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1 | CGROUPS |
2 | ------- | |
3 | ||
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4 | Written by Paul Menage <menage@google.com> based on |
5 | Documentation/cgroups/cpusets.txt | |
ddbcc7e8 PM |
6 | |
7 | Original copyright statements from cpusets.txt: | |
8 | Portions Copyright (C) 2004 BULL SA. | |
9 | Portions Copyright (c) 2004-2006 Silicon Graphics, Inc. | |
10 | Modified by Paul Jackson <pj@sgi.com> | |
11 | Modified by Christoph Lameter <clameter@sgi.com> | |
12 | ||
13 | CONTENTS: | |
14 | ========= | |
15 | ||
16 | 1. Control Groups | |
17 | 1.1 What are cgroups ? | |
18 | 1.2 Why are cgroups needed ? | |
19 | 1.3 How are cgroups implemented ? | |
20 | 1.4 What does notify_on_release do ? | |
21 | 1.5 How do I use cgroups ? | |
22 | 2. Usage Examples and Syntax | |
23 | 2.1 Basic Usage | |
24 | 2.2 Attaching processes | |
25 | 3. Kernel API | |
26 | 3.1 Overview | |
27 | 3.2 Synchronization | |
28 | 3.3 Subsystem API | |
29 | 4. Questions | |
30 | ||
31 | 1. Control Groups | |
d19e0583 | 32 | ================= |
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33 | |
34 | 1.1 What are cgroups ? | |
35 | ---------------------- | |
36 | ||
37 | Control Groups provide a mechanism for aggregating/partitioning sets of | |
38 | tasks, and all their future children, into hierarchical groups with | |
39 | specialized behaviour. | |
40 | ||
41 | Definitions: | |
42 | ||
43 | A *cgroup* associates a set of tasks with a set of parameters for one | |
44 | or more subsystems. | |
45 | ||
46 | A *subsystem* is a module that makes use of the task grouping | |
47 | facilities provided by cgroups to treat groups of tasks in | |
48 | particular ways. A subsystem is typically a "resource controller" that | |
49 | schedules a resource or applies per-cgroup limits, but it may be | |
50 | anything that wants to act on a group of processes, e.g. a | |
51 | virtualization subsystem. | |
52 | ||
53 | A *hierarchy* is a set of cgroups arranged in a tree, such that | |
54 | every task in the system is in exactly one of the cgroups in the | |
55 | hierarchy, and a set of subsystems; each subsystem has system-specific | |
56 | state attached to each cgroup in the hierarchy. Each hierarchy has | |
57 | an instance of the cgroup virtual filesystem associated with it. | |
58 | ||
59 | At any one time there may be multiple active hierachies of task | |
60 | cgroups. Each hierarchy is a partition of all tasks in the system. | |
61 | ||
62 | User level code may create and destroy cgroups by name in an | |
63 | instance of the cgroup virtual file system, specify and query to | |
64 | which cgroup a task is assigned, and list the task pids assigned to | |
65 | a cgroup. Those creations and assignments only affect the hierarchy | |
66 | associated with that instance of the cgroup file system. | |
67 | ||
68 | On their own, the only use for cgroups is for simple job | |
69 | tracking. The intention is that other subsystems hook into the generic | |
70 | cgroup support to provide new attributes for cgroups, such as | |
71 | accounting/limiting the resources which processes in a cgroup can | |
45ce80fb | 72 | access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allows |
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73 | you to associate a set of CPUs and a set of memory nodes with the |
74 | tasks in each cgroup. | |
75 | ||
76 | 1.2 Why are cgroups needed ? | |
77 | ---------------------------- | |
78 | ||
79 | There are multiple efforts to provide process aggregations in the | |
80 | Linux kernel, mainly for resource tracking purposes. Such efforts | |
81 | include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server | |
82 | namespaces. These all require the basic notion of a | |
83 | grouping/partitioning of processes, with newly forked processes ending | |
84 | in the same group (cgroup) as their parent process. | |
85 | ||
86 | The kernel cgroup patch provides the minimum essential kernel | |
87 | mechanisms required to efficiently implement such groups. It has | |
88 | minimal impact on the system fast paths, and provides hooks for | |
89 | specific subsystems such as cpusets to provide additional behaviour as | |
90 | desired. | |
91 | ||
92 | Multiple hierarchy support is provided to allow for situations where | |
93 | the division of tasks into cgroups is distinctly different for | |
94 | different subsystems - having parallel hierarchies allows each | |
95 | hierarchy to be a natural division of tasks, without having to handle | |
96 | complex combinations of tasks that would be present if several | |
