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1 | CGROUPS |
2 | ------- | |
3 | ||
45ce80fb LZ |
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 ? | |
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21 | 1.5 What does clone_children do ? |
22 | 1.6 How do I use cgroups ? | |
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23 | 2. Usage Examples and Syntax |
24 | 2.1 Basic Usage | |
25 | 2.2 Attaching processes | |
8ca712ea | 26 | 2.3 Mounting hierarchies by name |
0dea1168 | 27 | 2.4 Notification API |
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28 | 3. Kernel API |
29 | 3.1 Overview | |
30 | 3.2 Synchronization | |
31 | 3.3 Subsystem API | |
32 | 4. Questions | |
33 | ||
34 | 1. Control Groups | |
d19e0583 | 35 | ================= |
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36 | |
37 | 1.1 What are cgroups ? | |
38 | ---------------------- | |
39 | ||
40 | Control Groups provide a mechanism for aggregating/partitioning sets of | |
41 | tasks, and all their future children, into hierarchical groups with | |
42 | specialized behaviour. | |
43 | ||
44 | Definitions: | |
45 | ||
46 | A *cgroup* associates a set of tasks with a set of parameters for one | |
47 | or more subsystems. | |
48 | ||
49 | A *subsystem* is a module that makes use of the task grouping | |
50 | facilities provided by cgroups to treat groups of tasks in | |
51 | particular ways. A subsystem is typically a "resource controller" that | |
52 | schedules a resource or applies per-cgroup limits, but it may be | |
53 | anything that wants to act on a group of processes, e.g. a | |
54 | virtualization subsystem. | |
55 | ||
56 | A *hierarchy* is a set of cgroups arranged in a tree, such that | |
57 | every task in the system is in exactly one of the cgroups in the | |
58 | hierarchy, and a set of subsystems; each subsystem has system-specific | |
59 | state attached to each cgroup in the hierarchy. Each hierarchy has | |
60 | an instance of the cgroup virtual filesystem associated with it. | |
61 | ||
caa790ba | 62 | At any one time there may be multiple active hierarchies of task |
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63 | cgroups. Each hierarchy is a partition of all tasks in the system. |
64 | ||
65 | User level code may create and destroy cgroups by name in an | |
66 | instance of the cgroup virtual file system, specify and query to | |
67 | which cgroup a task is assigned, and list the task pids assigned to | |
68 | a cgroup. Those creations and assignments only affect the hierarchy | |
69 | associated with that instance of the cgroup file system. | |
70 | ||
71 | On their own, the only use for cgroups is for simple job | |
72 | tracking. The intention is that other subsystems hook into the generic | |
73 | cgroup support to provide new attributes for cgroups, such as | |
74 | accounting/limiting the resources which processes in a cgroup can | |
45ce80fb | 75 | access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allows |
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76 | you to associate a set of CPUs and a set of memory nodes with the |
77 | tasks in each cgroup. | |
78 | ||
79 | 1.2 Why are cgroups needed ? | |
80 | ---------------------------- | |
81 | ||
82 | There are multiple efforts to provide process aggregations in the | |
83 | Linux kernel, mainly for resource tracking purposes. Such efforts | |
84 | include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server | |
85 | namespaces. These all require the basic notion of a | |
86 | grouping/partitioning of processes, with newly forked processes ending | |
87 | in the same group (cgroup) as their parent process. | |
88 | ||
89 | The kernel cgroup patch provides the minimum essential kernel | |
90 | mechanisms required to efficiently implement such groups. It has | |
91 | minimal impact on the system fast paths, and provides hooks for | |
92 | specific subsystems such as cpusets to provide additional behaviour as | |
93 | desired. | |
94 | ||
95 | Multiple hierarchy support is provided to allow for situations where | |
96 | the division of tasks into cgroups is distinctly different for | |
97 | different subsystems - having parallel hierarchies allows each | |
98 | hierarchy to be a natural division of tasks, without having to handle | |
99 | complex combinations of tasks that would be present if several | |
100 | unrelated subsystems needed to be forced into the same tree of | |
101 | cgroups. | |
102 | ||
