blob: 4ea852345a474aa802335a964accb4ab78731822 [file] [log] [blame]
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 ?
21 1.5 How do I use cgroups ?
222. Usage Examples and Syntax
23 2.1 Basic Usage
24 2.2 Attaching processes
253. Kernel API
26 3.1 Overview
27 3.2 Synchronization
28 3.3 Subsystem API
294. Questions
30
311. Control Groups
Li Zefand19e0582008-02-23 15:24:08 -080032=================
Paul Menageddbcc7e2007-10-18 23:39:30 -070033
341.1 What are cgroups ?
35----------------------
36
37Control Groups provide a mechanism for aggregating/partitioning sets of
38tasks, and all their future children, into hierarchical groups with
39specialized behaviour.
40
41Definitions:
42
43A *cgroup* associates a set of tasks with a set of parameters for one
44or more subsystems.
45
46A *subsystem* is a module that makes use of the task grouping
47facilities provided by cgroups to treat groups of tasks in
48particular ways. A subsystem is typically a "resource controller" that
49schedules a resource or applies per-cgroup limits, but it may be
50anything that wants to act on a group of processes, e.g. a
51virtualization subsystem.
52
53A *hierarchy* is a set of cgroups arranged in a tree, such that
54every task in the system is in exactly one of the cgroups in the
55hierarchy, and a set of subsystems; each subsystem has system-specific
56state attached to each cgroup in the hierarchy. Each hierarchy has
57an instance of the cgroup virtual filesystem associated with it.
58
59At any one time there may be multiple active hierachies of task
60cgroups. Each hierarchy is a partition of all tasks in the system.
61
62User level code may create and destroy cgroups by name in an
63instance of the cgroup virtual file system, specify and query to
64which cgroup a task is assigned, and list the task pids assigned to
65a cgroup. Those creations and assignments only affect the hierarchy
66associated with that instance of the cgroup file system.
67
68On their own, the only use for cgroups is for simple job
69tracking. The intention is that other subsystems hook into the generic
70cgroup support to provide new attributes for cgroups, such as
71accounting/limiting the resources which processes in a cgroup can
Li Zefan45ce80f2009-01-15 13:50:59 -080072access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allows
Paul Menageddbcc7e2007-10-18 23:39:30 -070073you to associate a set of CPUs and a set of memory nodes with the
74tasks in each cgroup.
75
761.2 Why are cgroups needed ?
77----------------------------
78
79There are multiple efforts to provide process aggregations in the
80Linux kernel, mainly for resource tracking purposes. Such efforts
81include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server
82namespaces. These all require the basic notion of a
83grouping/partitioning of processes, with newly forked processes ending
84in the same group (cgroup) as their parent process.
85
86The kernel cgroup patch provides the minimum essential kernel
87mechanisms required to efficiently implement such groups. It has
88minimal impact on the system fast paths, and provides hooks for
89specific subsystems such as cpusets to provide additional behaviour as
90desired.
91
92Multiple hierarchy support is provided to allow for situations where
93the division of tasks into cgroups is distinctly different for
94different subsystems - having parallel hierarchies allows each
95hierarchy to be a natural division of tasks, without having to handle
96complex combinations of tasks that would be present if several
97unrelated subsystems needed to be forced into the same tree of
98cgroups.
99
100At one extreme, each resource controller or subsystem could be in a
101separate hierarchy; at the other extreme, all subsystems
102would be attached to the same hierarchy.
103
104As an example of a scenario (originally proposed by vatsa@in.ibm.com)
105that can benefit from multiple hierarchies, consider a large
106university server with various users - students, professors, system
107tasks etc. The resource planning for this server could be along the
108following lines:
109
110 CPU : Top cpuset
111 / \
112 CPUSet1 CPUSet2
113 | |
114 (Profs) (Students)
115
116 In addition (system tasks) are attached to topcpuset (so
117 that they can run anywhere) with a limit of 20%
118
119 Memory : Professors (50%), students (30%), system (20%)
120
121 Disk : Prof (50%), students (30%), system (20%)
122
123 Network : WWW browsing (20%), Network File System (60%), others (20%)
124 / \
125 Prof (15%) students (5%)
126
127Browsers like firefox/lynx go into the WWW network class, while (k)nfsd go
128into NFS network class.