97 | unrelated subsystems needed to be forced into the same tree of | |
98 | cgroups. | |
99 | ||
100 | At one extreme, each resource controller or subsystem could be in a | |
101 | separate hierarchy; at the other extreme, all subsystems | |
102 | would be attached to the same hierarchy. | |
103 | ||
104 | As an example of a scenario (originally proposed by vatsa@in.ibm.com) | |
105 | that can benefit from multiple hierarchies, consider a large | |
106 | university server with various users - students, professors, system | |
107 | tasks etc. The resource planning for this server could be along the | |
108 | following lines: | |
109 | ||
110 | CPU : Top cpuset | |
111 | / \ | |
112 | CPUSet1 CPUSet2 | |
113 | | | | |
114 | (Profs) (Students) | |
115 | ||
116 | In addition (system tasks) are attached to topcpuset (so | |
117 | that they can run anywhere) with a limit of 20% | |
118 | ||
119 | Memory : Professors (50%), students (30%), system (20%) | |
120 | ||
121 | Disk : Prof (50%), students (30%), system (20%) | |
122 | ||
123 | Network : WWW browsing (20%), Network File System (60%), others (20%) | |
124 | / \ | |
125 | Prof (15%) students (5%) | |
126 | ||
127 | Browsers like firefox/lynx go into the WWW network class, while (k)nfsd go | |
128 | into NFS network class. | |
129 | ||
130 | At the same time firefox/lynx will share an appropriate CPU/Memory class | |
131 | depending on who launched it (prof/student). | |
132 | ||
133 | With the ability to classify tasks differently for different resources | |
134 | (by putting those resource subsystems in different hierarchies) then | |
135 | the admin can easily set up a script which receives exec notifications | |
136 | and depending on who is launching the browser he can | |
137 | ||
138 | # echo browser_pid > /mnt/<restype>/<userclass>/tasks | |
139 | ||
140 | With only a single hierarchy, he now would potentially have to create | |
141 | a separate cgroup for every browser launched and associate it with | |
142 | approp network and other resource class. This may lead to | |
143 | proliferation of such cgroups. | |
144 | ||
145 | Also lets say that the administrator would like to give enhanced network | |
146 | access temporarily to a student's browser (since it is night and the user | |
d19e0583 | 147 | wants to do online gaming :)) OR give one of the students simulation |
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148 | apps enhanced CPU power, |
149 | ||
d19e0583 | 150 | With ability to write pids directly to resource classes, it's just a |
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151 | matter of : |
152 | ||
153 | # echo pid > /mnt/network/<new_class>/tasks | |
154 | (after some time) | |
155 | # echo pid > /mnt/network/<orig_class>/tasks | |
156 | ||
157 | Without this ability, he would have to split the cgroup into | |
158 | multiple separate ones and then associate the new cgroups with the | |
159 | new resource classes. | |
160 | ||
161 | ||
162 | ||
163 | 1.3 How are cgroups implemented ? | |
164 | --------------------------------- | |
165 | ||
166 | Control Groups extends the kernel as follows: | |
167 | ||
168 | - Each task in the system has a reference-counted pointer to a | |
169 | css_set. | |
170 | ||
171 | - A css_set contains a set of reference-counted pointers to | |
172 | cgroup_subsys_state objects, one for each cgroup subsystem | |
173 | registered in the system. There is no direct link from a task to | |
174 | the cgroup of which it's a member in each hierarchy, but this | |
175 | can be determined by following pointers through the | |
176 | cgroup_subsys_state objects. This is because accessing the | |
177 | subsystem state is something that's expected to happen frequently | |
178 | and in performance-critical code, whereas operations that require a | |
179 | task's actual cgroup assignments (in particular, moving between | |
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180 | cgroups) are less common. A linked list runs through the cg_list |
181 | field of each task_struct using the css_set, anchored at | |
182 | css_set->tasks. | |
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183 | |
184 | - A cgroup hierarchy filesystem can be mounted for browsing and | |
185 | manipulation from user space. | |
186 | ||
187 | - You can list all the tasks (by pid) attached to any cgroup. | |
188 | ||
189 | The implementation of cgroups requires a few, simple hooks | |
190 | into the rest of the kernel, none in performance critical paths: | |
191 | ||
192 | - in init/main.c, to initialize the root cgroups and initial | |
193 | css_set at system boot. | |
194 | ||
