Merge branch 'drm-core-next' of git://people.freedesktop.org/~airlied/linux
[linux-2.6-block.git] / Documentation / cgroups / cgroups.txt
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1 CGROUPS
2 -------
3
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4Written by Paul Menage <menage@google.com> based on
5Documentation/cgroups/cpusets.txt
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6
7Original copyright statements from cpusets.txt:
8Portions Copyright (C) 2004 BULL SA.
9Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
10Modified by Paul Jackson <pj@sgi.com>
11Modified by Christoph Lameter <clameter@sgi.com>
12
13CONTENTS:
14=========
15
161. 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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232. 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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283. Kernel API
29 3.1 Overview
30 3.2 Synchronization
31 3.3 Subsystem API
324. Questions
33
341. Control Groups
d19e0583 35=================
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36
371.1 What are cgroups ?
38----------------------
39
40Control Groups provide a mechanism for aggregating/partitioning sets of
41tasks, and all their future children, into hierarchical groups with
42specialized behaviour.
43
44Definitions:
45
46A *cgroup* associates a set of tasks with a set of parameters for one
47or more subsystems.
48
49A *subsystem* is a module that makes use of the task grouping
50facilities provided by cgroups to treat groups of tasks in
51particular ways. A subsystem is typically a "resource controller" that
52schedules a resource or applies per-cgroup limits, but it may be
53anything that wants to act on a group of processes, e.g. a
54virtualization subsystem.
55
56A *hierarchy* is a set of cgroups arranged in a tree, such that
57every task in the system is in exactly one of the cgroups in the
58hierarchy, and a set of subsystems; each subsystem has system-specific
59state attached to each cgroup in the hierarchy. Each hierarchy has
60an instance of the cgroup virtual filesystem associated with it.
61
caa790ba 62At any one time there may be multiple active hierarchies of task
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63cgroups. Each hierarchy is a partition of all tasks in the system.
64
65User level code may create and destroy cgroups by name in an
66instance of the cgroup virtual file system, specify and query to
67which cgroup a task is assigned, and list the task pids assigned to
68a cgroup. Those creations and assignments only affect the hierarchy
69associated with that instance of the cgroup file system.
70
71On their own, the only use for cgroups is for simple job
72tracking. The intention is that other subsystems hook into the generic
73cgroup support to provide new attributes for cgroups, such as
74accounting/limiting the resources which processes in a cgroup can
45ce80fb 75access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allows
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76you to associate a set of CPUs and a set of memory nodes with the
77tasks in each cgroup.
78
791.2 Why are cgroups needed ?
80----------------------------
81
82There are multiple efforts to provide process aggregations in the
83Linux kernel, mainly for resource tracking purposes. Such efforts
84include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server
85namespaces. These all require the basic notion of a
86grouping/partitioning of processes, with newly forked processes ending
87in the same group (cgroup) as their parent process.
88
89The kernel cgroup patch provides the minimum essential kernel
90mechanisms required to efficiently implement such groups. It has
91minimal impact on the system fast paths, and provides hooks for
92specific subsystems such as cpusets to provide additional behaviour as
93desired.
94
95Multiple hierarchy support is provided to allow for situations where
96the division of tasks into cgroups is distinctly different for
97different subsystems - having parallel hierarchies allows each
98hierarchy to be a natural division of tasks, without having to handle
99complex combinations of tasks that would be present if several
100unrelated subsystems needed to be forced into the same tree of
101cgroups.
102
103At one extreme, each resource controller or subsystem could be in a
104separate hierarchy; at the other extreme, all subsystems
105would be attached to the same hierarchy.
106
107As an example of a scenario (originally proposed by vatsa@in.ibm.com)
108that can benefit from multiple hierarchies, consider a large
109university server with various users - students, professors, system
110tasks etc. The resource planning for this server could be along the
111following 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%)
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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 130Browsers like Firefox/Lynx go into the WWW network class, while (k)nfsd go
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131into NFS network class.
132
caa790ba 133At the same time Firefox/Lynx will share an appropriate CPU/Memory class
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134depending on who launched it (prof/student).