129
130At the same time firefox/lynx will share an appropriate CPU/Memory class
131depending on who launched it (prof/student).
132
133With the ability to classify tasks differently for different resources
134(by putting those resource subsystems in different hierarchies) then
135the admin can easily set up a script which receives exec notifications
136and depending on who is launching the browser he can
137
138 # echo browser_pid > /mnt/<restype>/<userclass>/tasks
139
140With only a single hierarchy, he now would potentially have to create
141a separate cgroup for every browser launched and associate it with
142approp network and other resource class. This may lead to
143proliferation of such cgroups.
144
145Also lets say that the administrator would like to give enhanced network
146access temporarily to a student's browser (since it is night and the user
Li Zefand19e0582008-02-23 15:24:08 -0800147wants to do online gaming :)) OR give one of the students simulation
Paul Menageddbcc7e2007-10-18 23:39:30 -0700148apps enhanced CPU power,
149
Li Zefand19e0582008-02-23 15:24:08 -0800150With ability to write pids directly to resource classes, it's just a
Paul Menageddbcc7e2007-10-18 23:39:30 -0700151matter of :
152
153 # echo pid > /mnt/network/<new_class>/tasks
154 (after some time)
155 # echo pid > /mnt/network/<orig_class>/tasks
156
157Without this ability, he would have to split the cgroup into
158multiple separate ones and then associate the new cgroups with the
159new resource classes.
160
161
162
1631.3 How are cgroups implemented ?
164---------------------------------
165
166Control Groups extends the kernel as follows:
167
168 - Each task in the system has a reference-counted pointer to a
169 css_set.
170
171 - A css_set contains a set of reference-counted pointers to
172 cgroup_subsys_state objects, one for each cgroup subsystem
173 registered in the system. There is no direct link from a task to
174 the cgroup of which it's a member in each hierarchy, but this
175 can be determined by following pointers through the
176 cgroup_subsys_state objects. This is because accessing the
177 subsystem state is something that's expected to happen frequently
178 and in performance-critical code, whereas operations that require a
179 task's actual cgroup assignments (in particular, moving between
Paul Menage817929e2007-10-18 23:39:36 -0700180 cgroups) are less common. A linked list runs through the cg_list
181 field of each task_struct using the css_set, anchored at
182 css_set->tasks.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700183
184 - A cgroup hierarchy filesystem can be mounted for browsing and
185 manipulation from user space.
186
187 - You can list all the tasks (by pid) attached to any cgroup.
188
189The implementation of cgroups requires a few, simple hooks
190into the rest of the kernel, none in performance critical paths:
191
192 - in init/main.c, to initialize the root cgroups and initial
193 css_set at system boot.
194
195 - in fork and exit, to attach and detach a task from its css_set.
196
197In addition a new file system, of type "cgroup" may be mounted, to
198enable browsing and modifying the cgroups presently known to the
199kernel. When mounting a cgroup hierarchy, you may specify a
200comma-separated list of subsystems to mount as the filesystem mount
201options. By default, mounting the cgroup filesystem attempts to
202mount a hierarchy containing all registered subsystems.
203
204If an active hierarchy with exactly the same set of subsystems already
205exists, it will be reused for the new mount. If no existing hierarchy
206matches, and any of the requested subsystems are in use in an existing
207hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
208is activated, associated with the requested subsystems.
209
210It's not currently possible to bind a new subsystem to an active
211cgroup hierarchy, or to unbind a subsystem from an active cgroup
212hierarchy. This may be possible in future, but is fraught with nasty
213error-recovery issues.
214
215When a cgroup filesystem is unmounted, if there are any
216child cgroups created below the top-level cgroup, that hierarchy
217will remain active even though unmounted; if there are no
218child cgroups then the hierarchy will be deactivated.
219
220No new system calls are added for cgroups - all support for
221querying and modifying cgroups is via this cgroup file system.