195 | - in fork and exit, to attach and detach a task from its css_set. | |
196 | ||
197 | In addition a new file system, of type "cgroup" may be mounted, to | |
198 | enable browsing and modifying the cgroups presently known to the | |
199 | kernel. When mounting a cgroup hierarchy, you may specify a | |
200 | comma-separated list of subsystems to mount as the filesystem mount | |
201 | options. By default, mounting the cgroup filesystem attempts to | |
202 | mount a hierarchy containing all registered subsystems. | |
203 | ||
204 | If an active hierarchy with exactly the same set of subsystems already | |
205 | exists, it will be reused for the new mount. If no existing hierarchy | |
206 | matches, and any of the requested subsystems are in use in an existing | |
207 | hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy | |
208 | is activated, associated with the requested subsystems. | |
209 | ||
210 | It's not currently possible to bind a new subsystem to an active | |
211 | cgroup hierarchy, or to unbind a subsystem from an active cgroup | |
212 | hierarchy. This may be possible in future, but is fraught with nasty | |
213 | error-recovery issues. | |
214 | ||
215 | When a cgroup filesystem is unmounted, if there are any | |
216 | child cgroups created below the top-level cgroup, that hierarchy | |
217 | will remain active even though unmounted; if there are no | |
218 | child cgroups then the hierarchy will be deactivated. | |
219 | ||
220 | No new system calls are added for cgroups - all support for | |
221 | querying and modifying cgroups is via this cgroup file system. | |
222 | ||
223 | Each task under /proc has an added file named 'cgroup' displaying, | |
224 | for each active hierarchy, the subsystem names and the cgroup name | |
225 | as the path relative to the root of the cgroup file system. | |
226 | ||
227 | Each cgroup is represented by a directory in the cgroup file system | |
228 | containing the following files describing that cgroup: | |
229 | ||
230 | - tasks: list of tasks (by pid) attached to that cgroup | |
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231 | - notify_on_release flag: run the release agent on exit? |
232 | - release_agent: the path to use for release notifications (this file | |
233 | exists in the top cgroup only) | |
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234 | |
235 | Other subsystems such as cpusets may add additional files in each | |
d19e0583 | 236 | cgroup dir. |
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237 | |
238 | New cgroups are created using the mkdir system call or shell | |
239 | command. The properties of a cgroup, such as its flags, are | |
240 | modified by writing to the appropriate file in that cgroups | |
241 | directory, as listed above. | |
242 | ||
243 | The named hierarchical structure of nested cgroups allows partitioning | |
244 | a large system into nested, dynamically changeable, "soft-partitions". | |
245 | ||
246 | The attachment of each task, automatically inherited at fork by any | |
247 | children of that task, to a cgroup allows organizing the work load | |
248 | on a system into related sets of tasks. A task may be re-attached to | |
249 | any other cgroup, if allowed by the permissions on the necessary | |
250 | cgroup file system directories. | |
251 | ||
252 | When a task is moved from one cgroup to another, it gets a new | |
253 | css_set pointer - if there's an already existing css_set with the | |
254 | desired collection of cgroups then that group is reused, else a new | |
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255 | css_set is allocated. The appropriate existing css_set is located by |
256 | looking into a hash table. | |
ddbcc7e8 | 257 | |
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258 | To allow access from a cgroup to the css_sets (and hence tasks) |
259 | that comprise it, a set of cg_cgroup_link objects form a lattice; | |
260 | each cg_cgroup_link is linked into a list of cg_cgroup_links for | |
d19e0583 | 261 | a single cgroup on its cgrp_link_list field, and a list of |
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262 | cg_cgroup_links for a single css_set on its cg_link_list. |
263 | ||
264 | Thus the set of tasks in a cgroup can be listed by iterating over | |
265 | each css_set that references the cgroup, and sub-iterating over | |
266 | each css_set's task set. | |
267 | ||
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268 | The use of a Linux virtual file system (vfs) to represent the |
269 | cgroup hierarchy provides for a familiar permission and name space | |
270 | for cgroups, with a minimum of additional kernel code. | |