135
136With the ability to classify tasks differently for different resources
137(by putting those resource subsystems in different hierarchies) then
138the admin can easily set up a script which receives exec notifications
139and 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
143With only a single hierarchy, he now would potentially have to create
144a separate cgroup for every browser launched and associate it with
67de0162 145appropriate network and other resource class. This may lead to
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146proliferation of such cgroups.
147
148Also lets say that the administrator would like to give enhanced network
149access temporarily to a student's browser (since it is night and the user
d19e0583 150wants to do online gaming :)) OR give one of the students simulation
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151apps enhanced CPU power,
152
d19e0583 153With ability to write pids directly to resource classes, it's just a
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154matter 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
160Without this ability, he would have to split the cgroup into
161multiple separate ones and then associate the new cgroups with the
162new resource classes.
163
164
165
1661.3 How are cgroups implemented ?
167---------------------------------
168
169Control 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
192The implementation of cgroups requires a few, simple hooks
193into 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
200In addition a new file system, of type "cgroup" may be mounted, to
201enable browsing and modifying the cgroups presently known to the
202kernel. When mounting a cgroup hierarchy, you may specify a
203comma-separated list of subsystems to mount as the filesystem mount
204options. By default, mounting the cgroup filesystem attempts to
205mount a hierarchy containing all registered subsystems.
206
207If an active hierarchy with exactly the same set of subsystems already
208exists, it will be reused for the new mount. If no existing hierarchy
209matches, and any of the requested subsystems are in use in an existing
210hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
211is activated, associated with the requested subsystems.
212
213It's not currently possible to bind a new subsystem to an active
214cgroup hierarchy, or to unbind a subsystem from an active cgroup
215hierarchy. This may be possible in future, but is fraught with nasty
216error-recovery issues.
217
218When a cgroup filesystem is unmounted, if there are any
219child cgroups created below the top-level cgroup, that hierarchy
220will remain active even though unmounted; if there are no
221child cgroups then the hierarchy will be deactivated.
222
223No new system calls are added for cgroups - all support for
224querying and modifying cgroups is via this cgroup file system.
225
226Each task under /proc has an added file named 'cgroup' displaying,
227for each active hierarchy, the subsystem names and the cgroup name
228as the path relative to the root of the cgroup file system.
229
230Each cgroup is represented by a directory in the cgroup file system
231containing the following files describing that cgroup:
232
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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
245Other subsystems such as cpusets may add additional files in each
d19e0583 246cgroup dir.
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247
248New cgroups are created using the mkdir system call or shell
249command. The properties of a cgroup, such as its flags, are
250modified by writing to the appropriate file in that cgroups
251directory, as listed above.
252
253The named hierarchical structure of nested cgroups allows partitioning
254a large system into nested, dynamically changeable, "soft-partitions".
255
256The attachment of each task, automatically inherited at fork by any
257children of that task, to a cgroup allows organizing the work load
258on a system into related sets of tasks. A task may be re-attached to
259any other cgroup, if allowed by the permissions on the necessary
260cgroup file system directories.
261
262When a task is moved from one cgroup to another, it gets a new
263css_set pointer - if there's an already existing css_set with the
264desired collection of cgroups then that group is reused, else a new
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265css_set is allocated. The appropriate existing css_set is located by
266looking into a hash table.
ddbcc7e8 267
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268To allow access from a cgroup to the css_sets (and hence tasks)
269that comprise it, a set of cg_cgroup_link objects form a lattice;
270each cg_cgroup_link is linked into a list of cg_cgroup_links for
d19e0583 271a single cgroup on its cgrp_link_list field, and a list of
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272cg_cgroup_links for a single css_set on its cg_link_list.
273
274Thus the set of tasks in a cgroup can be listed by iterating over
275each css_set that references the cgroup, and sub-iterating over
276each css_set's task set.
277
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278The use of a Linux virtual file system (vfs) to represent the
279cgroup hierarchy provides for a familiar permission and name space
280for cgroups, with a minimum of additional kernel code.
281
2821.4 What does notify_on_release do ?
283------------------------------------
284
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285If the notify_on_release flag is enabled (1) in a cgroup, then
286whenever the last task in the cgroup leaves (exits or attaches to
287some other cgroup) and the last child cgroup of that cgroup
288is removed, then the kernel runs the command specified by the contents
289of the "release_agent" file in that hierarchy's root directory,
290supplying the pathname (relative to the mount point of the cgroup
291file system) of the abandoned cgroup. This enables automatic
292removal of abandoned cgroups. The default value of
293notify_on_release in the root cgroup at system boot is disabled
294(0). The default value of other cgroups at creation is the current
295value of their parents notify_on_release setting. The default value of
296a cgroup hierarchy's release_agent path is empty.
