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Based on kernel version 3.2. Page generated on 2012-01-05 23:28 EST.

1					CGROUPS
2					-------
3	
4	Written by Paul Menage <menage@google.com> based on
5	Documentation/cgroups/cpusets.txt
6	
7	Original copyright statements from cpusets.txt:
8	Portions Copyright (C) 2004 BULL SA.
9	Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
10	Modified by Paul Jackson <pj@sgi.com>
11	Modified by Christoph Lameter <clameter@sgi.com>
12	
13	CONTENTS:
14	=========
15	
16	1. Control Groups
17	  1.1 What are cgroups ?
18	  1.2 Why are cgroups needed ?
19	  1.3 How are cgroups implemented ?
20	  1.4 What does notify_on_release do ?
21	  1.5 What does clone_children do ?
22	  1.6 How do I use cgroups ?
23	2. Usage Examples and Syntax
24	  2.1 Basic Usage
25	  2.2 Attaching processes
26	  2.3 Mounting hierarchies by name
27	  2.4 Notification API
28	3. Kernel API
29	  3.1 Overview
30	  3.2 Synchronization
31	  3.3 Subsystem API
32	4. Questions
33	
34	1. Control Groups
35	=================
36	
37	1.1 What are cgroups ?
38	----------------------
39	
40	Control Groups provide a mechanism for aggregating/partitioning sets of
41	tasks, and all their future children, into hierarchical groups with
42	specialized behaviour.
43	
44	Definitions:
45	
46	A *cgroup* associates a set of tasks with a set of parameters for one
47	or more subsystems.
48	
49	A *subsystem* is a module that makes use of the task grouping
50	facilities provided by cgroups to treat groups of tasks in
51	particular ways. A subsystem is typically a "resource controller" that
52	schedules a resource or applies per-cgroup limits, but it may be
53	anything that wants to act on a group of processes, e.g. a
54	virtualization subsystem.
55	
56	A *hierarchy* is a set of cgroups arranged in a tree, such that
57	every task in the system is in exactly one of the cgroups in the
58	hierarchy, and a set of subsystems; each subsystem has system-specific
59	state attached to each cgroup in the hierarchy.  Each hierarchy has
60	an instance of the cgroup virtual filesystem associated with it.
61	
62	At any one time there may be multiple active hierarchies of task
63	cgroups. Each hierarchy is a partition of all tasks in the system.
64	
65	User level code may create and destroy cgroups by name in an
66	instance of the cgroup virtual file system, specify and query to
67	which cgroup a task is assigned, and list the task pids assigned to
68	a cgroup. Those creations and assignments only affect the hierarchy
69	associated with that instance of the cgroup file system.
70	
71	On their own, the only use for cgroups is for simple job
72	tracking. The intention is that other subsystems hook into the generic
73	cgroup support to provide new attributes for cgroups, such as
74	accounting/limiting the resources which processes in a cgroup can
75	access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allows
76	you to associate a set of CPUs and a set of memory nodes with the
77	tasks in each cgroup.
78	
79	1.2 Why are cgroups needed ?
80	----------------------------
81	
82	There are multiple efforts to provide process aggregations in the
83	Linux kernel, mainly for resource tracking purposes. Such efforts
84	include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server
85	namespaces. These all require the basic notion of a
86	grouping/partitioning of processes, with newly forked processes ending
87	in the same group (cgroup) as their parent process.
88	
89	The kernel cgroup patch provides the minimum essential kernel
90	mechanisms required to efficiently implement such groups. It has
91	minimal impact on the system fast paths, and provides hooks for
92	specific subsystems such as cpusets to provide additional behaviour as
93	desired.
94	
95	Multiple hierarchy support is provided to allow for situations where
96	the division of tasks into cgroups is distinctly different for
97	different subsystems - having parallel hierarchies allows each
98	hierarchy to be a natural division of tasks, without having to handle
99	complex combinations of tasks that would be present if several
100	unrelated subsystems needed to be forced into the same tree of
101	cgroups.
102	
103	At one extreme, each resource controller or subsystem could be in a
104	separate hierarchy; at the other extreme, all subsystems
105	would be attached to the same hierarchy.
106	
107	As an example of a scenario (originally proposed by vatsa@in.ibm.com)
108	that can benefit from multiple hierarchies, consider a large
109	university server with various users - students, professors, system
110	tasks etc. The resource planning for this server could be along the
111	following lines:
112	
113	       CPU :          "Top cpuset"
114	                       /       \
115	               CPUSet1         CPUSet2
116	                  |               |
117	               (Professors)    (Students)
118	
119	               In addition (system tasks) are attached to topcpuset (so
120	               that they can run anywhere) with a limit of 20%
121	
122	       Memory : Professors (50%), Students (30%), system (20%)
123	
124	       Disk : Professors (50%), Students (30%), system (20%)
125	
126	       Network : WWW browsing (20%), Network File System (60%), others (20%)
127	                               / \
128	               Professors (15%)  students (5%)
129	
130	Browsers like Firefox/Lynx go into the WWW network class, while (k)nfsd go
131	into NFS network class.
