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1 | RCU Torture Test Operation |
2 | ||
3 | ||
4 | CONFIG_RCU_TORTURE_TEST | |
5 | ||
6 | The CONFIG_RCU_TORTURE_TEST config option is available for all RCU | |
7 | implementations. It creates an rcutorture kernel module that can | |
8 | be loaded to run a torture test. The test periodically outputs | |
9 | status messages via printk(), which can be examined via the dmesg | |
72e9bb54 | 10 | command (perhaps grepping for "torture"). The test is started |
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11 | when the module is loaded, and stops when the module is unloaded. |
12 | ||
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13 | |
14 | MODULE PARAMETERS | |
15 | ||
16 | This module has the following parameters: | |
17 | ||
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18 | fqs_duration Duration (in microseconds) of artificially induced bursts |
19 | of force_quiescent_state() invocations. In RCU | |
20 | implementations having force_quiescent_state(), these | |
21 | bursts help force races between forcing a given grace | |
22 | period and that grace period ending on its own. | |
23 | ||
24 | fqs_holdoff Holdoff time (in microseconds) between consecutive calls | |
25 | to force_quiescent_state() within a burst. | |
26 | ||
27 | fqs_stutter Wait time (in seconds) between consecutive bursts | |
28 | of calls to force_quiescent_state(). | |
29 | ||
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30 | gp_normal Make the fake writers use normal synchronous grace-period |
31 | primitives. | |
32 | ||
33 | gp_exp Make the fake writers use expedited synchronous grace-period | |
34 | primitives. If both gp_normal and gp_exp are set, or | |
35 | if neither gp_normal nor gp_exp are set, then randomly | |
36 | choose the primitive so that about 50% are normal and | |
37 | 50% expedited. By default, neither are set, which | |
38 | gives best overall test coverage. | |
39 | ||
63cd758e | 40 | irqreader Says to invoke RCU readers from irq level. This is currently |
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41 | done via timers. Defaults to "1" for variants of RCU that |
42 | permit this. (Or, more accurately, variants of RCU that do | |
43 | -not- permit this know to ignore this variable.) | |
a241ec65 | 44 | |
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45 | n_barrier_cbs If this is nonzero, RCU barrier testing will be conducted, |
46 | in which case n_barrier_cbs specifies the number of | |
47 | RCU callbacks (and corresponding kthreads) to use for | |
48 | this testing. The value cannot be negative. If you | |
49 | specify this to be non-zero when torture_type indicates a | |
50 | synchronous RCU implementation (one for which a member of | |
51 | the synchronize_rcu() rather than the call_rcu() family is | |
52 | used -- see the documentation for torture_type below), an | |
53 | error will be reported and no testing will be carried out. | |
54 | ||
b772e1dd JT |
55 | nfakewriters This is the number of RCU fake writer threads to run. Fake |
56 | writer threads repeatedly use the synchronous "wait for | |
57 | current readers" function of the interface selected by | |
58 | torture_type, with a delay between calls to allow for various | |
59 | different numbers of writers running in parallel. | |
60 | nfakewriters defaults to 4, which provides enough parallelism | |
61 | to trigger special cases caused by multiple writers, such as | |
62 | the synchronize_srcu() early return optimization. | |
63 | ||
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64 | nreaders This is the number of RCU reading threads supported. |
65 | The default is twice the number of CPUs. Why twice? | |
66 | To properly exercise RCU implementations with preemptible | |
67 | read-side critical sections. | |
68 | ||
b58bdcca PM |
69 | onoff_interval |
70 | The number of seconds between each attempt to execute a | |
71 | randomly selected CPU-hotplug operation. Defaults to | |
72 | zero, which disables CPU hotplugging. In HOTPLUG_CPU=n | |
73 | kernels, rcutorture will silently refuse to do any | |
74 | CPU-hotplug operations regardless of what value is | |
75 | specified for onoff_interval. | |
76 | ||
9b9ec9b9 PM |
77 | onoff_holdoff The number of seconds to wait until starting CPU-hotplug |
78 | operations. This would normally only be used when | |
79 | rcutorture was built into the kernel and started | |
80 | automatically at boot time, in which case it is useful | |
81 | in order to avoid confusing boot-time code with CPUs | |