297
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2981.5 What does clone_children do ?
299---------------------------------
300
301If the clone_children flag is enabled (1) in a cgroup, then all
302cgroups created beneath will call the post_clone callbacks for each
303subsystem of the newly created cgroup. Usually when this callback is
304implemented for a subsystem, it copies the values of the parent
305subsystem, this is the case for the cpuset.
306
3071.6 How do I use cgroups ?
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308--------------------------
309
310To start a new job that is to be contained within a cgroup, using
311the "cpuset" cgroup subsystem, the steps are something like:
312
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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
323For example, the following sequence of commands will setup a cgroup
324named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
325and then start a subshell 'sh' in that cgroup:
326
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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
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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
3412. Usage Examples and Syntax
342============================
343
3442.1 Basic Usage
345---------------
346
347Creating, modifying, using the cgroups can be done through the cgroup
348virtual filesystem.
349
caa790ba 350To mount a cgroup hierarchy with all available subsystems, type:
f6e07d38 351# mount -t cgroup xxx /sys/fs/cgroup
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352
353The "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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356Note: Some subsystems do not work without some user input first. For instance,
357if cpusets are enabled the user will have to populate the cpus and mems files
358for each new cgroup created before that group can be used.
359
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360As explained in section `1.2 Why are cgroups needed?' you should create
361different hierarchies of cgroups for each single resource or group of
362resources you want to control. Therefore, you should mount a tmpfs on
363/sys/fs/cgroup and create directories for each cgroup resource or resource
364group.
365
366# mount -t tmpfs cgroup_root /sys/fs/cgroup
367# mkdir /sys/fs/cgroup/rg1
368
595f4b69 369To mount a cgroup hierarchy with just the cpuset and memory
ddbcc7e8 370subsystems, type:
f6e07d38 371# mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1
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372
373To change the set of subsystems bound to a mounted hierarchy, just
374remount with different options:
f6e07d38 375# mount -o remount,cpuset,blkio hier1 /sys/fs/cgroup/rg1
ddbcc7e8 376
1bdcd78e 377Now memory is removed from the hierarchy and blkio is added.
b6719ec1 378
1bdcd78e 379Note this will add blkio to the hierarchy but won't remove memory or
b6719ec1 380cpuset, 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
383To 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
387Note that specifying 'release_agent' more than once will return failure.
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388
389Note that changing the set of subsystems is currently only supported
390when the hierarchy consists of a single (root) cgroup. Supporting
391the ability to arbitrarily bind/unbind subsystems from an existing
392cgroup hierarchy is intended to be implemented in the future.
393
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394Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
395tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
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396is the cgroup that holds the whole system.
397
b6719ec1 398If 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
401It can also be changed via remount.
402
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403If 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
407Now you want to do something with this cgroup.
408# cd my_cgroup
409
410In this directory you can find several files:
411# ls
7823da36 412cgroup.procs notify_on_release tasks
d19e0583 413(plus whatever files added by the attached subsystems)
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414
415Now attach your shell to this cgroup:
416# /bin/echo $$ > tasks
417
418You can also create cgroups inside your cgroup by using mkdir in this
419directory.
420# mkdir my_sub_cs
421
422To remove a cgroup, just use rmdir:
423# rmdir my_sub_cs
424
425This will fail if the cgroup is in use (has cgroups inside, or
426has processes attached, or is held alive by other subsystem-specific
427reference).
428
4292.2 Attaching processes
430-----------------------
431
432# /bin/echo PID > tasks
433
434Note that it is PID, not PIDs. You can only attach ONE task at a time.
435If 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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442You can attach the current shell task by echoing 0:
443
444# echo 0 > tasks
445
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446You can use the cgroup.procs file instead of the tasks file to move all
447threads in a threadgroup at once. Echoing the pid of any task in a
448threadgroup to cgroup.procs causes all tasks in that threadgroup to be
449be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
450in the writing task's threadgroup.
