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Paul Menageddbcc7e2007-10-18 23:39:30 -07001 CGROUPS
2 -------
3
Li Zefan45ce80f2009-01-15 13:50:59 -08004Written by Paul Menage <menage@google.com> based on
5Documentation/cgroups/cpusets.txt
Paul Menageddbcc7e2007-10-18 23:39:30 -07006
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 ?
Daniel Lezcano97978e62010-10-27 15:33:35 -070021 1.5 What does clone_children do ?
22 1.6 How do I use cgroups ?
Paul Menageddbcc7e2007-10-18 23:39:30 -0700232. Usage Examples and Syntax
24 2.1 Basic Usage
25 2.2 Attaching processes
Kirill A. Shutemov8ca712e2010-03-10 15:22:10 -080026 2.3 Mounting hierarchies by name
Paul Menageddbcc7e2007-10-18 23:39:30 -0700273. Kernel API
28 3.1 Overview
29 3.2 Synchronization
30 3.3 Subsystem API
Aristeu Rozanski19ec2562012-09-11 16:28:10 -0400314. Extended attributes usage
325. Questions
Paul Menageddbcc7e2007-10-18 23:39:30 -070033
341. Control Groups
Li Zefand19e0582008-02-23 15:24:08 -080035=================
Paul Menageddbcc7e2007-10-18 23:39:30 -070036
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
Chris Samuelcaa790b2009-01-17 00:01:18 +110062At any one time there may be multiple active hierarchies of task
Paul Menageddbcc7e2007-10-18 23:39:30 -070063cgroups. Each hierarchy is a partition of all tasks in the system.
64
Michael Kerrisk83b061f2012-09-11 13:20:20 +020065User-level code may create and destroy cgroups by name in an
Paul Menageddbcc7e2007-10-18 23:39:30 -070066instance of the cgroup virtual file system, specify and query to
Michael Kerrisk83b061f2012-09-11 13:20:20 +020067which cgroup a task is assigned, and list the task PIDs assigned to
Paul Menageddbcc7e2007-10-18 23:39:30 -070068a 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
Michael Kerrisk83b061f2012-09-11 13:20:20 +020075access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allow
Paul Menageddbcc7e2007-10-18 23:39:30 -070076you 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
Michael Kerrisk83b061f2012-09-11 13:20:20 +020083Linux kernel, mainly for resource-tracking purposes. Such efforts
Paul Menageddbcc7e2007-10-18 23:39:30 -070084include 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
Michael Kerrisk83b061f2012-09-11 13:20:20 +020087up in the same group (cgroup) as their parent process.
Paul Menageddbcc7e2007-10-18 23:39:30 -070088
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
Geunsik Lim6ad85232011-04-04 15:10:45 -0700113 CPU : "Top cpuset"
Paul Menageddbcc7e2007-10-18 23:39:30 -0700114 / \
115 CPUSet1 CPUSet2
Geunsik Lim6ad85232011-04-04 15:10:45 -0700116 | |
117 (Professors) (Students)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700118
119 In addition (system tasks) are attached to topcpuset (so
120 that they can run anywhere) with a limit of 20%
121
Geunsik Lim6ad85232011-04-04 15:10:45 -0700122 Memory : Professors (50%), Students (30%), system (20%)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700123
Geunsik Lim6ad85232011-04-04 15:10:45 -0700124 Disk : Professors (50%), Students (30%), system (20%)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700125
126 Network : WWW browsing (20%), Network File System (60%), others (20%)
127 / \
Geunsik Lim6ad85232011-04-04 15:10:45 -0700128 Professors (15%) students (5%)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700129
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200130Browsers like Firefox/Lynx go into the WWW network class, while (k)nfsd goes
131into the NFS network class.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700132
Chris Samuelcaa790b2009-01-17 00:01:18 +1100133At the same time Firefox/Lynx will share an appropriate CPU/Memory class
Paul Menageddbcc7e2007-10-18 23:39:30 -0700134depending on who launched it (prof/student).