222
223Each task under /proc has an added file named 'cgroup' displaying,
224for each active hierarchy, the subsystem names and the cgroup name
225as the path relative to the root of the cgroup file system.
226
227Each cgroup is represented by a directory in the cgroup file system
228containing the following files describing that cgroup:
229
230 - tasks: list of tasks (by pid) attached to that cgroup
Li Zefand19e0582008-02-23 15:24:08 -0800231 - notify_on_release flag: run the release agent on exit?
232 - release_agent: the path to use for release notifications (this file
233 exists in the top cgroup only)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700234
235Other subsystems such as cpusets may add additional files in each
Li Zefand19e0582008-02-23 15:24:08 -0800236cgroup dir.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700237
238New cgroups are created using the mkdir system call or shell
239command. The properties of a cgroup, such as its flags, are
240modified by writing to the appropriate file in that cgroups
241directory, as listed above.
242
243The named hierarchical structure of nested cgroups allows partitioning
244a large system into nested, dynamically changeable, "soft-partitions".
245
246The attachment of each task, automatically inherited at fork by any
247children of that task, to a cgroup allows organizing the work load
248on a system into related sets of tasks. A task may be re-attached to
249any other cgroup, if allowed by the permissions on the necessary
250cgroup file system directories.
251
252When a task is moved from one cgroup to another, it gets a new
253css_set pointer - if there's an already existing css_set with the
254desired collection of cgroups then that group is reused, else a new
Li Zefanb851ee72009-02-18 14:48:14 -0800255css_set is allocated. The appropriate existing css_set is located by
256looking into a hash table.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700257
Paul Menage817929e2007-10-18 23:39:36 -0700258To allow access from a cgroup to the css_sets (and hence tasks)
259that comprise it, a set of cg_cgroup_link objects form a lattice;
260each cg_cgroup_link is linked into a list of cg_cgroup_links for
Li Zefand19e0582008-02-23 15:24:08 -0800261a single cgroup on its cgrp_link_list field, and a list of
Paul Menage817929e2007-10-18 23:39:36 -0700262cg_cgroup_links for a single css_set on its cg_link_list.
263
264Thus the set of tasks in a cgroup can be listed by iterating over
265each css_set that references the cgroup, and sub-iterating over
266each css_set's task set.
267
Paul Menageddbcc7e2007-10-18 23:39:30 -0700268The use of a Linux virtual file system (vfs) to represent the
269cgroup hierarchy provides for a familiar permission and name space
270for cgroups, with a minimum of additional kernel code.
271
2721.4 What does notify_on_release do ?
273------------------------------------
274
Paul Menageddbcc7e2007-10-18 23:39:30 -0700275If the notify_on_release flag is enabled (1) in a cgroup, then
276whenever the last task in the cgroup leaves (exits or attaches to
277some other cgroup) and the last child cgroup of that cgroup
278is removed, then the kernel runs the command specified by the contents
279of the "release_agent" file in that hierarchy's root directory,
280supplying the pathname (relative to the mount point of the cgroup
281file system) of the abandoned cgroup. This enables automatic
282removal of abandoned cgroups. The default value of
283notify_on_release in the root cgroup at system boot is disabled
284(0). The default value of other cgroups at creation is the current
285value of their parents notify_on_release setting. The default value of
286a cgroup hierarchy's release_agent path is empty.
287
2881.5 How do I use cgroups ?
289--------------------------
290
291To start a new job that is to be contained within a cgroup, using
292the "cpuset" cgroup subsystem, the steps are something like:
293
294 1) mkdir /dev/cgroup
295 2) mount -t cgroup -ocpuset cpuset /dev/cgroup
296 3) Create the new cgroup by doing mkdir's and write's (or echo's) in
297 the /dev/cgroup virtual file system.
298 4) Start a task that will be the "founding father" of the new job.
299 5) Attach that task to the new cgroup by writing its pid to the
300 /dev/cgroup tasks file for that cgroup.
301 6) fork, exec or clone the job tasks from this founding father task.
