | - cpudump |
2476 | When the function is hit, it will dump the contents of the ftrace | |
2477 | ring buffer for the current CPU to the console. Unlike the "dump" | |
2478 | command, it only prints out the contents of the ring buffer for the | |
2479 | CPU that executed the function that triggered the dump. | |
2480 | ||
eb6d42ea SR |
2481 | trace_pipe |
2482 | ---------- | |
2483 | ||
5752674e IM |
2484 | The trace_pipe outputs the same content as the trace file, but |
2485 | the effect on the tracing is different. Every read from | |
2486 | trace_pipe is consumed. This means that subsequent reads will be | |
2487 | different. The trace is live. | |
eb6d42ea | 2488 | |
156f5a78 GL |
2489 | # echo function > current_tracer |
2490 | # cat trace_pipe > /tmp/trace.out & | |
eb6d42ea | 2491 | [1] 4153 |
6752ab4a | 2492 | # echo 1 > tracing_on |
eb6d42ea | 2493 | # usleep 1 |
6752ab4a | 2494 | # echo 0 > tracing_on |
156f5a78 | 2495 | # cat trace |
9b803c0f | 2496 | # tracer: function |
eb6d42ea | 2497 | # |
8d016091 SRRH |
2498 | # entries-in-buffer/entries-written: 0/0 #P:4 |
2499 | # | |
2500 | # _-----=> irqs-off | |
2501 | # / _----=> need-resched | |
2502 | # | / _---=> hardirq/softirq | |
2503 | # || / _--=> preempt-depth | |
2504 | # ||| / delay | |
2505 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
2506 | # | | | |||| | | | |
eb6d42ea SR |
2507 | |
2508 | # | |
2509 | # cat /tmp/trace.out | |
8d016091 SRRH |
2510 | bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write |
2511 | bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock | |
2512 | bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify | |
2513 | bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify | |
2514 | bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify | |
2515 | bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock | |
2516 | bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock | |
2517 | bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify | |
2518 | bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath | |
eb6d42ea SR |
2519 | |
2520 | ||
5752674e | 2521 | Note, reading the trace_pipe file will block until more input is |
8d016091 | 2522 | added. |
eb6d42ea SR |
2523 | |
2524 | trace entries | |
2525 | ------------- | |
2526 | ||
5752674e IM |
2527 | Having too much or not enough data can be troublesome in |
2528 | diagnosing an issue in the kernel. The file buffer_size_kb is | |
2529 | used to modify the size of the internal trace buffers. The | |
2530 | number listed is the number of entries that can be recorded per | |
8d016091 | 2531 | CPU. To know the full size, multiply the number of possible CPUs |
5752674e | 2532 | with the number of entries. |
eb6d42ea | 2533 | |
156f5a78 | 2534 | # cat buffer_size_kb |
1696b2b0 | 2535 | 1408 (units kilobytes) |
eb6d42ea | 2536 | |
8d016091 SRRH |
2537 | Or simply read buffer_total_size_kb |
2538 | ||
2539 | # cat buffer_total_size_kb | |
2540 | 5632 | |
2541 | ||
2542 | To modify the buffer, simple echo in a number (in 1024 byte segments). | |
eb6d42ea | 2543 | |
156f5a78 GL |
2544 | # echo 10000 > buffer_size_kb |
2545 | # cat buffer_size_kb | |
1696b2b0 | 2546 | 10000 (units kilobytes) |
eb6d42ea | 2547 | |
8d016091 SRRH |
2548 | It will try to allocate as much as possible. If you allocate too |
2549 | much, it can cause Out-Of-Memory to trigger. | |
eb6d42ea | 2550 | |
156f5a78 | 2551 | # echo 1000000000000 > buffer_size_kb |
eb6d42ea | 2552 | -bash: echo: write error: Cannot allocate memory |
156f5a78 | 2553 | # cat buffer_size_kb |
eb6d42ea SR |
2554 | 85 |
2555 | ||
8d016091 SRRH |
2556 | The per_cpu buffers can be changed individually as well: |
2557 | ||
2558 | # echo 10000 > per_cpu/cpu0/buffer_size_kb | |
2559 | # echo 100 > per_cpu/cpu1/buffer_size_kb | |
2560 | ||
2561 | When the per_cpu buffers are not the same, the buffer_size_kb | |
2562 | at the top level will just show an X | |
2563 | ||
2564 | # cat buffer_size_kb | |
2565 | X | |
2566 | ||
2567 | This is where the buffer_total_size_kb is useful: | |
2568 | ||
2569 | # cat buffer_total_size_kb | |
2570 | 12916 | |
2571 | ||
2572 | Writing to the top level buffer_size_kb will reset all the buffers | |
2573 | to be the same again. | |
2574 | ||
c1043fcd HT |
2575 | Snapshot |