271 | ||
272 | 1.4 What does notify_on_release do ? | |
273 | ------------------------------------ | |
274 | ||
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275 | If the notify_on_release flag is enabled (1) in a cgroup, then |
276 | whenever the last task in the cgroup leaves (exits or attaches to | |
277 | some other cgroup) and the last child cgroup of that cgroup | |
278 | is removed, then the kernel runs the command specified by the contents | |
279 | of the "release_agent" file in that hierarchy's root directory, | |
280 | supplying the pathname (relative to the mount point of the cgroup | |
281 | file system) of the abandoned cgroup. This enables automatic | |
282 | removal of abandoned cgroups. The default value of | |
283 | notify_on_release in the root cgroup at system boot is disabled | |
284 | (0). The default value of other cgroups at creation is the current | |
285 | value of their parents notify_on_release setting. The default value of | |
286 | a cgroup hierarchy's release_agent path is empty. | |
287 | ||
288 | 1.5 How do I use cgroups ? | |
289 | -------------------------- | |
290 | ||
291 | To start a new job that is to be contained within a cgroup, using | |
292 | the "cpuset" cgroup subsystem, the steps are something like: | |
293 | ||
294 | 1) mkdir /dev/cgroup | |
295 | 2) mount -t cgroup -ocpuset cpuset /dev/cgroup | |
296 | 3) Create the new cgroup by doing mkdir's and write's (or echo's) in | |
297 | the /dev/cgroup virtual file system. | |
298 | 4) Start a task that will be the "founding father" of the new job. | |
299 | 5) Attach that task to the new cgroup by writing its pid to the | |
300 | /dev/cgroup tasks file for that cgroup. | |
301 | 6) fork, exec or clone the job tasks from this founding father task. | |
302 | ||
303 | For example, the following sequence of commands will setup a cgroup | |
304 | named "Charlie", containing just CPUs 2 and 3, and Memory Node 1, | |
305 | and then start a subshell 'sh' in that cgroup: | |
306 | ||
307 | mount -t cgroup cpuset -ocpuset /dev/cgroup | |
308 | cd /dev/cgroup | |
309 | mkdir Charlie | |
310 | cd Charlie | |
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311 | /bin/echo 2-3 > cpuset.cpus |
312 | /bin/echo 1 > cpuset.mems | |
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313 | /bin/echo $$ > tasks |
314 | sh | |
315 | # The subshell 'sh' is now running in cgroup Charlie | |
316 | # The next line should display '/Charlie' | |
317 | cat /proc/self/cgroup | |
318 | ||
319 | 2. Usage Examples and Syntax | |
320 | ============================ | |
321 | ||
322 | 2.1 Basic Usage | |
323 | --------------- | |
324 | ||
325 | Creating, modifying, using the cgroups can be done through the cgroup | |
326 | virtual filesystem. | |
327 | ||
328 | To mount a cgroup hierarchy will all available subsystems, type: | |
329 | # mount -t cgroup xxx /dev/cgroup | |
330 | ||
331 | The "xxx" is not interpreted by the cgroup code, but will appear in | |
332 | /proc/mounts so may be any useful identifying string that you like. | |
333 | ||
334 | To mount a cgroup hierarchy with just the cpuset and numtasks | |
335 | subsystems, type: | |
336 | # mount -t cgroup -o cpuset,numtasks hier1 /dev/cgroup | |
337 | ||
338 | To change the set of subsystems bound to a mounted hierarchy, just | |
339 | remount with different options: | |
340 | ||
341 | # mount -o remount,cpuset,ns /dev/cgroup | |
342 | ||
343 | Note that changing the set of subsystems is currently only supported | |
344 | when the hierarchy consists of a single (root) cgroup. Supporting | |
345 | the ability to arbitrarily bind/unbind subsystems from an existing | |
346 | cgroup hierarchy is intended to be implemented in the future. | |
347 | ||
348 | Then under /dev/cgroup you can find a tree that corresponds to the | |
349 | tree of the cgroups in the system. For instance, /dev/cgroup | |
350 | is the cgroup that holds the whole system. | |
351 | ||
352 | If you want to create a new cgroup under /dev/cgroup: | |
353 | # cd /dev/cgroup | |
354 | # mkdir my_cgroup | |
355 | ||
356 | Now you want to do something with this cgroup. | |
357 | # cd my_cgroup | |
358 | ||
359 | In this directory you can find several files: | |
360 | # ls | |
18e7f1f0 | 361 | notify_on_release tasks |
d19e0583 | 362 | (plus whatever files added by the attached subsystems) |
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363 | |
364 | Now attach your shell to this cgroup: | |
365 | # /bin/echo $$ > tasks | |
366 | ||
367 | You can also create cgroups inside your cgroup by using mkdir in this | |
368 | directory. | |
369 | # mkdir my_sub_cs | |