82 | coming and going. | |
83 | ||
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84 | shuffle_interval |
85 | The number of seconds to keep the test threads affinitied | |
86 | to a particular subset of the CPUs, defaults to 3 seconds. | |
87 | Used in conjunction with test_no_idle_hz. | |
88 | ||
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89 | shutdown_secs The number of seconds to run the test before terminating |
90 | the test and powering off the system. The default is | |
91 | zero, which disables test termination and system shutdown. | |
92 | This capability is useful for automated testing. | |
93 | ||
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94 | stall_cpu The number of seconds that a CPU should be stalled while |
95 | within both an rcu_read_lock() and a preempt_disable(). | |
96 | This stall happens only once per rcutorture run. | |
97 | If you need multiple stalls, use modprobe and rmmod to | |
98 | repeatedly run rcutorture. The default for stall_cpu | |
99 | is zero, which prevents rcutorture from stalling a CPU. | |
100 | ||
101 | Note that attempts to rmmod rcutorture while the stall | |
102 | is ongoing will hang, so be careful what value you | |
103 | choose for this module parameter! In addition, too-large | |
104 | values for stall_cpu might well induce failures and | |
105 | warnings in other parts of the kernel. You have been | |
106 | warned! | |
107 | ||
108 | stall_cpu_holdoff | |
109 | The number of seconds to wait after rcutorture starts | |
110 | before stalling a CPU. Defaults to 10 seconds. | |
111 | ||
a241ec65 PM |
112 | stat_interval The number of seconds between output of torture |
113 | statistics (via printk()). Regardless of the interval, | |
114 | statistics are printed when the module is unloaded. | |
115 | Setting the interval to zero causes the statistics to | |
116 | be printed -only- when the module is unloaded, and this | |
117 | is the default. | |
118 | ||
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119 | stutter The length of time to run the test before pausing for this |
120 | same period of time. Defaults to "stutter=5", so as | |
121 | to run and pause for (roughly) five-second intervals. | |
122 | Specifying "stutter=0" causes the test to run continuously | |
123 | without pausing, which is the old default behavior. | |
124 | ||
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125 | test_boost Whether or not to test the ability of RCU to do priority |
126 | boosting. Defaults to "test_boost=1", which performs | |
127 | RCU priority-inversion testing only if the selected | |
128 | RCU implementation supports priority boosting. Specifying | |
129 | "test_boost=0" never performs RCU priority-inversion | |
130 | testing. Specifying "test_boost=2" performs RCU | |
131 | priority-inversion testing even if the selected RCU | |
132 | implementation does not support RCU priority boosting, | |
133 | which can be used to test rcutorture's ability to | |
134 | carry out RCU priority-inversion testing. | |
135 | ||
136 | test_boost_interval | |
137 | The number of seconds in an RCU priority-inversion test | |
138 | cycle. Defaults to "test_boost_interval=7". It is | |
139 | usually wise for this value to be relatively prime to | |
140 | the value selected for "stutter". | |
141 | ||
142 | test_boost_duration | |
143 | The number of seconds to do RCU priority-inversion testing | |
144 | within any given "test_boost_interval". Defaults to | |
145 | "test_boost_duration=4". | |
146 | ||
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147 | test_no_idle_hz Whether or not to test the ability of RCU to operate in |
148 | a kernel that disables the scheduling-clock interrupt to | |
149 | idle CPUs. Boolean parameter, "1" to test, "0" otherwise. | |
f85d6c71 | 150 | Defaults to omitting this test. |
29766f1e | 151 | |
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152 | torture_type The type of RCU to test, with string values as follows: |
153 | ||
b672adf8 PM |
154 | "rcu": rcu_read_lock(), rcu_read_unlock() and call_rcu(), |
155 | along with expedited, synchronous, and polling | |
156 | variants. | |
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157 | |
158 | "rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and | |
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159 | call_rcu_bh(), along with expedited and synchronous |
160 | variants. | |
63cd758e | 161 | |
b672adf8 PM |
162 | "rcu_busted": This tests an intentionally incorrect version |