2576 | -------- | |
2577 | CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature | |
2578 | available to all non latency tracers. (Latency tracers which | |
2579 | record max latency, such as "irqsoff" or "wakeup", can't use | |
2580 | this feature, since those are already using the snapshot | |
2581 | mechanism internally.) | |
2582 | ||
2583 | Snapshot preserves a current trace buffer at a particular point | |
2584 | in time without stopping tracing. Ftrace swaps the current | |
2585 | buffer with a spare buffer, and tracing continues in the new | |
2586 | current (=previous spare) buffer. | |
2587 | ||
2588 | The following debugfs files in "tracing" are related to this | |
2589 | feature: | |
2590 | ||
2591 | snapshot: | |
2592 | ||
2593 | This is used to take a snapshot and to read the output | |
2594 | of the snapshot. Echo 1 into this file to allocate a | |
2595 | spare buffer and to take a snapshot (swap), then read | |
2596 | the snapshot from this file in the same format as | |
2597 | "trace" (described above in the section "The File | |
2598 | System"). Both reads snapshot and tracing are executable | |
2599 | in parallel. When the spare buffer is allocated, echoing | |
2600 | 0 frees it, and echoing else (positive) values clear the | |
2601 | snapshot contents. | |
2602 | More details are shown in the table below. | |
2603 | ||
2604 | status\input | 0 | 1 | else | | |
2605 | --------------+------------+------------+------------+ | |
1abccd74 | 2606 | not allocated |(do nothing)| alloc+swap |(do nothing)| |
c1043fcd HT |
2607 | --------------+------------+------------+------------+ |
2608 | allocated | free | swap | clear | | |
2609 | --------------+------------+------------+------------+ | |
2610 | ||
2611 | Here is an example of using the snapshot feature. | |
2612 | ||
2613 | # echo 1 > events/sched/enable | |
2614 | # echo 1 > snapshot | |
2615 | # cat snapshot | |
2616 | # tracer: nop | |
2617 | # | |
2618 | # entries-in-buffer/entries-written: 71/71 #P:8 | |
2619 | # | |
2620 | # _-----=> irqs-off | |
2621 | # / _----=> need-resched | |
2622 | # | / _---=> hardirq/softirq | |
2623 | # || / _--=> preempt-depth | |
2624 | # ||| / delay | |
2625 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
2626 | # | | | |||| | | | |
2627 | <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120 | |
2628 | sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120 | |
2629 | [...] | |
2630 | <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120 | |
2631 | ||
2632 | # cat trace | |
2633 | # tracer: nop | |
2634 | # | |
2635 | # entries-in-buffer/entries-written: 77/77 #P:8 | |
2636 | # | |
2637 | # _-----=> irqs-off | |
2638 | # / _----=> need-resched | |
2639 | # | / _---=> hardirq/softirq | |
2640 | # || / _--=> preempt-depth | |
2641 | # ||| / delay | |
2642 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
2643 | # | | | |||| | | | |
2644 | <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120 | |
2645 | snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120 | |
2646 | [...] | |
2647 | ||
2648 | ||
2649 | If you try to use this snapshot feature when current tracer is | |
2650 | one of the latency tracers, you will get the following results. | |
2651 | ||
2652 | # echo wakeup > current_tracer | |
2653 | # echo 1 > snapshot | |
2654 | bash: echo: write error: Device or resource busy | |
2655 | # cat snapshot | |
2656 | cat: snapshot: Device or resource busy | |
2657 | ||
8d016091 SRRH |
2658 | |
2659 | Instances | |
2660 | --------- | |
2661 | In the debugfs tracing directory is a directory called "instances". | |
2662 | This directory can have new directories created inside of it using | |
2663 | mkdir, and removing directories with rmdir. The directory created | |
2664 | with mkdir in this directory will already contain files and other | |
2665 | directories after it is created. | |
2666 | ||
2667 | # mkdir instances/foo | |
2668 | # ls instances/foo | |
2669 | buffer_size_kb buffer_total_size_kb events free_buffer per_cpu | |
2670 | set_event snapshot trace trace_clock trace_marker trace_options | |
2671 | trace_pipe tracing_on | |
2672 | ||
2673 | As you can see, the new directory looks similar to the tracing directory | |
2674 | itself. In fact, it is very similar, except that the buffer and | |