370 | ||
371 | To remove a cgroup, just use rmdir: | |
372 | # rmdir my_sub_cs | |
373 | ||
374 | This will fail if the cgroup is in use (has cgroups inside, or | |
375 | has processes attached, or is held alive by other subsystem-specific | |
376 | reference). | |
377 | ||
378 | 2.2 Attaching processes | |
379 | ----------------------- | |
380 | ||
381 | # /bin/echo PID > tasks | |
382 | ||
383 | Note that it is PID, not PIDs. You can only attach ONE task at a time. | |
384 | If you have several tasks to attach, you have to do it one after another: | |
385 | ||
386 | # /bin/echo PID1 > tasks | |
387 | # /bin/echo PID2 > tasks | |
388 | ... | |
389 | # /bin/echo PIDn > tasks | |
390 | ||
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391 | You can attach the current shell task by echoing 0: |
392 | ||
393 | # echo 0 > tasks | |
394 | ||
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395 | 3. Kernel API |
396 | ============= | |
397 | ||
398 | 3.1 Overview | |
399 | ------------ | |
400 | ||
401 | Each kernel subsystem that wants to hook into the generic cgroup | |
402 | system needs to create a cgroup_subsys object. This contains | |
403 | various methods, which are callbacks from the cgroup system, along | |
404 | with a subsystem id which will be assigned by the cgroup system. | |
405 | ||
406 | Other fields in the cgroup_subsys object include: | |
407 | ||
408 | - subsys_id: a unique array index for the subsystem, indicating which | |
d19e0583 | 409 | entry in cgroup->subsys[] this subsystem should be managing. |
ddbcc7e8 | 410 | |
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411 | - name: should be initialized to a unique subsystem name. Should be |
412 | no longer than MAX_CGROUP_TYPE_NAMELEN. | |
ddbcc7e8 | 413 | |
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414 | - early_init: indicate if the subsystem needs early initialization |
415 | at system boot. | |
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416 | |
417 | Each cgroup object created by the system has an array of pointers, | |
418 | indexed by subsystem id; this pointer is entirely managed by the | |
419 | subsystem; the generic cgroup code will never touch this pointer. | |
420 | ||
421 | 3.2 Synchronization | |
422 | ------------------- | |
423 | ||
424 | There is a global mutex, cgroup_mutex, used by the cgroup | |
425 | system. This should be taken by anything that wants to modify a | |
426 | cgroup. It may also be taken to prevent cgroups from being | |
427 | modified, but more specific locks may be more appropriate in that | |
428 | situation. | |
429 | ||
430 | See kernel/cgroup.c for more details. | |
431 | ||
432 | Subsystems can take/release the cgroup_mutex via the functions | |
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433 | cgroup_lock()/cgroup_unlock(). |
434 | ||
435 | Accessing a task's cgroup pointer may be done in the following ways: | |
436 | - while holding cgroup_mutex | |
437 | - while holding the task's alloc_lock (via task_lock()) | |
438 | - inside an rcu_read_lock() section via rcu_dereference() | |
439 | ||
440 | 3.3 Subsystem API | |
d19e0583 | 441 | ----------------- |
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442 | |
443 | Each subsystem should: | |
444 | ||
445 | - add an entry in linux/cgroup_subsys.h | |
446 | - define a cgroup_subsys object called <name>_subsys | |
447 | ||
448 | Each subsystem may export the following methods. The only mandatory | |
449 | methods are create/destroy. Any others that are null are presumed to | |
450 | be successful no-ops. | |
451 | ||
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452 | struct cgroup_subsys_state *create(struct cgroup_subsys *ss, |
453 | struct cgroup *cgrp) | |
8dc4f3e1 | 454 | (cgroup_mutex held by caller) |
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455 | |
456 | Called to create a subsystem state object for a cgroup. The | |
457 | subsystem should allocate its subsystem state object for the passed | |
458 | cgroup, returning a pointer to the new object on success or a | |
459 | negative error code. On success, the subsystem pointer should point to | |
460 | a structure of type cgroup_subsys_state (typically embedded in a | |
461 | larger subsystem-specific object), which will be initialized by the | |
462 | cgroup system. Note that this will be called at initialization to | |
463 | create the root subsystem state for this subsystem; this case can be | |
464 | identified by the passed cgroup object having a NULL parent (since | |
465 | it's the root of the hierarchy) and may be an appropriate place for | |
466 | initialization code. | |
467 | ||