163 | of RCU in order to help test rcutorture itself. | |
bdf2a436 | 164 | |
63cd758e | 165 | "srcu": srcu_read_lock(), srcu_read_unlock() and |
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166 | call_srcu(), along with expedited and |
167 | synchronous variants. | |
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168 | |
169 | "sched": preempt_disable(), preempt_enable(), and | |
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170 | call_rcu_sched(), along with expedited, |
171 | synchronous, and polling variants. | |
63cd758e | 172 | |
b672adf8 PM |
173 | "tasks": voluntary context switch and call_rcu_tasks(), |
174 | along with expedited and synchronous variants. | |
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175 | |
176 | Defaults to "rcu". | |
72e9bb54 | 177 | |
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178 | verbose Enable debug printk()s. Default is disabled. |
179 | ||
180 | ||
181 | OUTPUT | |
182 | ||
183 | The statistics output is as follows: | |
184 | ||
63cd758e | 185 | rcu-torture:--- Start of test: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 |
fae4b54f | 186 | rcu-torture: rtc: (null) ver: 155441 tfle: 0 rta: 155441 rtaf: 8884 rtf: 155440 rtmbe: 0 rtbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 3055767 |
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187 | rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0 |
188 | rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0 | |
189 | rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0 | |
190 | rcu-torture:--- End of test: SUCCESS: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 | |
a241ec65 | 191 | |
72e9bb54 | 192 | The command "dmesg | grep torture:" will extract this information on |
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193 | most systems. On more esoteric configurations, it may be necessary to |
194 | use other commands to access the output of the printk()s used by | |
195 | the RCU torture test. The printk()s use KERN_ALERT, so they should | |
196 | be evident. ;-) | |
197 | ||
63cd758e PM |
198 | The first and last lines show the rcutorture module parameters, and the |
199 | last line shows either "SUCCESS" or "FAILURE", based on rcutorture's | |
200 | automatic determination as to whether RCU operated correctly. | |
201 | ||
a241ec65 PM |
202 | The entries are as follows: |
203 | ||
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204 | o "rtc": The hexadecimal address of the structure currently visible |
205 | to readers. | |
206 | ||
63cd758e | 207 | o "ver": The number of times since boot that the RCU writer task |
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208 | has changed the structure visible to readers. |
209 | ||
210 | o "tfle": If non-zero, indicates that the "torture freelist" | |
63cd758e | 211 | containing structures to be placed into the "rtc" area is empty. |
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212 | This condition is important, since it can fool you into thinking |
213 | that RCU is working when it is not. :-/ | |
214 | ||
215 | o "rta": Number of structures allocated from the torture freelist. | |
216 | ||
217 | o "rtaf": Number of allocations from the torture freelist that have | |
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218 | failed due to the list being empty. It is not unusual for this |
219 | to be non-zero, but it is bad for it to be a large fraction of | |
220 | the value indicated by "rta". | |
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221 | |
222 | o "rtf": Number of frees into the torture freelist. | |
223 | ||
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224 | o "rtmbe": A non-zero value indicates that rcutorture believes that |
225 | rcu_assign_pointer() and rcu_dereference() are not working | |
226 | correctly. This value should be zero. | |
227 | ||
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228 | o "rtbe": A non-zero value indicates that one of the rcu_barrier() |
229 | family of functions is not working correctly. | |
230 | ||
63cd758e PM |
231 | o "rtbke": rcutorture was unable to create the real-time kthreads |
232 | used to force RCU priority inversion. This value should be zero. | |
233 | ||
234 | o "rtbre": Although rcutorture successfully created the kthreads | |
235 | used to force RCU priority inversion, it was unable to set them | |
236 | to the real-time priority level of 1. This value should be zero. | |
237 | ||
238 | o "rtbf": The number of times that RCU priority boosting failed | |