132	
133	At the same time Firefox/Lynx will share an appropriate CPU/Memory class
134	depending on who launched it (prof/student).
135	
136	With the ability to classify tasks differently for different resources
137	(by putting those resource subsystems in different hierarchies) then
138	the admin can easily set up a script which receives exec notifications
139	and depending on who is launching the browser he can
140	
141	    # echo browser_pid > /sys/fs/cgroup/<restype>/<userclass>/tasks
142	
143	With only a single hierarchy, he now would potentially have to create
144	a separate cgroup for every browser launched and associate it with
145	appropriate network and other resource class.  This may lead to
146	proliferation of such cgroups.
147	
148	Also lets say that the administrator would like to give enhanced network
149	access temporarily to a student's browser (since it is night and the user
150	wants to do online gaming :))  OR give one of the students simulation
151	apps enhanced CPU power,
152	
153	With ability to write pids directly to resource classes, it's just a
154	matter of :
155	
156	       # echo pid > /sys/fs/cgroup/network/<new_class>/tasks
157	       (after some time)
158	       # echo pid > /sys/fs/cgroup/network/<orig_class>/tasks
159	
160	Without this ability, he would have to split the cgroup into
161	multiple separate ones and then associate the new cgroups with the
162	new resource classes.
163	
164	
165	
166	1.3 How are cgroups implemented ?
167	---------------------------------
168	
169	Control Groups extends the kernel as follows:
170	
171	 - Each task in the system has a reference-counted pointer to a
172	   css_set.
173	
174	 - A css_set contains a set of reference-counted pointers to
175	   cgroup_subsys_state objects, one for each cgroup subsystem
176	   registered in the system. There is no direct link from a task to
177	   the cgroup of which it's a member in each hierarchy, but this
178	   can be determined by following pointers through the
179	   cgroup_subsys_state objects. This is because accessing the
180	   subsystem state is something that's expected to happen frequently
181	   and in performance-critical code, whereas operations that require a
182	   task's actual cgroup assignments (in particular, moving between
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.
186	
187	 - A cgroup hierarchy filesystem can be mounted  for browsing and
188	   manipulation from user space.
189	
190	 - You can list all the tasks (by pid) attached to any cgroup.
191	
192	The implementation of cgroups requires a few, simple hooks
193	into the rest of the kernel, none in performance critical paths:
194	
195	 - in init/main.c, to initialize the root cgroups and initial
196	   css_set at system boot.
197	
198	 - in fork and exit, to attach and detach a task from its css_set.
199	
200	In addition a new file system, of type "cgroup" may be mounted, to
201	enable browsing and modifying the cgroups presently known to the
202	kernel.  When mounting a cgroup hierarchy, you may specify a
203	comma-separated list of subsystems to mount as the filesystem mount
204	options.  By default, mounting the cgroup filesystem attempts to
205	mount a hierarchy containing all registered subsystems.
206	
207	If an active hierarchy with exactly the same set of subsystems already
208	exists, it will be reused for the new mount. If no existing hierarchy
209	matches, and any of the requested subsystems are in use in an existing
210	hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
211	is activated, associated with the requested subsystems.
212	
213	It's not currently possible to bind a new subsystem to an active
214	cgroup hierarchy, or to unbind a subsystem from an active cgroup
215	hierarchy. This may be possible in future, but is fraught with nasty
216	error-recovery issues.
217	
218	When a cgroup filesystem is unmounted, if there are any
219	child cgroups created below the top-level cgroup, that hierarchy
220	will remain active even though unmounted; if there are no
221	child cgroups then the hierarchy will be deactivated.
222	
223	No new system calls are added for cgroups - all support for
224	querying and modifying cgroups is via this cgroup file system.
225	
226	Each task under /proc has an added file named 'cgroup' displaying,
227	for each active hierarchy, the subsystem names and the cgroup name
228	as the path relative to the root of the cgroup file system.
229	
230	Each cgroup is represented by a directory in the cgroup file system
231	containing the following files describing that cgroup:
232	
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.
239	   Writing a thread group id into this file moves all threads in that
240	   group into this cgroup.
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)
244	
245	Other subsystems such as cpusets may add additional files in each
246	cgroup dir.
247	
248	New cgroups are created using the mkdir system call or shell
249	command.  The properties of a cgroup, such as its flags, are
250	modified by writing to the appropriate file in that cgroups
251	directory, as listed above.
252	
253	The named hierarchical structure of nested cgroups allows partitioning
254	a large system into nested, dynamically changeable, "soft-partitions".