103 | At one extreme, each resource controller or subsystem could be in a | |
104 | separate hierarchy; at the other extreme, all subsystems | |
105 | would be attached to the same hierarchy. | |
106 | ||
107 | As an example of a scenario (originally proposed by vatsa@in.ibm.com) | |
108 | that can benefit from multiple hierarchies, consider a large | |
109 | university server with various users - students, professors, system | |
110 | tasks etc. The resource planning for this server could be along the | |
111 | following lines: | |
112 | ||
6ad85239 | 113 | CPU : "Top cpuset" |
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114 | / \ |
115 | CPUSet1 CPUSet2 | |
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116 | | | |
117 | (Professors) (Students) | |
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118 | |
119 | In addition (system tasks) are attached to topcpuset (so | |
120 | that they can run anywhere) with a limit of 20% | |
121 | ||
6ad85239 | 122 | Memory : Professors (50%), Students (30%), system (20%) |
ddbcc7e8 | 123 | |
6ad85239 | 124 | Disk : Professors (50%), Students (30%), system (20%) |
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125 | |
126 | Network : WWW browsing (20%), Network File System (60%), others (20%) | |
127 | / \ | |
6ad85239 | 128 | Professors (15%) students (5%) |
ddbcc7e8 | 129 | |
caa790ba | 130 | Browsers like Firefox/Lynx go into the WWW network class, while (k)nfsd go |
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131 | into NFS network class. |
132 | ||
caa790ba | 133 | At the same time Firefox/Lynx will share an appropriate CPU/Memory class |
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134 | depending on who launched it (prof/student). |
135 | ||
136 | With the ability to classify tasks differently for different resources | |
137 | (by putting those resource subsystems in different hierarchies) then | |
138 | the admin can easily set up a script which receives exec notifications | |
139 | and depending on who is launching the browser he can | |
140 | ||
f6e07d38 | 141 | # echo browser_pid > /sys/fs/cgroup/<restype>/<userclass>/tasks |
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142 | |
143 | With only a single hierarchy, he now would potentially have to create | |
144 | a separate cgroup for every browser launched and associate it with | |
145 | approp network and other resource class. This may lead to | |
146 | proliferation of such cgroups. | |
147 | ||
148 | Also lets say that the administrator would like to give enhanced network | |
149 | access temporarily to a student's browser (since it is night and the user | |
d19e0583 | 150 | wants to do online gaming :)) OR give one of the students simulation |
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151 | apps enhanced CPU power, |
152 | ||
d19e0583 | 153 | With ability to write pids directly to resource classes, it's just a |
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154 | matter of : |
155 | ||
f6e07d38 | 156 | # echo pid > /sys/fs/cgroup/network/<new_class>/tasks |
ddbcc7e8 | 157 | (after some time) |
f6e07d38 | 158 | # echo pid > /sys/fs/cgroup/network/<orig_class>/tasks |
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159 | |
160 | Without this ability, he would have to split the cgroup into | |
161 | multiple separate ones and then associate the new cgroups with the | |
162 | new resource classes. | |
163 | ||
164 | ||
165 | ||
166 | 1.3 How are cgroups implemented ? | |
167 | --------------------------------- | |
168 | ||
169 | Control Groups extends the kernel as follows: | |
170 | ||
171 | - Each task in the system has a reference-counted pointer to a | |
172 | css_set. | |
173 | ||
174 | - A css_set contains a set of reference-counted pointers to | |
175 | cgroup_subsys_state objects, one for each cgroup subsystem | |
176 | registered in the system. There is no direct link from a task to | |
177 | the cgroup of which it's a member in each hierarchy, but this | |
178 | can be determined by following pointers through the | |
179 | cgroup_subsys_state objects. This is because accessing the | |
180 | subsystem state is something that's expected to happen frequently | |
181 | and in performance-critical code, whereas operations that require a | |
182 | task's actual cgroup assignments (in particular, moving between | |
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183 | cgroups) are less common. A linked list runs through the cg_list |
184 | field of each task_struct using the css_set, anchored at | |