135
136With the ability to classify tasks differently for different resources
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200137(by putting those resource subsystems in different hierarchies),
Paul Menageddbcc7e2007-10-18 23:39:30 -0700138the admin can easily set up a script which receives exec notifications
139and depending on who is launching the browser he can
140
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700141 # echo browser_pid > /sys/fs/cgroup/<restype>/<userclass>/tasks
Paul Menageddbcc7e2007-10-18 23:39:30 -0700142
143With only a single hierarchy, he now would potentially have to create
144a separate cgroup for every browser launched and associate it with
Jörg Sommer67de0162011-06-15 13:00:47 -0700145appropriate network and other resource class. This may lead to
Paul Menageddbcc7e2007-10-18 23:39:30 -0700146proliferation of such cgroups.
147
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200148Also let's say that the administrator would like to give enhanced network
Paul Menageddbcc7e2007-10-18 23:39:30 -0700149access temporarily to a student's browser (since it is night and the user
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200150wants to do online gaming :)) OR give one of the student's simulation
151apps enhanced CPU power.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700152
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200153With ability to write PIDs directly to resource classes, it's just a
154matter of:
Paul Menageddbcc7e2007-10-18 23:39:30 -0700155
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700156 # echo pid > /sys/fs/cgroup/network/<new_class>/tasks
Paul Menageddbcc7e2007-10-18 23:39:30 -0700157 (after some time)
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700158 # echo pid > /sys/fs/cgroup/network/<orig_class>/tasks
Paul Menageddbcc7e2007-10-18 23:39:30 -0700159
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200160Without this ability, the administrator would have to split the cgroup into
Paul Menageddbcc7e2007-10-18 23:39:30 -0700161multiple 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
Paul Menage817929e2007-10-18 23:39:36 -0700183 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.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700186
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200187 - A cgroup hierarchy filesystem can be mounted for browsing and
Paul Menageddbcc7e2007-10-18 23:39:30 -0700188 manipulation from user space.
189
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200190 - You can list all the tasks (by PID) attached to any cgroup.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700191
192The implementation of cgroups requires a few, simple hooks
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200193into the rest of the kernel, none in performance-critical paths:
Paul Menageddbcc7e2007-10-18 23:39:30 -0700194
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
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200200In addition, a new file system of type "cgroup" may be mounted, to
Paul Menageddbcc7e2007-10-18 23:39:30 -0700201enable 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
Tejun Heo26d5bbe2013-04-12 10:29:04 -0700213It'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.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700217
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
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200233 - 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
Paul Menage7823da32009-10-07 16:32:26 -0700235 moves the thread into this cgroup.
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200236 - cgroup.procs: list of thread group IDs in the cgroup. This list is
237 not guaranteed to be sorted or free of duplicate TGIDs, and userspace
Paul Menage7823da32009-10-07 16:32:26 -0700238 should sort/uniquify the list if this property is required.
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200239 Writing a thread group ID into this file moves all threads in that
Ben Blum74a11662011-05-26 16:25:20 -0700240 group into this cgroup.
Li Zefand19e0582008-02-23 15:24:08 -0800241 - 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)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700244
245Other subsystems such as cpusets may add additional files in each
Li Zefand19e0582008-02-23 15:24:08 -0800246cgroup dir.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700247
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
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200264desired collection of cgroups then that group is reused, otherwise a new
Li Zefanb851ee72009-02-18 14:48:14 -0800265css_set is allocated. The appropriate existing css_set is located by
266looking into a hash table.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700267
Paul Menage817929e2007-10-18 23:39:36 -0700268To 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
Li Zefand19e0582008-02-23 15:24:08 -0800271a single cgroup on its cgrp_link_list field, and a list of
Paul Menage817929e2007-10-18 23:39:36 -0700272cg_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
Paul Menageddbcc7e2007-10-18 23:39:30 -0700278The 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
Paul Menageddbcc7e2007-10-18 23:39:30 -0700285If 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
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200295value of their parents' notify_on_release settings. The default value of
Paul Menageddbcc7e2007-10-18 23:39:30 -0700296a cgroup hierarchy's release_agent path is empty.