302
303For example, the following sequence of commands will setup a cgroup
304named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
305and then start a subshell 'sh' in that cgroup:
306
307 mount -t cgroup cpuset -ocpuset /dev/cgroup
308 cd /dev/cgroup
309 mkdir Charlie
310 cd Charlie
Dhaval Giani0f146a72008-05-12 14:02:31 -0700311 /bin/echo 2-3 > cpuset.cpus
312 /bin/echo 1 > cpuset.mems
Paul Menageddbcc7e2007-10-18 23:39:30 -0700313 /bin/echo $$ > tasks
314 sh
315 # The subshell 'sh' is now running in cgroup Charlie
316 # The next line should display '/Charlie'
317 cat /proc/self/cgroup
318
3192. Usage Examples and Syntax
320============================
321
3222.1 Basic Usage
323---------------
324
325Creating, modifying, using the cgroups can be done through the cgroup
326virtual filesystem.
327
328To mount a cgroup hierarchy will all available subsystems, type:
329# mount -t cgroup xxx /dev/cgroup
330
331The "xxx" is not interpreted by the cgroup code, but will appear in
332/proc/mounts so may be any useful identifying string that you like.
333
334To mount a cgroup hierarchy with just the cpuset and numtasks
335subsystems, type:
Li Zefanb6719ec2009-04-02 16:57:28 -0700336# mount -t cgroup -o cpuset,memory hier1 /dev/cgroup
Paul Menageddbcc7e2007-10-18 23:39:30 -0700337
338To change the set of subsystems bound to a mounted hierarchy, just
339remount with different options:
Li Zefanb6719ec2009-04-02 16:57:28 -0700340# mount -o remount,cpuset,ns hier1 /dev/cgroup
Paul Menageddbcc7e2007-10-18 23:39:30 -0700341
Li Zefanb6719ec2009-04-02 16:57:28 -0700342Now memory is removed from the hierarchy and ns is added.
343
344Note this will add ns to the hierarchy but won't remove memory or
345cpuset, because the new options are appended to the old ones:
346# mount -o remount,ns /dev/cgroup
347
348To Specify a hierarchy's release_agent:
349# mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \
350 xxx /dev/cgroup
351
352Note that specifying 'release_agent' more than once will return failure.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700353
354Note that changing the set of subsystems is currently only supported
355when the hierarchy consists of a single (root) cgroup. Supporting
356the ability to arbitrarily bind/unbind subsystems from an existing
357cgroup hierarchy is intended to be implemented in the future.
358
359Then under /dev/cgroup you can find a tree that corresponds to the
360tree of the cgroups in the system. For instance, /dev/cgroup
361is the cgroup that holds the whole system.
362
Li Zefanb6719ec2009-04-02 16:57:28 -0700363If you want to change the value of release_agent:
364# echo "/sbin/new_release_agent" > /dev/cgroup/release_agent
365
366It can also be changed via remount.
367
Paul Menageddbcc7e2007-10-18 23:39:30 -0700368If you want to create a new cgroup under /dev/cgroup:
369# cd /dev/cgroup
370# mkdir my_cgroup
371
372Now you want to do something with this cgroup.
373# cd my_cgroup
374
375In this directory you can find several files:
376# ls
Li Zefan18e7f1f2009-01-07 18:07:32 -0800377notify_on_release tasks
Li Zefand19e0582008-02-23 15:24:08 -0800378(plus whatever files added by the attached subsystems)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700379
380Now attach your shell to this cgroup:
381# /bin/echo $$ > tasks
382
383You can also create cgroups inside your cgroup by using mkdir in this
384directory.
385# mkdir my_sub_cs
386
387To remove a cgroup, just use rmdir:
388# rmdir my_sub_cs
389
390This will fail if the cgroup is in use (has cgroups inside, or
391has processes attached, or is held alive by other subsystem-specific
392reference).
393
3942.2 Attaching processes
395-----------------------
396
397# /bin/echo PID > tasks
398
399Note that it is PID, not PIDs. You can only attach ONE task at a time.