185 | css_set->tasks. | |
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186 | |
187 | - A cgroup hierarchy filesystem can be mounted for browsing and | |
188 | manipulation from user space. | |
189 | ||
190 | - You can list all the tasks (by pid) attached to any cgroup. | |
191 | ||
192 | The implementation of cgroups requires a few, simple hooks | |
193 | into the rest of the kernel, none in performance critical paths: | |
194 | ||
195 | - in init/main.c, to initialize the root cgroups and initial | |
196 | css_set at system boot. | |
197 | ||
198 | - in fork and exit, to attach and detach a task from its css_set. | |
199 | ||
200 | In addition a new file system, of type "cgroup" may be mounted, to | |
201 | enable browsing and modifying the cgroups presently known to the | |
202 | kernel. When mounting a cgroup hierarchy, you may specify a | |
203 | comma-separated list of subsystems to mount as the filesystem mount | |
204 | options. By default, mounting the cgroup filesystem attempts to | |
205 | mount a hierarchy containing all registered subsystems. | |
206 | ||
207 | If an active hierarchy with exactly the same set of subsystems already | |
208 | exists, it will be reused for the new mount. If no existing hierarchy | |
209 | matches, and any of the requested subsystems are in use in an existing | |
210 | hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy | |
211 | is activated, associated with the requested subsystems. | |
212 | ||
213 | It's not currently possible to bind a new subsystem to an active | |
214 | cgroup hierarchy, or to unbind a subsystem from an active cgroup | |
215 | hierarchy. This may be possible in future, but is fraught with nasty | |
216 | error-recovery issues. | |
217 | ||
218 | When a cgroup filesystem is unmounted, if there are any | |
219 | child cgroups created below the top-level cgroup, that hierarchy | |
220 | will remain active even though unmounted; if there are no | |
221 | child cgroups then the hierarchy will be deactivated. | |
222 | ||
223 | No new system calls are added for cgroups - all support for | |
224 | querying and modifying cgroups is via this cgroup file system. | |
225 | ||
226 | Each task under /proc has an added file named 'cgroup' displaying, | |
227 | for each active hierarchy, the subsystem names and the cgroup name | |
228 | as the path relative to the root of the cgroup file system. | |
229 | ||
230 | Each cgroup is represented by a directory in the cgroup file system | |
231 | containing the following files describing that cgroup: | |
232 | ||
7823da36 PM |
233 | - tasks: list of tasks (by pid) attached to that cgroup. This list |
234 | is not guaranteed to be sorted. Writing a thread id into this file | |
235 | moves the thread into this cgroup. | |
236 | - cgroup.procs: list of tgids in the cgroup. This list is not | |
237 | guaranteed to be sorted or free of duplicate tgids, and userspace | |
238 | should sort/uniquify the list if this property is required. | |
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239 | Writing a thread group id into this file moves all threads in that |
240 | group into this cgroup. | |
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241 | - notify_on_release flag: run the release agent on exit? |
242 | - release_agent: the path to use for release notifications (this file | |
243 | exists in the top cgroup only) | |
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244 | |
245 | Other subsystems such as cpusets may add additional files in each | |
d19e0583 | 246 | cgroup dir. |
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247 | |
248 | New cgroups are created using the mkdir system call or shell | |
249 | command. The properties of a cgroup, such as its flags, are | |
250 | modified by writing to the appropriate file in that cgroups | |
251 | directory, as listed above. | |
252 | ||
253 | The named hierarchical structure of nested cgroups allows partitioning | |
254 | a large system into nested, dynamically changeable, "soft-partitions". | |
255 | ||
256 | The attachment of each task, automatically inherited at fork by any | |
257 | children of that task, to a cgroup allows organizing the work load | |
258 | on a system into related sets of tasks. A task may be re-attached to | |
259 | any other cgroup, if allowed by the permissions on the necessary | |
260 | cgroup file system directories. | |
261 | ||
262 | When a task is moved from one cgroup to another, it gets a new | |