297
Daniel Lezcano97978e62010-10-27 15:33:35 -07002981.5 What does clone_children do ?
299---------------------------------
300
Tejun Heo2260e7f2012-11-19 08:13:38 -0800301This flag only affects the cpuset controller. If the clone_children
302flag is enabled (1) in a cgroup, a new cpuset cgroup will copy its
303configuration from the parent during initialization.
Daniel Lezcano97978e62010-10-27 15:33:35 -0700304
3051.6 How do I use cgroups ?
Paul Menageddbcc7e2007-10-18 23:39:30 -0700306--------------------------
307
308To start a new job that is to be contained within a cgroup, using
309the "cpuset" cgroup subsystem, the steps are something like:
310
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700311 1) mount -t tmpfs cgroup_root /sys/fs/cgroup
312 2) mkdir /sys/fs/cgroup/cpuset
313 3) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
314 4) Create the new cgroup by doing mkdir's and write's (or echo's) in
SeongJae Park845502d2014-12-03 20:53:44 +0900315 the /sys/fs/cgroup/cpuset virtual file system.
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700316 5) Start a task that will be the "founding father" of the new job.
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200317 6) Attach that task to the new cgroup by writing its PID to the
SeongJae Park845502d2014-12-03 20:53:44 +0900318 /sys/fs/cgroup/cpuset tasks file for that cgroup.
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700319 7) fork, exec or clone the job tasks from this founding father task.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700320
321For example, the following sequence of commands will setup a cgroup
322named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
323and then start a subshell 'sh' in that cgroup:
324
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700325 mount -t tmpfs cgroup_root /sys/fs/cgroup
326 mkdir /sys/fs/cgroup/cpuset
327 mount -t cgroup cpuset -ocpuset /sys/fs/cgroup/cpuset
328 cd /sys/fs/cgroup/cpuset
Paul Menageddbcc7e2007-10-18 23:39:30 -0700329 mkdir Charlie
330 cd Charlie
Dhaval Giani0f146a72008-05-12 14:02:31 -0700331 /bin/echo 2-3 > cpuset.cpus
332 /bin/echo 1 > cpuset.mems
Paul Menageddbcc7e2007-10-18 23:39:30 -0700333 /bin/echo $$ > tasks
334 sh
335 # The subshell 'sh' is now running in cgroup Charlie
336 # The next line should display '/Charlie'
337 cat /proc/self/cgroup
338
3392. Usage Examples and Syntax
340============================
341
3422.1 Basic Usage
343---------------
344
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200345Creating, modifying, using cgroups can be done through the cgroup
Paul Menageddbcc7e2007-10-18 23:39:30 -0700346virtual filesystem.
347
Chris Samuelcaa790b2009-01-17 00:01:18 +1100348To mount a cgroup hierarchy with all available subsystems, type:
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700349# mount -t cgroup xxx /sys/fs/cgroup
Paul Menageddbcc7e2007-10-18 23:39:30 -0700350
351The "xxx" is not interpreted by the cgroup code, but will appear in
352/proc/mounts so may be any useful identifying string that you like.
353
Eric B Munsonbb6405e2011-03-15 16:12:18 -0700354Note: Some subsystems do not work without some user input first. For instance,
355if cpusets are enabled the user will have to populate the cpus and mems files
356for each new cgroup created before that group can be used.
357
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700358As explained in section `1.2 Why are cgroups needed?' you should create
359different hierarchies of cgroups for each single resource or group of
360resources you want to control. Therefore, you should mount a tmpfs on
361/sys/fs/cgroup and create directories for each cgroup resource or resource
362group.