239 | to resolve RCU priority inversion. | |
240 | ||
241 | o "rtb": The number of times that rcutorture attempted to force | |
242 | an RCU priority inversion condition. If you are testing RCU | |
243 | priority boosting via the "test_boost" module parameter, this | |
244 | value should be non-zero. | |
245 | ||
246 | o "nt": The number of times rcutorture ran RCU read-side code from | |
247 | within a timer handler. This value should be non-zero only | |
248 | if you specified the "irqreader" module parameter. | |
249 | ||
a241ec65 PM |
250 | o "Reader Pipe": Histogram of "ages" of structures seen by readers. |
251 | If any entries past the first two are non-zero, RCU is broken. | |
252 | And rcutorture prints the error flag string "!!!" to make sure | |
253 | you notice. The age of a newly allocated structure is zero, | |
254 | it becomes one when removed from reader visibility, and is | |
255 | incremented once per grace period subsequently -- and is freed | |
256 | after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. | |
257 | ||
258 | The output displayed above was taken from a correctly working | |
259 | RCU. If you want to see what it looks like when broken, break | |
260 | it yourself. ;-) | |
261 | ||
262 | o "Reader Batch": Another histogram of "ages" of structures seen | |
263 | by readers, but in terms of counter flips (or batches) rather | |
264 | than in terms of grace periods. The legal number of non-zero | |
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265 | entries is again two. The reason for this separate view is that |
266 | it is sometimes easier to get the third entry to show up in the | |
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267 | "Reader Batch" list than in the "Reader Pipe" list. |
268 | ||
269 | o "Free-Block Circulation": Shows the number of torture structures | |
270 | that have reached a given point in the pipeline. The first element | |
271 | should closely correspond to the number of structures allocated, | |
272 | the second to the number that have been removed from reader view, | |
273 | and all but the last remaining to the corresponding number of | |
274 | passes through a grace period. The last entry should be zero, | |
275 | as it is only incremented if a torture structure's counter | |
276 | somehow gets incremented farther than it should. | |
277 | ||
b2896d2e | 278 | Different implementations of RCU can provide implementation-specific |
4de5f89e | 279 | additional information. For example, Tree SRCU provides the following |
63cd758e | 280 | additional line: |
b2896d2e | 281 | |
4de5f89e | 282 | srcud-torture: Tree SRCU per-CPU(idx=0): 0(35,-21) 1(-4,24) 2(1,1) 3(-26,20) 4(28,-47) 5(-9,4) 6(-10,14) 7(-14,11) T(1,6) |
b2896d2e | 283 | |
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284 | This line shows the per-CPU counter state, in this case for Tree SRCU |
285 | using a dynamically allocated srcu_struct (hence "srcud-" rather than | |
286 | "srcu-"). The numbers in parentheses are the values of the "old" and | |
287 | "current" counters for the corresponding CPU. The "idx" value maps the | |
288 | "old" and "current" values to the underlying array, and is useful for | |
289 | debugging. The final "T" entry contains the totals of the counters. | |
240ebbf8 | 290 | |
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291 | |
292 | USAGE | |
293 | ||
294 | The following script may be used to torture RCU: | |
295 | ||
296 | #!/bin/sh | |
297 | ||
298 | modprobe rcutorture | |
105617da | 299 | sleep 3600 |
a241ec65 | 300 | rmmod rcutorture |
72e9bb54 | 301 | dmesg | grep torture: |
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302 | |
303 | The output can be manually inspected for the error flag of "!!!". | |
304 | One could of course create a more elaborate script that automatically | |
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305 | checked for such errors. The "rmmod" command forces a "SUCCESS", |
306 | "FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first | |
307 | two are self-explanatory, while the last indicates that while there | |
308 | were no RCU failures, CPU-hotplug problems were detected. | |
4de5f89e PM |
309 | |
310 | However, the tools/testing/selftests/rcutorture/bin/kvm.sh script | |
311 | provides better automation, including automatic failure analysis. | |
312 | It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd. | |
313 | See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions | |
314 | on setting up such an initrd. |