2675 | events are agnostic from the main director, or from any other | |
2676 | instances that are created. | |
2677 | ||
2678 | The files in the new directory work just like the files with the | |
2679 | same name in the tracing directory except the buffer that is used | |
2680 | is a separate and new buffer. The files affect that buffer but do not | |
2681 | affect the main buffer with the exception of trace_options. Currently, | |
2682 | the trace_options affect all instances and the top level buffer | |
2683 | the same, but this may change in future releases. That is, options | |
2684 | may become specific to the instance they reside in. | |
2685 | ||
2686 | Notice that none of the function tracer files are there, nor is | |
2687 | current_tracer and available_tracers. This is because the buffers | |
2688 | can currently only have events enabled for them. | |
2689 | ||
2690 | # mkdir instances/foo | |
2691 | # mkdir instances/bar | |
2692 | # mkdir instances/zoot | |
2693 | # echo 100000 > buffer_size_kb | |
2694 | # echo 1000 > instances/foo/buffer_size_kb | |
2695 | # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb | |
2696 | # echo function > current_trace | |
2697 | # echo 1 > instances/foo/events/sched/sched_wakeup/enable | |
2698 | # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable | |
2699 | # echo 1 > instances/foo/events/sched/sched_switch/enable | |
2700 | # echo 1 > instances/bar/events/irq/enable | |
2701 | # echo 1 > instances/zoot/events/syscalls/enable | |
2702 | # cat trace_pipe | |
2703 | CPU:2 [LOST 11745 EVENTS] | |
2704 | bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist | |
2705 | bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave | |
2706 | bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist | |
2707 | bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist | |
2708 | bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock | |
2709 | bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype | |
2710 | bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist | |
2711 | bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist | |
2712 | bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics | |
2713 | bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics | |
2714 | bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process | |
2715 | [...] | |
2716 | ||
2717 | # cat instances/foo/trace_pipe | |
2718 | bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 | |
2719 | bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 | |
2720 | <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003 | |
2721 | <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120 | |
2722 | rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120 | |
2723 | bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 | |
2724 | bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 | |
2725 | bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120 | |
2726 | kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001 | |
2727 | kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120 | |
2728 | [...] | |
2729 | ||
2730 | # cat instances/bar/trace_pipe | |
2731 | migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX] | |
2732 | <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX] | |
2733 | bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER] | |
2734 | bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU] | |
2735 | bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER] | |
2736 | bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER] | |
2737 | bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU] | |
2738 | bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU] | |
2739 | sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4 | |
2740 | sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled | |
2741 | sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0 | |
2742 | sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled | |
2743 | [...] | |
2744 | ||
2745 | # cat instances/zoot/trace | |
2746 | # tracer: nop | |
2747 | # | |
2748 | # entries-in-buffer/entries-written: 18996/18996 #P:4 | |
2749 | # | |
2750 | # _-----=> irqs-off | |
2751 | # / _----=> need-resched | |
2752 | # | / _---=> hardirq/softirq | |