363
364# mount -t tmpfs cgroup_root /sys/fs/cgroup
365# mkdir /sys/fs/cgroup/rg1
366
Trevor Woerner595f4b62010-05-26 14:42:35 -0700367To mount a cgroup hierarchy with just the cpuset and memory
Paul Menageddbcc7e2007-10-18 23:39:30 -0700368subsystems, type:
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700369# mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1
Paul Menageddbcc7e2007-10-18 23:39:30 -0700370
Daniel Wagner9a8054a2012-07-10 10:49:18 +0200371While remounting cgroups is currently supported, it is not recommend
372to use it. Remounting allows changing bound subsystems and
373release_agent. Rebinding is hardly useful as it only works when the
374hierarchy is empty and release_agent itself should be replaced with
375conventional fsnotify. The support for remounting will be removed in
376the future.
Li Zefanb6719ec2009-04-02 16:57:28 -0700377
378To Specify a hierarchy's release_agent:
379# mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700380 xxx /sys/fs/cgroup/rg1
Li Zefanb6719ec2009-04-02 16:57:28 -0700381
382Note that specifying 'release_agent' more than once will return failure.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700383
Tejun Heo26d5bbe2013-04-12 10:29:04 -0700384Note that changing the set of subsystems is currently only supported
385when the hierarchy consists of a single (root) cgroup. Supporting
386the ability to arbitrarily bind/unbind subsystems from an existing
387cgroup hierarchy is intended to be implemented in the future.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700388
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700389Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
390tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
Paul Menageddbcc7e2007-10-18 23:39:30 -0700391is the cgroup that holds the whole system.
392
Li Zefanb6719ec2009-04-02 16:57:28 -0700393If you want to change the value of release_agent:
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700394# echo "/sbin/new_release_agent" > /sys/fs/cgroup/rg1/release_agent
Li Zefanb6719ec2009-04-02 16:57:28 -0700395
396It can also be changed via remount.
397
Jörg Sommerf6e07d32011-06-15 12:59:45 -0700398If you want to create a new cgroup under /sys/fs/cgroup/rg1:
399# cd /sys/fs/cgroup/rg1
Paul Menageddbcc7e2007-10-18 23:39:30 -0700400# mkdir my_cgroup
401
402Now you want to do something with this cgroup.
403# cd my_cgroup
404
405In this directory you can find several files:
406# ls
Paul Menage7823da32009-10-07 16:32:26 -0700407cgroup.procs notify_on_release tasks
Li Zefand19e0582008-02-23 15:24:08 -0800408(plus whatever files added by the attached subsystems)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700409
410Now attach your shell to this cgroup:
411# /bin/echo $$ > tasks
412
413You can also create cgroups inside your cgroup by using mkdir in this
414directory.
415# mkdir my_sub_cs
416
417To remove a cgroup, just use rmdir:
418# rmdir my_sub_cs
419
420This will fail if the cgroup is in use (has cgroups inside, or
421has processes attached, or is held alive by other subsystem-specific
422reference).
423
4242.2 Attaching processes
425-----------------------
426
427# /bin/echo PID > tasks
428
429Note that it is PID, not PIDs. You can only attach ONE task at a time.
430If you have several tasks to attach, you have to do it one after another:
431
432# /bin/echo PID1 > tasks
433# /bin/echo PID2 > tasks
434 ...
435# /bin/echo PIDn > tasks
436
Li Zefanbef67c52008-07-04 09:59:55 -0700437You can attach the current shell task by echoing 0:
438
439# echo 0 > tasks
440
Ben Blum74a11662011-05-26 16:25:20 -0700441You can use the cgroup.procs file instead of the tasks file to move all
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200442threads in a threadgroup at once. Echoing the PID of any task in a
Ben Blum74a11662011-05-26 16:25:20 -0700443threadgroup to cgroup.procs causes all tasks in that threadgroup to be
Rami Rosen1ae65ae2013-04-03 20:32:04 +0300444attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
Ben Blum74a11662011-05-26 16:25:20 -0700445in the writing task's threadgroup.
446
Eric B Munsonbb6405e2011-03-15 16:12:18 -0700447Note: Since every task is always a member of exactly one cgroup in each
448mounted hierarchy, to remove a task from its current cgroup you must
449move it into a new cgroup (possibly the root cgroup) by writing to the
450new cgroup's tasks file.