400If you have several tasks to attach, you have to do it one after another:
401
402# /bin/echo PID1 > tasks
403# /bin/echo PID2 > tasks
404 ...
405# /bin/echo PIDn > tasks
406
Li Zefanbef67c52008-07-04 09:59:55 -0700407You can attach the current shell task by echoing 0:
408
409# echo 0 > tasks
410
Paul Menageddbcc7e2007-10-18 23:39:30 -07004113. Kernel API
412=============
413
4143.1 Overview
415------------
416
417Each kernel subsystem that wants to hook into the generic cgroup
418system needs to create a cgroup_subsys object. This contains
419various methods, which are callbacks from the cgroup system, along
420with a subsystem id which will be assigned by the cgroup system.
421
422Other fields in the cgroup_subsys object include:
423
424- subsys_id: a unique array index for the subsystem, indicating which
Li Zefand19e0582008-02-23 15:24:08 -0800425 entry in cgroup->subsys[] this subsystem should be managing.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700426
Li Zefand19e0582008-02-23 15:24:08 -0800427- name: should be initialized to a unique subsystem name. Should be
428 no longer than MAX_CGROUP_TYPE_NAMELEN.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700429
Li Zefand19e0582008-02-23 15:24:08 -0800430- early_init: indicate if the subsystem needs early initialization
431 at system boot.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700432
433Each cgroup object created by the system has an array of pointers,
434indexed by subsystem id; this pointer is entirely managed by the
435subsystem; the generic cgroup code will never touch this pointer.
436
4373.2 Synchronization
438-------------------
439
440There is a global mutex, cgroup_mutex, used by the cgroup
441system. This should be taken by anything that wants to modify a
442cgroup. It may also be taken to prevent cgroups from being
443modified, but more specific locks may be more appropriate in that
444situation.
445
446See kernel/cgroup.c for more details.
447
448Subsystems can take/release the cgroup_mutex via the functions
Paul Menageddbcc7e2007-10-18 23:39:30 -0700449cgroup_lock()/cgroup_unlock().
450
451Accessing a task's cgroup pointer may be done in the following ways:
452- while holding cgroup_mutex
453- while holding the task's alloc_lock (via task_lock())
454- inside an rcu_read_lock() section via rcu_dereference()
455
4563.3 Subsystem API
Li Zefand19e0582008-02-23 15:24:08 -0800457-----------------
Paul Menageddbcc7e2007-10-18 23:39:30 -0700458
459Each subsystem should:
460
461- add an entry in linux/cgroup_subsys.h
462- define a cgroup_subsys object called <name>_subsys
463
464Each subsystem may export the following methods. The only mandatory
465methods are create/destroy. Any others that are null are presumed to
466be successful no-ops.
467
Li Zefand19e0582008-02-23 15:24:08 -0800468struct cgroup_subsys_state *create(struct cgroup_subsys *ss,
469 struct cgroup *cgrp)
Paul Menage8dc4f3e2008-02-07 00:13:45 -0800470(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700471
472Called to create a subsystem state object for a cgroup. The
473subsystem should allocate its subsystem state object for the passed
474cgroup, returning a pointer to the new object on success or a
475negative error code. On success, the subsystem pointer should point to
476a structure of type cgroup_subsys_state (typically embedded in a
477larger subsystem-specific object), which will be initialized by the
478cgroup system. Note that this will be called at initialization to
479create the root subsystem state for this subsystem; this case can be
480identified by the passed cgroup object having a NULL parent (since
481it's the root of the hierarchy) and may be an appropriate place for
482initialization code.
483
Li Zefand19e0582008-02-23 15:24:08 -0800484void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
Paul Menage8dc4f3e2008-02-07 00:13:45 -0800485(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700486
Paul Menage8dc4f3e2008-02-07 00:13:45 -0800487The cgroup system is about to destroy the passed cgroup; the subsystem
488should do any necessary cleanup and free its subsystem state
489object. By the time this method is called, the cgroup has already been
490unlinked from the file system and from the child list of its parent;
491cgroup->parent is still valid. (Note - can also be called for a
492newly-created cgroup if an error occurs after this subsystem's
493create() method has been called for the new cgroup).