255	
256	The attachment of each task, automatically inherited at fork by any
257	children of that task, to a cgroup allows organizing the work load
258	on a system into related sets of tasks.  A task may be re-attached to
259	any other cgroup, if allowed by the permissions on the necessary
260	cgroup file system directories.
261	
262	When a task is moved from one cgroup to another, it gets a new
263	css_set pointer - if there's an already existing css_set with the
264	desired collection of cgroups then that group is reused, else a new
265	css_set is allocated. The appropriate existing css_set is located by
266	looking into a hash table.
267	
268	To allow access from a cgroup to the css_sets (and hence tasks)
269	that comprise it, a set of cg_cgroup_link objects form a lattice;
270	each cg_cgroup_link is linked into a list of cg_cgroup_links for
271	a single cgroup on its cgrp_link_list field, and a list of
272	cg_cgroup_links for a single css_set on its cg_link_list.
273	
274	Thus the set of tasks in a cgroup can be listed by iterating over
275	each css_set that references the cgroup, and sub-iterating over
276	each css_set's task set.
277	
278	The use of a Linux virtual file system (vfs) to represent the
279	cgroup hierarchy provides for a familiar permission and name space
280	for cgroups, with a minimum of additional kernel code.
281	
282	1.4 What does notify_on_release do ?
283	------------------------------------
284	
285	If the notify_on_release flag is enabled (1) in a cgroup, then
286	whenever the last task in the cgroup leaves (exits or attaches to
287	some other cgroup) and the last child cgroup of that cgroup
288	is removed, then the kernel runs the command specified by the contents
289	of the "release_agent" file in that hierarchy's root directory,
290	supplying the pathname (relative to the mount point of the cgroup
291	file system) of the abandoned cgroup.  This enables automatic
292	removal of abandoned cgroups.  The default value of
293	notify_on_release in the root cgroup at system boot is disabled
294	(0).  The default value of other cgroups at creation is the current
295	value of their parents notify_on_release setting. The default value of
296	a cgroup hierarchy's release_agent path is empty.
297	
298	1.5 What does clone_children do ?
299	---------------------------------
300	
301	If the clone_children flag is enabled (1) in a cgroup, then all
302	cgroups created beneath will call the post_clone callbacks for each
303	subsystem of the newly created cgroup. Usually when this callback is
304	implemented for a subsystem, it copies the values of the parent
305	subsystem, this is the case for the cpuset.
306	
307	1.6 How do I use cgroups ?
308	--------------------------
309	
310	To start a new job that is to be contained within a cgroup, using
311	the "cpuset" cgroup subsystem, the steps are something like:
312	
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.
322	
323	For example, the following sequence of commands will setup a cgroup
324	named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
325	and then start a subshell 'sh' in that cgroup:
326	
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
331	  mkdir Charlie
332	  cd Charlie
333	  /bin/echo 2-3 > cpuset.cpus
334	  /bin/echo 1 > cpuset.mems
335	  /bin/echo $$ > tasks
336	  sh
337	  # The subshell 'sh' is now running in cgroup Charlie
338	  # The next line should display '/Charlie'
339	  cat /proc/self/cgroup
340	
341	2. Usage Examples and Syntax
342	============================
343	
344	2.1 Basic Usage
345	---------------
346	
347	Creating, modifying, using the cgroups can be done through the cgroup
348	virtual filesystem.
349	
350	To mount a cgroup hierarchy with all available subsystems, type:
351	# mount -t cgroup xxx /sys/fs/cgroup
352	
353	The "xxx" is not interpreted by the cgroup code, but will appear in
354	/proc/mounts so may be any useful identifying string that you like.
355	
356	Note: Some subsystems do not work without some user input first.  For instance,
357	if cpusets are enabled the user will have to populate the cpus and mems files
358	for each new cgroup created before that group can be used.
359	
360	As explained in section `1.2 Why are cgroups needed?' you should create
361	different hierarchies of cgroups for each single resource or group of
362	resources you want to control. Therefore, you should mount a tmpfs on
363	/sys/fs/cgroup and create directories for each cgroup resource or resource
364	group.
365	
366	# mount -t tmpfs cgroup_root /sys/fs/cgroup
367	# mkdir /sys/fs/cgroup/rg1
368	
369	To mount a cgroup hierarchy with just the cpuset and memory
370	subsystems, type:
371	# mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1
372	
373	To change the set of subsystems bound to a mounted hierarchy, just
374	remount with different options:
375	# mount -o remount,cpuset,blkio hier1 /sys/fs/cgroup/rg1
376	
377	Now memory is removed from the hierarchy and blkio is added.
378	
379	Note this will add blkio to the hierarchy but won't remove memory or
380	cpuset, because the new options are appended to the old ones:
381	# mount -o remount,blkio /sys/fs/cgroup/rg1
382	
383	To Specify a hierarchy's release_agent:
384	# mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \
385	  xxx /sys/fs/cgroup/rg1
386	
387	Note that specifying 'release_agent' more than once will return failure.
388	
389	Note that changing the set of subsystems is currently only supported
390	when the hierarchy consists of a single (root) cgroup. Supporting
391	the ability to arbitrarily bind/unbind subsystems from an existing
392	cgroup hierarchy is intended to be implemented in the future.