451
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452Note: Since every task is always a member of exactly one cgroup in each
453mounted hierarchy, to remove a task from its current cgroup you must
454move it into a new cgroup (possibly the root cgroup) by writing to the
455new cgroup's tasks file.
456
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457Note: Due to some restrictions enforced by some cgroup subsystems, moving
458a process to another cgroup can fail.
bb6405ea 459
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4602.3 Mounting hierarchies by name
461--------------------------------
462
463Passing the name=<x> option when mounting a cgroups hierarchy
464associates the given name with the hierarchy. This can be used when
465mounting a pre-existing hierarchy, in order to refer to it by name
466rather than by its set of active subsystems. Each hierarchy is either
467nameless, or has a unique name.
468
469The name should match [\w.-]+
470
471When passing a name=<x> option for a new hierarchy, you need to
472specify subsystems manually; the legacy behaviour of mounting all
473subsystems when none are explicitly specified is not supported when
474you give a subsystem a name.
475
476The name of the subsystem appears as part of the hierarchy description
477in /proc/mounts and /proc/<pid>/cgroups.
478
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4792.4 Notification API
480--------------------
481
482There is mechanism which allows to get notifications about changing
483status of a cgroup.
484
485To 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
491eventfd will be woken up by control file implementation or when the
492cgroup is removed.
493
494To unregister notification handler just close eventfd.
495
496NOTE: Support of notifications should be implemented for the control
497file. See documentation for the subsystem.
c6d57f33 498
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4993. Kernel API
500=============
501
5023.1 Overview
503------------
504
505Each kernel subsystem that wants to hook into the generic cgroup
506system needs to create a cgroup_subsys object. This contains
507various methods, which are callbacks from the cgroup system, along
508with a subsystem id which will be assigned by the cgroup system.
509
510Other 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
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515- name: should be initialized to a unique subsystem name. Should be
516 no longer than MAX_CGROUP_TYPE_NAMELEN.
ddbcc7e8 517
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518- early_init: indicate if the subsystem needs early initialization
519 at system boot.
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520
521Each cgroup object created by the system has an array of pointers,
522indexed by subsystem id; this pointer is entirely managed by the
523subsystem; the generic cgroup code will never touch this pointer.
524
5253.2 Synchronization
526-------------------
527
528There is a global mutex, cgroup_mutex, used by the cgroup
529system. This should be taken by anything that wants to modify a
530cgroup. It may also be taken to prevent cgroups from being
531modified, but more specific locks may be more appropriate in that
532situation.
533
534See kernel/cgroup.c for more details.
535
536Subsystems can take/release the cgroup_mutex via the functions
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537cgroup_lock()/cgroup_unlock().
538
539Accessing 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
5443.3 Subsystem API
d19e0583 545-----------------
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546
547Each subsystem should:
548
549- add an entry in linux/cgroup_subsys.h
550- define a cgroup_subsys object called <name>_subsys
551
e6a1105b 552If a subsystem can be compiled as a module, it should also have in its
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553module initcall a call to cgroup_load_subsys(), and in its exitcall a
554call to cgroup_unload_subsys(). It should also set its_subsys.module =
555THIS_MODULE in its .c file.
e6a1105b 556
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557Each subsystem may export the following methods. The only mandatory
558methods are create/destroy. Any others that are null are presumed to
559be successful no-ops.
560
761b3ef5 561struct cgroup_subsys_state *create(struct cgroup *cgrp)
8dc4f3e1 562(cgroup_mutex held by caller)
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563
564Called to create a subsystem state object for a cgroup. The
565subsystem should allocate its subsystem state object for the passed
566cgroup, returning a pointer to the new object on success or a
567negative error code. On success, the subsystem pointer should point to
568a structure of type cgroup_subsys_state (typically embedded in a
569larger subsystem-specific object), which will be initialized by the
570cgroup system. Note that this will be called at initialization to
571create the root subsystem state for this subsystem; this case can be
572identified by the passed cgroup object having a NULL parent (since
573it's the root of the hierarchy) and may be an appropriate place for
574initialization code.