Paul Menageddbcc7e2007-10-18 23:39:30 -0700494
KAMEZAWA Hiroyukiec64f512009-04-02 16:57:26 -0700495int pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp);
Li Zefand19e0582008-02-23 15:24:08 -0800496
497Called before checking the reference count on each subsystem. This may
498be useful for subsystems which have some extra references even if
KAMEZAWA Hiroyukiec64f512009-04-02 16:57:26 -0700499there are not tasks in the cgroup. If pre_destroy() returns error code,
500rmdir() will fail with it. From this behavior, pre_destroy() can be
501called multiple times against a cgroup.
Li Zefand19e0582008-02-23 15:24:08 -0800502
503int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
Paul Menageddbcc7e2007-10-18 23:39:30 -0700504 struct task_struct *task)
Paul Menage8dc4f3e2008-02-07 00:13:45 -0800505(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700506
507Called prior to moving a task into a cgroup; if the subsystem
508returns an error, this will abort the attach operation. If a NULL
509task is passed, then a successful result indicates that *any*
510unspecified task can be moved into the cgroup. Note that this isn't
511called on a fork. If this method returns 0 (success) then this should
512remain valid while the caller holds cgroup_mutex.
513
Li Zefand19e0582008-02-23 15:24:08 -0800514void attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
515 struct cgroup *old_cgrp, struct task_struct *task)
Li Zefan18e7f1f2009-01-07 18:07:32 -0800516(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700517
518Called after the task has been attached to the cgroup, to allow any
519post-attachment activity that requires memory allocations or blocking.
520
521void fork(struct cgroup_subsy *ss, struct task_struct *task)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700522
Li Zefane8d55fd2008-04-29 01:00:13 -0700523Called when a task is forked into a cgroup.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700524
525void exit(struct cgroup_subsys *ss, struct task_struct *task)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700526
Li Zefand19e0582008-02-23 15:24:08 -0800527Called during task exit.
Paul Menageddbcc7e2007-10-18 23:39:30 -0700528
Li Zefand19e0582008-02-23 15:24:08 -0800529int populate(struct cgroup_subsys *ss, struct cgroup *cgrp)
Li Zefan18e7f1f2009-01-07 18:07:32 -0800530(cgroup_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700531
532Called after creation of a cgroup to allow a subsystem to populate
533the cgroup directory with file entries. The subsystem should make
534calls to cgroup_add_file() with objects of type cftype (see
535include/linux/cgroup.h for details). Note that although this
536method can return an error code, the error code is currently not
537always handled well.
538
Li Zefand19e0582008-02-23 15:24:08 -0800539void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp)
Li Zefan18e7f1f2009-01-07 18:07:32 -0800540(cgroup_mutex held by caller)
Paul Menage697f4162007-10-18 23:39:34 -0700541
542Called at the end of cgroup_clone() to do any paramater
543initialization which might be required before a task could attach. For
544example in cpusets, no task may attach before 'cpus' and 'mems' are set
545up.
546
Paul Menageddbcc7e2007-10-18 23:39:30 -0700547void bind(struct cgroup_subsys *ss, struct cgroup *root)
Paul Menage999cd8a2009-01-07 18:08:36 -0800548(cgroup_mutex and ss->hierarchy_mutex held by caller)
Paul Menageddbcc7e2007-10-18 23:39:30 -0700549
550Called when a cgroup subsystem is rebound to a different hierarchy
551and root cgroup. Currently this will only involve movement between
552the default hierarchy (which never has sub-cgroups) and a hierarchy
553that is being created/destroyed (and hence has no sub-cgroups).
554
5554. Questions
556============
557
558Q: what's up with this '/bin/echo' ?
559A: bash's builtin 'echo' command does not check calls to write() against
560 errors. If you use it in the cgroup file system, you won't be
561 able to tell whether a command succeeded or failed.
562
563Q: When I attach processes, only the first of the line gets really attached !
564A: We can only return one error code per call to write(). So you should also
565 put only ONE pid.
566