263 | css_set pointer - if there's an already existing css_set with the | |
264 | desired collection of cgroups then that group is reused, else a new | |
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265 | css_set is allocated. The appropriate existing css_set is located by |
266 | looking into a hash table. | |
ddbcc7e8 | 267 | |
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268 | To allow access from a cgroup to the css_sets (and hence tasks) |
269 | that comprise it, a set of cg_cgroup_link objects form a lattice; | |
270 | each cg_cgroup_link is linked into a list of cg_cgroup_links for | |
d19e0583 | 271 | a single cgroup on its cgrp_link_list field, and a list of |
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272 | cg_cgroup_links for a single css_set on its cg_link_list. |
273 | ||
274 | Thus the set of tasks in a cgroup can be listed by iterating over | |
275 | each css_set that references the cgroup, and sub-iterating over | |
276 | each css_set's task set. | |
277 | ||
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278 | The use of a Linux virtual file system (vfs) to represent the |
279 | cgroup hierarchy provides for a familiar permission and name space | |
280 | for cgroups, with a minimum of additional kernel code. | |
281 | ||
282 | 1.4 What does notify_on_release do ? | |
283 | ------------------------------------ | |
284 | ||
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285 | If the notify_on_release flag is enabled (1) in a cgroup, then |
286 | whenever the last task in the cgroup leaves (exits or attaches to | |
287 | some other cgroup) and the last child cgroup of that cgroup | |
288 | is removed, then the kernel runs the command specified by the contents | |
289 | of the "release_agent" file in that hierarchy's root directory, | |
290 | supplying the pathname (relative to the mount point of the cgroup | |
291 | file system) of the abandoned cgroup. This enables automatic | |
292 | removal of abandoned cgroups. The default value of | |
293 | notify_on_release in the root cgroup at system boot is disabled | |
294 | (0). The default value of other cgroups at creation is the current | |
295 | value of their parents notify_on_release setting. The default value of | |
296 | a cgroup hierarchy's release_agent path is empty. | |
297 | ||
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298 | 1.5 What does clone_children do ? |
299 | --------------------------------- | |
300 | ||
301 | If the clone_children flag is enabled (1) in a cgroup, then all | |
302 | cgroups created beneath will call the post_clone callbacks for each | |
303 | subsystem of the newly created cgroup. Usually when this callback is | |
304 | implemented for a subsystem, it copies the values of the parent | |
305 | subsystem, this is the case for the cpuset. | |
306 | ||
307 | 1.6 How do I use cgroups ? | |
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308 | -------------------------- |
309 | ||
310 | To start a new job that is to be contained within a cgroup, using | |
311 | the "cpuset" cgroup subsystem, the steps are something like: | |
312 | ||
f6e07d38 JS |
313 | 1) mount -t tmpfs cgroup_root /sys/fs/cgroup |
314 | 2) mkdir /sys/fs/cgroup/cpuset | |
315 | 3) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset | |
316 | 4) Create the new cgroup by doing mkdir's and write's (or echo's) in | |
317 | the /sys/fs/cgroup virtual file system. | |
318 | 5) Start a task that will be the "founding father" of the new job. | |
319 | 6) Attach that task to the new cgroup by writing its pid to the | |
320 | /sys/fs/cgroup/cpuset/tasks file for that cgroup. | |
321 | 7) fork, exec or clone the job tasks from this founding father task. | |
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322 | |
323 | For example, the following sequence of commands will setup a cgroup | |
324 | named "Charlie", containing just CPUs 2 and 3, and Memory Node 1, | |
325 | and then start a subshell 'sh' in that cgroup: | |
326 | ||
f6e07d38 JS |
327 | mount -t tmpfs cgroup_root /sys/fs/cgroup |
328 | mkdir /sys/fs/cgroup/cpuset | |
329 | mount -t cgroup cpuset -ocpuset /sys/fs/cgroup/cpuset | |
330 | cd /sys/fs/cgroup/cpuset | |
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331 | mkdir Charlie |
332 | cd Charlie | |
0f146a76 DG |
333 | /bin/echo 2-3 > cpuset.cpus |
334 | /bin/echo 1 > cpuset.mems | |
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335 | /bin/echo $$ > tasks |
336 | sh | |
337 | # The subshell 'sh' is now running in cgroup Charlie | |