451
Li Zefan5fe69d72011-11-04 11:22:05 -0700452Note: Due to some restrictions enforced by some cgroup subsystems, moving
453a process to another cgroup can fail.
Eric B Munsonbb6405e2011-03-15 16:12:18 -0700454
Paul Menagec6d57f32009-09-23 15:56:19 -07004552.3 Mounting hierarchies by name
456--------------------------------
457
458Passing the name=<x> option when mounting a cgroups hierarchy
459associates the given name with the hierarchy. This can be used when
460mounting a pre-existing hierarchy, in order to refer to it by name
461rather than by its set of active subsystems. Each hierarchy is either
462nameless, or has a unique name.
463
464The name should match [\w.-]+
465
466When passing a name=<x> option for a new hierarchy, you need to
467specify subsystems manually; the legacy behaviour of mounting all
468subsystems when none are explicitly specified is not supported when
469you give a subsystem a name.
470
471The name of the subsystem appears as part of the hierarchy description
472in /proc/mounts and /proc/<pid>/cgroups.
473
474
Paul Menageddbcc7e2007-10-18 23:39:30 -07004753. Kernel API
476=============
477
4783.1 Overview
479------------
480
481Each kernel subsystem that wants to hook into the generic cgroup
482system needs to create a cgroup_subsys object. This contains
483various methods, which are callbacks from the cgroup system, along
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200484with a subsystem ID which will be assigned by the cgroup system.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700485
486Other fields in the cgroup_subsys object include:
487
488- subsys_id: a unique array index for the subsystem, indicating which
Li Zefand19e0582008-02-23 15:24:08 -0800489 entry in cgroup->subsys[] this subsystem should be managing.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700490
Li Zefand19e0582008-02-23 15:24:08 -0800491- name: should be initialized to a unique subsystem name. Should be
492 no longer than MAX_CGROUP_TYPE_NAMELEN.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700493
Li Zefand19e0582008-02-23 15:24:08 -0800494- early_init: indicate if the subsystem needs early initialization
495 at system boot.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700496
497Each cgroup object created by the system has an array of pointers,
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200498indexed by subsystem ID; this pointer is entirely managed by the
Paul Menageddbcc7e2007-10-18 23:39:30 -0700499subsystem; the generic cgroup code will never touch this pointer.
500
5013.2 Synchronization
502-------------------
503
504There is a global mutex, cgroup_mutex, used by the cgroup
505system. This should be taken by anything that wants to modify a
506cgroup. It may also be taken to prevent cgroups from being
507modified, but more specific locks may be more appropriate in that
508situation.
509
510See kernel/cgroup.c for more details.
511
512Subsystems can take/release the cgroup_mutex via the functions
Paul Menageddbcc7e2007-10-18 23:39:30 -0700513cgroup_lock()/cgroup_unlock().
514
515Accessing a task's cgroup pointer may be done in the following ways:
516- while holding cgroup_mutex
517- while holding the task's alloc_lock (via task_lock())
518- inside an rcu_read_lock() section via rcu_dereference()
519
5203.3 Subsystem API
Li Zefand19e0582008-02-23 15:24:08 -0800521-----------------
Paul Menageddbcc7e2007-10-18 23:39:30 -0700522
523Each subsystem should:
524
525- add an entry in linux/cgroup_subsys.h
526- define a cgroup_subsys object called <name>_subsys
527
Ben Blume6a11052010-03-10 15:22:09 -0800528If a subsystem can be compiled as a module, it should also have in its
Ben Blumcf5d5942010-03-10 15:22:09 -0800529module initcall a call to cgroup_load_subsys(), and in its exitcall a
530call to cgroup_unload_subsys(). It should also set its_subsys.module =
531THIS_MODULE in its .c file.
Ben Blume6a11052010-03-10 15:22:09 -0800532
Paul Menageddbcc7e2007-10-18 23:39:30 -0700533Each subsystem may export the following methods. The only mandatory
Tejun Heo92fb9742012-11-19 08:13:38 -0800534methods are css_alloc/free. Any others that are null are presumed to
Paul Menageddbcc7e2007-10-18 23:39:30 -0700535be successful no-ops.