575
761b3ef5 576void destroy(struct cgroup *cgrp)
8dc4f3e1 577(cgroup_mutex held by caller)
ddbcc7e8 578
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579The cgroup system is about to destroy the passed cgroup; the subsystem
580should do any necessary cleanup and free its subsystem state
581object. By the time this method is called, the cgroup has already been
582unlinked from the file system and from the child list of its parent;
583cgroup->parent is still valid. (Note - can also be called for a
584newly-created cgroup if an error occurs after this subsystem's
585create() method has been called for the new cgroup).
ddbcc7e8 586
761b3ef5 587int pre_destroy(struct cgroup *cgrp);
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588
589Called before checking the reference count on each subsystem. This may
590be useful for subsystems which have some extra references even if
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591there are not tasks in the cgroup. If pre_destroy() returns error code,
592rmdir() will fail with it. From this behavior, pre_destroy() can be
593called multiple times against a cgroup.
d19e0583 594
761b3ef5 595int can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
8dc4f3e1 596(cgroup_mutex held by caller)
ddbcc7e8 597
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598Called prior to moving one or more tasks into a cgroup; if the
599subsystem returns an error, this will abort the attach operation.
600@tset contains the tasks to be attached and is guaranteed to have at
601least one task in it.
602
603If there are multiple tasks in the taskset, then:
604 - it's guaranteed that all are from the same thread group
605 - @tset contains all tasks from the thread group whether or not
606 they're switching cgroups
607 - the first task is the leader
608
609Each @tset entry also contains the task's old cgroup and tasks which
610aren't switching cgroup can be skipped easily using the
611cgroup_taskset_for_each() iterator. Note that this isn't called on a
612fork. If this method returns 0 (success) then this should remain valid
613while the caller holds cgroup_mutex and it is ensured that either
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614attach() or cancel_attach() will be called in future.
615
761b3ef5 616void cancel_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
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617(cgroup_mutex held by caller)
618
619Called when a task attach operation has failed after can_attach() has succeeded.
620A subsystem whose can_attach() has some side-effects should provide this
88393161 621function, so that the subsystem can implement a rollback. If not, not necessary.
2468c723 622This will be called only about subsystems whose can_attach() operation have
2f7ee569 623succeeded. The parameters are identical to can_attach().
2468c723 624
761b3ef5 625void attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
18e7f1f0 626(cgroup_mutex held by caller)
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627
628Called after the task has been attached to the cgroup, to allow any
629post-attachment activity that requires memory allocations or blocking.
2f7ee569 630The parameters are identical to can_attach().
f780bdb7 631
761b3ef5 632void fork(struct task_struct *task)
ddbcc7e8 633
e8d55fde 634Called when a task is forked into a cgroup.
ddbcc7e8 635
761b3ef5 636void exit(struct task_struct *task)
ddbcc7e8 637
d19e0583 638Called during task exit.
ddbcc7e8 639
761b3ef5 640int populate(struct cgroup *cgrp)
18e7f1f0 641(cgroup_mutex held by caller)
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642
643Called after creation of a cgroup to allow a subsystem to populate
644the cgroup directory with file entries. The subsystem should make
645calls to cgroup_add_file() with objects of type cftype (see
646include/linux/cgroup.h for details). Note that although this
647method can return an error code, the error code is currently not
648always handled well.
649
761b3ef5 650void post_clone(struct cgroup *cgrp)
18e7f1f0 651(cgroup_mutex held by caller)
697f4161 652
a77aea92 653Called during cgroup_create() to do any parameter
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654initialization which might be required before a task could attach. For
655example in cpusets, no task may attach before 'cpus' and 'mems' are set
656up.
657
761b3ef5 658void bind(struct cgroup *root)
999cd8a4 659(cgroup_mutex and ss->hierarchy_mutex held by caller)
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660
661Called when a cgroup subsystem is rebound to a different hierarchy
662and root cgroup. Currently this will only involve movement between
663the default hierarchy (which never has sub-cgroups) and a hierarchy
664that is being created/destroyed (and hence has no sub-cgroups).
665
6664. Questions
667============
668
669Q: what's up with this '/bin/echo' ?
670A: bash's builtin 'echo' command does not check calls to write() against
671 errors. If you use it in the cgroup file system, you won't be
672 able to tell whether a command succeeded or failed.
673
674Q: When I attach processes, only the first of the line gets really attached !
675A: We can only return one error code per call to write(). So you should also
676 put only ONE pid.
677