338 | # The next line should display '/Charlie' | |
339 | cat /proc/self/cgroup | |
340 | ||
341 | 2. Usage Examples and Syntax | |
342 | ============================ | |
343 | ||
344 | 2.1 Basic Usage | |
345 | --------------- | |
346 | ||
347 | Creating, modifying, using the cgroups can be done through the cgroup | |
348 | virtual filesystem. | |
349 | ||
caa790ba | 350 | To mount a cgroup hierarchy with all available subsystems, type: |
f6e07d38 | 351 | # mount -t cgroup xxx /sys/fs/cgroup |
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352 | |
353 | The "xxx" is not interpreted by the cgroup code, but will appear in | |
354 | /proc/mounts so may be any useful identifying string that you like. | |
355 | ||
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356 | Note: Some subsystems do not work without some user input first. For instance, |
357 | if cpusets are enabled the user will have to populate the cpus and mems files | |
358 | for each new cgroup created before that group can be used. | |
359 | ||
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360 | As explained in section `1.2 Why are cgroups needed?' you should create |
361 | different hierarchies of cgroups for each single resource or group of | |
362 | resources you want to control. Therefore, you should mount a tmpfs on | |
363 | /sys/fs/cgroup and create directories for each cgroup resource or resource | |
364 | group. | |
365 | ||
366 | # mount -t tmpfs cgroup_root /sys/fs/cgroup | |
367 | # mkdir /sys/fs/cgroup/rg1 | |
368 | ||
595f4b69 | 369 | To mount a cgroup hierarchy with just the cpuset and memory |
ddbcc7e8 | 370 | subsystems, type: |
f6e07d38 | 371 | # mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1 |
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372 | |
373 | To change the set of subsystems bound to a mounted hierarchy, just | |
374 | remount with different options: | |
f6e07d38 | 375 | # mount -o remount,cpuset,blkio hier1 /sys/fs/cgroup/rg1 |
ddbcc7e8 | 376 | |
1bdcd78e | 377 | Now memory is removed from the hierarchy and blkio is added. |
b6719ec1 | 378 | |
1bdcd78e | 379 | Note this will add blkio to the hierarchy but won't remove memory or |
b6719ec1 | 380 | cpuset, because the new options are appended to the old ones: |
f6e07d38 | 381 | # mount -o remount,blkio /sys/fs/cgroup/rg1 |
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382 | |
383 | To Specify a hierarchy's release_agent: | |
384 | # mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \ | |
f6e07d38 | 385 | xxx /sys/fs/cgroup/rg1 |
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386 | |
387 | Note that specifying 'release_agent' more than once will return failure. | |
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388 | |
389 | Note that changing the set of subsystems is currently only supported | |
390 | when the hierarchy consists of a single (root) cgroup. Supporting | |
391 | the ability to arbitrarily bind/unbind subsystems from an existing | |
392 | cgroup hierarchy is intended to be implemented in the future. | |
393 | ||
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394 | Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the |
395 | tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1 | |
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396 | is the cgroup that holds the whole system. |
397 | ||
b6719ec1 | 398 | If you want to change the value of release_agent: |
f6e07d38 | 399 | # echo "/sbin/new_release_agent" > /sys/fs/cgroup/rg1/release_agent |
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400 | |
401 | It can also be changed via remount. | |
402 | ||
f6e07d38 JS |
403 | If you want to create a new cgroup under /sys/fs/cgroup/rg1: |
404 | # cd /sys/fs/cgroup/rg1 | |
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405 | # mkdir my_cgroup |
406 | ||
407 | Now you want to do something with this cgroup. | |
408 | # cd my_cgroup | |
409 | ||
410 | In this directory you can find several files: | |
411 | # ls | |
7823da36 | 412 | cgroup.procs notify_on_release tasks |
d19e0583 | 413 | (plus whatever files added by the attached subsystems) |
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414 | |
415 | Now attach your shell to this cgroup: | |
416 | # /bin/echo $$ > tasks | |
417 | ||
418 | You can also create cgroups inside your cgroup by using mkdir in this | |
419 | directory. | |
420 | # mkdir my_sub_cs | |
421 | ||
422 | To remove a cgroup, just use rmdir: | |
423 | # rmdir my_sub_cs | |
424 | ||