536
Tejun Heo92fb9742012-11-19 08:13:38 -0800537struct cgroup_subsys_state *css_alloc(struct cgroup *cgrp)
Paul Menage8dc4f3e2008-02-07 00:13:45 -0800538(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700539
Tejun Heo92fb9742012-11-19 08:13:38 -0800540Called to allocate a subsystem state object for a cgroup. The
Paul Menageddbcc7e2007-10-18 23:39:30 -0700541subsystem should allocate its subsystem state object for the passed
542cgroup, returning a pointer to the new object on success or a
Tejun Heo92fb9742012-11-19 08:13:38 -0800543ERR_PTR() value. On success, the subsystem pointer should point to
Paul Menageddbcc7e2007-10-18 23:39:30 -0700544a structure of type cgroup_subsys_state (typically embedded in a
545larger subsystem-specific object), which will be initialized by the
546cgroup system. Note that this will be called at initialization to
547create the root subsystem state for this subsystem; this case can be
548identified by the passed cgroup object having a NULL parent (since
549it's the root of the hierarchy) and may be an appropriate place for
550initialization code.
551
Tejun Heo92fb9742012-11-19 08:13:38 -0800552int css_online(struct cgroup *cgrp)
Paul Menage8dc4f3e2008-02-07 00:13:45 -0800553(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700554
Tejun Heo92fb9742012-11-19 08:13:38 -0800555Called after @cgrp successfully completed all allocations and made
556visible to cgroup_for_each_child/descendant_*() iterators. The
557subsystem may choose to fail creation by returning -errno. This
558callback can be used to implement reliable state sharing and
559propagation along the hierarchy. See the comment on
560cgroup_for_each_descendant_pre() for details.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700561
Tejun Heo92fb9742012-11-19 08:13:38 -0800562void css_offline(struct cgroup *cgrp);
Li Zefand7eeac12013-03-12 15:35:59 -0700563(cgroup_mutex held by caller)
Li Zefand19e0582008-02-23 15:24:08 -0800564
Tejun Heo92fb9742012-11-19 08:13:38 -0800565This is the counterpart of css_online() and called iff css_online()
566has succeeded on @cgrp. This signifies the beginning of the end of
567@cgrp. @cgrp is being removed and the subsystem should start dropping
568all references it's holding on @cgrp. When all references are dropped,
569cgroup removal will proceed to the next step - css_free(). After this
570callback, @cgrp should be considered dead to the subsystem.
571
572void css_free(struct cgroup *cgrp)
573(cgroup_mutex held by caller)
574
575The cgroup system is about to free @cgrp; the subsystem should free
576its subsystem state object. By the time this method is called, @cgrp
577is completely unused; @cgrp->parent is still valid. (Note - can also
578be called for a newly-created cgroup if an error occurs after this
579subsystem's create() method has been called for the new cgroup).
Li Zefand19e0582008-02-23 15:24:08 -0800580
Li Zefan761b3ef2012-01-31 13:47:36 +0800581int can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
Paul Menage8dc4f3e2008-02-07 00:13:45 -0800582(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700583
Tejun Heo2f7ee562011-12-12 18:12:21 -0800584Called prior to moving one or more tasks into a cgroup; if the
585subsystem returns an error, this will abort the attach operation.
586@tset contains the tasks to be attached and is guaranteed to have at
587least one task in it.
588
589If there are multiple tasks in the taskset, then:
590 - it's guaranteed that all are from the same thread group
591 - @tset contains all tasks from the thread group whether or not
592 they're switching cgroups
593 - the first task is the leader
594
595Each @tset entry also contains the task's old cgroup and tasks which
596aren't switching cgroup can be skipped easily using the
597cgroup_taskset_for_each() iterator. Note that this isn't called on a
598fork. If this method returns 0 (success) then this should remain valid
599while the caller holds cgroup_mutex and it is ensured that either
Ben Blumf780bdb2011-05-26 16:25:19 -0700600attach() or cancel_attach() will be called in future.