d19e0583 | 468 | void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp) |
8dc4f3e1 | 469 | (cgroup_mutex held by caller) |
ddbcc7e8 | 470 | |
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471 | The cgroup system is about to destroy the passed cgroup; the subsystem |
472 | should do any necessary cleanup and free its subsystem state | |
473 | object. By the time this method is called, the cgroup has already been | |
474 | unlinked from the file system and from the child list of its parent; | |
475 | cgroup->parent is still valid. (Note - can also be called for a | |
476 | newly-created cgroup if an error occurs after this subsystem's | |
477 | create() method has been called for the new cgroup). | |
ddbcc7e8 | 478 | |
d19e0583 | 479 | void pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp); |
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480 | |
481 | Called before checking the reference count on each subsystem. This may | |
482 | be useful for subsystems which have some extra references even if | |
483 | there are not tasks in the cgroup. | |
484 | ||
485 | int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp, | |
ddbcc7e8 | 486 | struct task_struct *task) |
8dc4f3e1 | 487 | (cgroup_mutex held by caller) |
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488 | |
489 | Called prior to moving a task into a cgroup; if the subsystem | |
490 | returns an error, this will abort the attach operation. If a NULL | |
491 | task is passed, then a successful result indicates that *any* | |
492 | unspecified task can be moved into the cgroup. Note that this isn't | |
493 | called on a fork. If this method returns 0 (success) then this should | |
494 | remain valid while the caller holds cgroup_mutex. | |
495 | ||
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496 | void attach(struct cgroup_subsys *ss, struct cgroup *cgrp, |
497 | struct cgroup *old_cgrp, struct task_struct *task) | |
18e7f1f0 | 498 | (cgroup_mutex held by caller) |
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499 | |
500 | Called after the task has been attached to the cgroup, to allow any | |
501 | post-attachment activity that requires memory allocations or blocking. | |
502 | ||
503 | void fork(struct cgroup_subsy *ss, struct task_struct *task) | |
ddbcc7e8 | 504 | |
e8d55fde | 505 | Called when a task is forked into a cgroup. |
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506 | |
507 | void exit(struct cgroup_subsys *ss, struct task_struct *task) | |
ddbcc7e8 | 508 | |
d19e0583 | 509 | Called during task exit. |
ddbcc7e8 | 510 | |
d19e0583 | 511 | int populate(struct cgroup_subsys *ss, struct cgroup *cgrp) |
18e7f1f0 | 512 | (cgroup_mutex held by caller) |
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513 | |
514 | Called after creation of a cgroup to allow a subsystem to populate | |
515 | the cgroup directory with file entries. The subsystem should make | |
516 | calls to cgroup_add_file() with objects of type cftype (see | |
517 | include/linux/cgroup.h for details). Note that although this | |
518 | method can return an error code, the error code is currently not | |
519 | always handled well. | |
520 | ||
d19e0583 | 521 | void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp) |
18e7f1f0 | 522 | (cgroup_mutex held by caller) |
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523 | |
524 | Called at the end of cgroup_clone() to do any paramater | |
525 | initialization which might be required before a task could attach. For | |
526 | example in cpusets, no task may attach before 'cpus' and 'mems' are set | |
527 | up. | |
528 | ||
ddbcc7e8 | 529 | void bind(struct cgroup_subsys *ss, struct cgroup *root) |
999cd8a4 | 530 | (cgroup_mutex and ss->hierarchy_mutex held by caller) |
ddbcc7e8 PM |
531 | |
532 | Called when a cgroup subsystem is rebound to a different hierarchy | |
533 | and root cgroup. Currently this will only involve movement between | |
534 | the default hierarchy (which never has sub-cgroups) and a hierarchy | |
535 | that is being created/destroyed (and hence has no sub-cgroups). | |
536 | ||
537 | 4. Questions | |
538 | ============ | |
539 | ||
540 | Q: what's up with this '/bin/echo' ? | |
541 | A: bash's builtin 'echo' command does not check calls to write() against | |
542 | errors. If you use it in the cgroup file system, you won't be | |
543 | able to tell whether a command succeeded or failed. | |
544 | ||
545 | Q: When I attach processes, only the first of the line gets really attached ! | |
546 | A: We can only return one error code per call to write(). So you should also | |
547 | put only ONE pid. | |
548 |