2753 | # || / _--=> preempt-depth | |
2754 | # ||| / delay | |
2755 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
2756 | # | | | |||| | | | |
2757 | bash-1998 [000] d... 140.733501: sys_write -> 0x2 | |
2758 | bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1) | |
2759 | bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1 | |
2760 | bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0) | |
2761 | bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1 | |
2762 | bash-1998 [000] d... 140.733510: sys_close(fd: a) | |
2763 | bash-1998 [000] d... 140.733510: sys_close -> 0x0 | |
2764 | bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8) | |
2765 | bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0 | |
2766 | bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8) | |
2767 | bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0 | |
2768 | ||
2769 | You can see that the trace of the top most trace buffer shows only | |
2770 | the function tracing. The foo instance displays wakeups and task | |
2771 | switches. | |
2772 | ||
2773 | To remove the instances, simply delete their directories: | |
2774 | ||
2775 | # rmdir instances/foo | |
2776 | # rmdir instances/bar | |
2777 | # rmdir instances/zoot | |
2778 | ||
2779 | Note, if a process has a trace file open in one of the instance | |
2780 | directories, the rmdir will fail with EBUSY. | |
2781 | ||
2782 | ||
2783 | Stack trace | |
5752674e | 2784 | ----------- |
8d016091 SRRH |
2785 | Since the kernel has a fixed sized stack, it is important not to |
2786 | waste it in functions. A kernel developer must be conscience of | |
2787 | what they allocate on the stack. If they add too much, the system | |
2788 | can be in danger of a stack overflow, and corruption will occur, | |
2789 | usually leading to a system panic. | |
2790 | ||
2791 | There are some tools that check this, usually with interrupts | |
2792 | periodically checking usage. But if you can perform a check | |
2793 | at every function call that will become very useful. As ftrace provides | |
2794 | a function tracer, it makes it convenient to check the stack size | |
2795 | at every function call. This is enabled via the stack tracer. | |
2796 | ||
2797 | CONFIG_STACK_TRACER enables the ftrace stack tracing functionality. | |
2798 | To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled. | |
2799 | ||
2800 | # echo 1 > /proc/sys/kernel/stack_tracer_enabled | |
2801 | ||
2802 | You can also enable it from the kernel command line to trace | |
2803 | the stack size of the kernel during boot up, by adding "stacktrace" | |
2804 | to the kernel command line parameter. | |
2805 | ||
2806 | After running it for a few minutes, the output looks like: | |
2807 | ||
2808 | # cat stack_max_size | |
2809 | 2928 | |
2810 | ||
2811 | # cat stack_trace | |
2812 | Depth Size Location (18 entries) | |
2813 | ----- ---- -------- | |
2814 | 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac | |
2815 | 1) 2704 160 find_busiest_group+0x31/0x1f1 | |
2816 | 2) 2544 256 load_balance+0xd9/0x662 | |
2817 | 3) 2288 80 idle_balance+0xbb/0x130 | |
2818 | 4) 2208 128 __schedule+0x26e/0x5b9 | |
2819 | 5) 2080 16 schedule+0x64/0x66 | |
2820 | 6) 2064 128 schedule_timeout+0x34/0xe0 | |
2821 | 7) 1936 112 wait_for_common+0x97/0xf1 | |
2822 | 8) 1824 16 wait_for_completion+0x1d/0x1f | |
2823 | 9) 1808 128 flush_work+0xfe/0x119 | |
2824 | 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20 | |
2825 | 11) 1664 48 input_available_p+0x1d/0x5c | |
2826 | 12) 1616 48 n_tty_poll+0x6d/0x134 | |
2827 | 13) 1568 64 tty_poll+0x64/0x7f | |
2828 | 14) 1504 880 do_select+0x31e/0x511 | |
2829 | 15) 624 400 core_sys_select+0x177/0x216 | |
2830 | 16) 224 96 sys_select+0x91/0xb9 | |
2831 | 17) 128 128 system_call_fastpath+0x16/0x1b | |
2832 | ||
2833 | Note, if -mfentry is being used by gcc, functions get traced before | |
2834 | they set up the stack frame. This means that leaf level functions | |
2835 | are not tested by the stack tracer when -mfentry is used. | |
2836 | ||
2837 | Currently, -mfentry is used by gcc 4.6.0 and above on x86 only. | |
2838 | ||
2839 | --------- | |
5752674e IM |
2840 | |
2841 | More details can be found in the source code, in the | |
baf20b3e | 2842 | kernel/trace/*.c files. |