601
Tejun Heob4536f0ca2014-07-08 18:02:57 -0400602void css_reset(struct cgroup_subsys_state *css)
603(cgroup_mutex held by caller)
604
605An optional operation which should restore @css's configuration to the
606initial state. This is currently only used on the unified hierarchy
607when a subsystem is disabled on a cgroup through
608"cgroup.subtree_control" but should remain enabled because other
609subsystems depend on it. cgroup core makes such a css invisible by
610removing the associated interface files and invokes this callback so
611that the hidden subsystem can return to the initial neutral state.
612This prevents unexpected resource control from a hidden css and
613ensures that the configuration is in the initial state when it is made
614visible again later.
615
Li Zefan761b3ef2012-01-31 13:47:36 +0800616void cancel_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
Daisuke Nishimura2468c722010-03-10 15:22:03 -0800617(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
Thomas Weber88393162010-03-16 11:47:56 +0100621function, so that the subsystem can implement a rollback. If not, not necessary.
Daisuke Nishimura2468c722010-03-10 15:22:03 -0800622This will be called only about subsystems whose can_attach() operation have
Tejun Heo2f7ee562011-12-12 18:12:21 -0800623succeeded. The parameters are identical to can_attach().
Daisuke Nishimura2468c722010-03-10 15:22:03 -0800624
Li Zefan761b3ef2012-01-31 13:47:36 +0800625void attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
Li Zefan18e7f1f2009-01-07 18:07:32 -0800626(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700627
628Called after the task has been attached to the cgroup, to allow any
629post-attachment activity that requires memory allocations or blocking.
Tejun Heo2f7ee562011-12-12 18:12:21 -0800630The parameters are identical to can_attach().
Ben Blumf780bdb2011-05-26 16:25:19 -0700631
Li Zefan761b3ef2012-01-31 13:47:36 +0800632void fork(struct task_struct *task)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700633
Li Zefane8d55fd2008-04-29 01:00:13 -0700634Called when a task is forked into a cgroup.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700635
Li Zefan761b3ef2012-01-31 13:47:36 +0800636void exit(struct task_struct *task)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700637
Li Zefand19e0582008-02-23 15:24:08 -0800638Called during task exit.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700639
Tejun Heo26d5bbe2013-04-12 10:29:04 -0700640void bind(struct cgroup *root)
641(cgroup_mutex held by caller)
642
643Called when a cgroup subsystem is rebound to a different hierarchy
644and root cgroup. Currently this will only involve movement between
645the default hierarchy (which never has sub-cgroups) and a hierarchy
646that is being created/destroyed (and hence has no sub-cgroups).
Paul Menageddbcc7e2007-10-18 23:39:30 -0700647
Aristeu Rozanski19ec2562012-09-11 16:28:10 -04006484. Extended attribute usage
649===========================
650
651cgroup filesystem supports certain types of extended attributes in its
652directories and files. The current supported types are:
653 - Trusted (XATTR_TRUSTED)
654 - Security (XATTR_SECURITY)
655
656Both require CAP_SYS_ADMIN capability to set.
657
658Like in tmpfs, the extended attributes in cgroup filesystem are stored
659using kernel memory and it's advised to keep the usage at minimum. This
660is the reason why user defined extended attributes are not supported, since
661any user can do it and there's no limit in the value size.
662
663The current known users for this feature are SELinux to limit cgroup usage
664in containers and systemd for assorted meta data like main PID in a cgroup
665(systemd creates a cgroup per service).
666
6675. Questions
Paul Menageddbcc7e2007-10-18 23:39:30 -0700668============
669
670Q: what's up with this '/bin/echo' ?
671A: bash's builtin 'echo' command does not check calls to write() against
672 errors. If you use it in the cgroup file system, you won't be
673 able to tell whether a command succeeded or failed.
674
675Q: When I attach processes, only the first of the line gets really attached !
676A: We can only return one error code per call to write(). So you should also
Michael Kerrisk83b061f2012-09-11 13:20:20 +0200677 put only ONE PID.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700678