425 | This will fail if the cgroup is in use (has cgroups inside, or | |
426 | has processes attached, or is held alive by other subsystem-specific | |
427 | reference). | |
428 | ||
429 | 2.2 Attaching processes | |
430 | ----------------------- | |
431 | ||
432 | # /bin/echo PID > tasks | |
433 | ||
434 | Note that it is PID, not PIDs. You can only attach ONE task at a time. | |
435 | If you have several tasks to attach, you have to do it one after another: | |
436 | ||
437 | # /bin/echo PID1 > tasks | |
438 | # /bin/echo PID2 > tasks | |
439 | ... | |
440 | # /bin/echo PIDn > tasks | |
441 | ||
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442 | You can attach the current shell task by echoing 0: |
443 | ||
444 | # echo 0 > tasks | |
445 | ||
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446 | You can use the cgroup.procs file instead of the tasks file to move all |
447 | threads in a threadgroup at once. Echoing the pid of any task in a | |
448 | threadgroup to cgroup.procs causes all tasks in that threadgroup to be | |
449 | be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks | |
450 | in the writing task's threadgroup. | |
451 | ||
bb6405ea EM |
452 | Note: Since every task is always a member of exactly one cgroup in each |
453 | mounted hierarchy, to remove a task from its current cgroup you must | |
454 | move it into a new cgroup (possibly the root cgroup) by writing to the | |
455 | new cgroup's tasks file. | |
456 | ||
457 | Note: If the ns cgroup is active, moving a process to another cgroup can | |
458 | fail. | |
459 | ||
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460 | 2.3 Mounting hierarchies by name |
461 | -------------------------------- | |
462 | ||
463 | Passing the name=<x> option when mounting a cgroups hierarchy | |
464 | associates the given name with the hierarchy. This can be used when | |
465 | mounting a pre-existing hierarchy, in order to refer to it by name | |
466 | rather than by its set of active subsystems. Each hierarchy is either | |
467 | nameless, or has a unique name. | |
468 | ||
469 | The name should match [\w.-]+ | |
470 | ||
471 | When passing a name=<x> option for a new hierarchy, you need to | |
472 | specify subsystems manually; the legacy behaviour of mounting all | |
473 | subsystems when none are explicitly specified is not supported when | |
474 | you give a subsystem a name. | |
475 | ||
476 | The name of the subsystem appears as part of the hierarchy description | |
477 | in /proc/mounts and /proc/<pid>/cgroups. | |
478 | ||
0dea1168 KS |
479 | 2.4 Notification API |
480 | -------------------- | |
481 | ||
482 | There is mechanism which allows to get notifications about changing | |
483 | status of a cgroup. | |
484 | ||
485 | To register new notification handler you need: | |
486 | - create a file descriptor for event notification using eventfd(2); | |
487 | - open a control file to be monitored (e.g. memory.usage_in_bytes); | |
488 | - write "<event_fd> <control_fd> <args>" to cgroup.event_control. | |
489 | Interpretation of args is defined by control file implementation; | |
490 | ||
491 | eventfd will be woken up by control file implementation or when the | |
492 | cgroup is removed. | |
493 | ||
494 | To unregister notification handler just close eventfd. | |
495 | ||
496 | NOTE: Support of notifications should be implemented for the control | |
497 | file. See documentation for the subsystem. | |
c6d57f33 | 498 | |
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499 | 3. Kernel API |
500 | ============= | |
501 | ||
502 | 3.1 Overview | |
503 | ------------ | |
504 | ||
505 | Each kernel subsystem that wants to hook into the generic cgroup | |
506 | system needs to create a cgroup_subsys object. This contains | |
507 | various methods, which are callbacks from the cgroup system, along | |
508 | with a subsystem id which will be assigned by the cgroup system. | |
509 | ||
510 | Other fields in the cgroup_subsys object include: | |
511 | ||
512 | - subsys_id: a unique array index for the subsystem, indicating which | |
d19e0583 | 513 | entry in cgroup->subsys[] this subsystem should be managing. |
ddbcc7e8 | 514 | |
d19e0583 LZ |
515 | - name: should be initialized to a unique subsystem name. Should be |
516 | no longer than MAX_CGROUP_TYPE_NAMELEN. | |
ddbcc7e8 | 517 | |
d19e0583 LZ |
518 | - early_init: indicate if the subsystem needs early initialization |
519 | at system boot. | |
ddbcc7e8 PM |
520 | |
521 | Each cgroup object created by the system has an array of pointers, | |
522 | indexed by subsystem id; this pointer is entirely managed by the | |
523 | subsystem; the generic cgroup code will never touch this pointer. | |
524 | ||
525 | 3.2 Synchronization | |
526 | ------------------- | |
527 | ||
528 | There is a global mutex, cgroup_mutex, used by the cgroup | |
529 | system. This should be taken by anything that wants to modify a | |
530 | cgroup. It may also be taken to prevent cgroups from being | |
531 | modified, but more specific locks may be more appropriate in that | |
532 | situation. | |
533 | ||
534 | See kernel/cgroup.c for more details. | |
535 | ||
536 | Subsystems can take/release the cgroup_mutex via the functions | |
ddbcc7e8 PM |
537 | cgroup_lock()/cgroup_unlock(). |
538 | ||
539 | Accessing a task's cgroup pointer may be done in the following ways: | |
540 | - while holding cgroup_mutex | |
541 | - while holding the task's alloc_lock (via task_lock()) | |
542 | - inside an rcu_read_lock() section via rcu_dereference() | |
543 | ||
544 | 3.3 Subsystem API | |
d19e0583 | 545 | ----------------- |
ddbcc7e8 PM |
546 | |
547 | Each subsystem should: | |
548 | ||
549 | - add an entry in linux/cgroup_subsys.h | |
550 | - define a cgroup_subsys object called <name>_subsys | |
551 | ||
e6a1105b | 552 | If a subsystem can be compiled as a module, it should also have in its |
cf5d5941 BB |
553 | module initcall a call to cgroup_load_subsys(), and in its exitcall a |
554 | call to cgroup_unload_subsys(). It should also set its_subsys.module = | |
555 | THIS_MODULE in its .c file. | |
e6a1105b | 556 | |
ddbcc7e8 PM |
557 | Each subsystem may export the following methods. The only mandatory |
558 | methods are create/destroy. Any others that are null are presumed to | |
559 | be successful no-ops. | |
560 | ||
d19e0583 LZ |
561 | struct cgroup_subsys_state *create(struct cgroup_subsys *ss, |
562 | struct cgroup *cgrp) | |
8dc4f3e1 | 563 | (cgroup_mutex held by caller) |
ddbcc7e8 PM |
564 | |
565 | Called to create a subsystem state object for a cgroup. The | |
566 | subsystem should allocate its subsystem state object for the passed | |
567 | cgroup, returning a pointer to the new object on success or a | |
568 | negative error code. On success, the subsystem pointer should point to | |
569 | a structure of type cgroup_subsys_state (typically embedded in a | |
570 | larger subsystem-specific object), which will be initialized by the | |
571 | cgroup system. Note that this will be called at initialization to | |
572 | create the root subsystem state for this subsystem; this case can be | |
573 | identified by the passed cgroup object having a NULL parent (since | |
574 | it's the root of the hierarchy) and may be an appropriate place for | |
575 | initialization code. | |
576 | ||
d19e0583 | 577 | void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp) |
8dc4f3e1 | 578 | (cgroup_mutex held by caller) |
ddbcc7e8 | 579 | |
8dc4f3e1 PM |
580 | The cgroup system is about to destroy the passed cgroup; the subsystem |
581 | should do any necessary cleanup and free its subsystem state | |
582 | object. By the time this method is called, the cgroup has already been | |
583 | unlinked from the file system and from the child list of its parent; | |
584 | cgroup->parent is still valid. (Note - can also be called for a | |
585 | newly-created cgroup if an error occurs after this subsystem's | |
586 | create() method has been called for the new cgroup). | |
ddbcc7e8 | 587 | |
ec64f515 | 588 | int pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp); |
d19e0583 LZ |
589 | |
590 | Called before checking the reference count on each subsystem. This may | |
591 | be useful for subsystems which have some extra references even if | |
ec64f515 KH |
592 | there are not tasks in the cgroup. If pre_destroy() returns error code, |
593 | rmdir() will fail with it. From this behavior, pre_destroy() can be | |
594 | called multiple times against a cgroup. | |
d19e0583 LZ |
595 | |
596 | int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp, | |
f780bdb7 | 597 | struct task_struct *task) |
8dc4f3e1 | 598 | (cgroup_mutex held by caller) |
ddbcc7e8 PM |
599 | |
600 | Called prior to moving a task into a cgroup; if the subsystem | |
601 | returns an error, this will abort the attach operation. If a NULL | |
602 | task is passed, then a successful result indicates that *any* | |
603 | unspecified task can be moved into the cgroup. Note that this isn't | |
604 | called on a fork. If this method returns 0 (success) then this should | |
2468c723 | 605 | remain valid while the caller holds cgroup_mutex and it is ensured that either |
f780bdb7 BB |
606 | attach() or cancel_attach() will be called in future. |
607 | ||
608 | int can_attach_task(struct cgroup *cgrp, struct task_struct *tsk); | |
609 | (cgroup_mutex held by caller) | |
610 | ||
611 | As can_attach, but for operations that must be run once per task to be | |
612 | attached (possibly many when using cgroup_attach_proc). Called after | |
613 | can_attach. | |
ddbcc7e8 | 614 | |
2468c723 DN |
615 | void cancel_attach(struct cgroup_subsys *ss, struct cgroup *cgrp, |
616 | struct task_struct *task, bool threadgroup) | |
617 | (cgroup_mutex held by caller) | |
618 | ||
619 | Called when a task attach operation has failed after can_attach() has succeeded. | |
620 | A subsystem whose can_attach() has some side-effects should provide this | |
88393161 | 621 | function, so that the subsystem can implement a rollback. If not, not necessary. |
2468c723 DN |
622 | This will be called only about subsystems whose can_attach() operation have |
623 | succeeded. | |
624 | ||
f780bdb7 BB |
625 | void pre_attach(struct cgroup *cgrp); |
626 | (cgroup_mutex held by caller) | |
627 | ||
628 | For any non-per-thread attachment work that needs to happen before | |
629 | attach_task. Needed by cpuset. | |
630 | ||
d19e0583 | 631 | void attach(struct cgroup_subsys *ss, struct cgroup *cgrp, |
f780bdb7 | 632 | struct cgroup *old_cgrp, struct task_struct *task) |
18e7f1f0 | 633 | (cgroup_mutex held by caller) |
ddbcc7e8 PM |
634 | |
635 | Called after the task has been attached to the cgroup, to allow any | |
636 | post-attachment activity that requires memory allocations or blocking. | |
f780bdb7 BB |
637 | |
638 | void attach_task(struct cgroup *cgrp, struct task_struct *tsk); | |
639 | (cgroup_mutex held by caller) | |
640 | ||
641 | As attach, but for operations that must be run once per task to be attached, | |
642 | like can_attach_task. Called before attach. Currently does not support any | |
be367d09 | 643 | subsystem that might need the old_cgrp for every thread in the group. |
ddbcc7e8 PM |
644 | |
645 | void fork(struct cgroup_subsy *ss, struct task_struct *task) | |
ddbcc7e8 | 646 | |
e8d55fde | 647 | Called when a task is forked into a cgroup. |
ddbcc7e8 PM |
648 | |
649 | void exit(struct cgroup_subsys *ss, struct task_struct *task) | |
ddbcc7e8 | 650 | |
d19e0583 | 651 | Called during task exit. |
ddbcc7e8 | 652 | |
d19e0583 | 653 | int populate(struct cgroup_subsys *ss, struct cgroup *cgrp) |
18e7f1f0 | 654 | (cgroup_mutex held by caller) |
ddbcc7e8 PM |
655 | |
656 | Called after creation of a cgroup to allow a subsystem to populate | |
657 | the cgroup directory with file entries. The subsystem should make | |
658 | calls to cgroup_add_file() with objects of type cftype (see | |
659 | include/linux/cgroup.h for details). Note that although this | |
660 | method can return an error code, the error code is currently not | |
661 | always handled well. | |
662 | ||
d19e0583 | 663 | void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp) |
18e7f1f0 | 664 | (cgroup_mutex held by caller) |
697f4161 | 665 | |
a77aea92 | 666 | Called during cgroup_create() to do any parameter |
697f4161 PM |
667 | initialization which might be required before a task could attach. For |
668 | example in cpusets, no task may attach before 'cpus' and 'mems' are set | |
669 | up. | |
670 | ||
ddbcc7e8 | 671 | void bind(struct cgroup_subsys *ss, struct cgroup *root) |
999cd8a4 | 672 | (cgroup_mutex and ss->hierarchy_mutex held by caller) |
ddbcc7e8 PM |
673 | |
674 | Called when a cgroup subsystem is rebound to a different hierarchy | |
675 | and root cgroup. Currently this will only involve movement between | |
676 | the default hierarchy (which never has sub-cgroups) and a hierarchy | |
677 | that is being created/destroyed (and hence has no sub-cgroups). | |
678 | ||
679 | 4. Questions | |
680 | ============ | |
681 | ||
682 | Q: what's up with this '/bin/echo' ? | |
683 | A: bash's builtin 'echo' command does not check calls to write() against | |
684 | errors. If you use it in the cgroup file system, you won't be | |
685 | able to tell whether a command succeeded or failed. | |
686 | ||
687 | Q: When I attach processes, only the first of the line gets really attached ! | |
688 | A: We can only return one error code per call to write(). So you should also | |
689 | put only ONE pid. | |
690 |