393	
394	Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
395	tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
396	is the cgroup that holds the whole system.
397	
398	If you want to change the value of release_agent:
399	# echo "/sbin/new_release_agent" > /sys/fs/cgroup/rg1/release_agent
400	
401	It can also be changed via remount.
402	
403	If you want to create a new cgroup under /sys/fs/cgroup/rg1:
404	# cd /sys/fs/cgroup/rg1
405	# mkdir my_cgroup
406	
407	Now you want to do something with this cgroup.
408	# cd my_cgroup
409	
410	In this directory you can find several files:
411	# ls
412	cgroup.procs notify_on_release tasks
413	(plus whatever files added by the attached subsystems)
414	
415	Now attach your shell to this cgroup:
416	# /bin/echo $$ > tasks
417	
418	You can also create cgroups inside your cgroup by using mkdir in this
419	directory.
420	# mkdir my_sub_cs
421	
422	To remove a cgroup, just use rmdir:
423	# rmdir my_sub_cs
424	
425	This will fail if the cgroup is in use (has cgroups inside, or
426	has processes attached, or is held alive by other subsystem-specific
427	reference).
428	
429	2.2 Attaching processes
430	-----------------------
431	
432	# /bin/echo PID > tasks
433	
434	Note that it is PID, not PIDs. You can only attach ONE task at a time.
435	If you have several tasks to attach, you have to do it one after another:
436	
437	# /bin/echo PID1 > tasks
438	# /bin/echo PID2 > tasks
439		...
440	# /bin/echo PIDn > tasks
441	
442	You can attach the current shell task by echoing 0:
443	
444	# echo 0 > tasks
445	
446	You can use the cgroup.procs file instead of the tasks file to move all
447	threads in a threadgroup at once. Echoing the pid of any task in a
448	threadgroup to cgroup.procs causes all tasks in that threadgroup to be
449	be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
450	in the writing task's threadgroup.
451	
452	Note: Since every task is always a member of exactly one cgroup in each
453	mounted hierarchy, to remove a task from its current cgroup you must
454	move it into a new cgroup (possibly the root cgroup) by writing to the
455	new cgroup's tasks file.
456	
457	Note: Due to some restrictions enforced by some cgroup subsystems, moving
458	a process to another cgroup can fail.
459	
460	2.3 Mounting hierarchies by name
461	--------------------------------
462	
463	Passing the name=<x> option when mounting a cgroups hierarchy
464	associates the given name with the hierarchy.  This can be used when
465	mounting a pre-existing hierarchy, in order to refer to it by name
466	rather than by its set of active subsystems.  Each hierarchy is either
467	nameless, or has a unique name.
468	
469	The name should match [\w.-]+
470	
471	When passing a name=<x> option for a new hierarchy, you need to
472	specify subsystems manually; the legacy behaviour of mounting all
473	subsystems when none are explicitly specified is not supported when
474	you give a subsystem a name.
475	
476	The name of the subsystem appears as part of the hierarchy description
477	in /proc/mounts and /proc/<pid>/cgroups.
478	
479	2.4 Notification API
480	--------------------
481	
482	There is mechanism which allows to get notifications about changing
483	status of a cgroup.
484	
485	To register new notification handler you need:
486	 - create a file descriptor for event notification using eventfd(2);
487	 - open a control file to be monitored (e.g. memory.usage_in_bytes);
488	 - write "<event_fd> <control_fd> <args>" to cgroup.event_control.
489	   Interpretation of args is defined by control file implementation;
490	
491	eventfd will be woken up by control file implementation or when the
492	cgroup is removed.
493	
494	To unregister notification handler just close eventfd.
495	
496	NOTE: Support of notifications should be implemented for the control
497	file. See documentation for the subsystem.
498	
499	3. Kernel API
500	=============
501	
502	3.1 Overview
503	------------
504	
505	Each kernel subsystem that wants to hook into the generic cgroup
506	system needs to create a cgroup_subsys object. This contains
507	various methods, which are callbacks from the cgroup system, along
508	with a subsystem id which will be assigned by the cgroup system.
509	
510	Other fields in the cgroup_subsys object include:
511	
512	- subsys_id: a unique array index for the subsystem, indicating which
513	  entry in cgroup->subsys[] this subsystem should be managing.
514	
515	- name: should be initialized to a unique subsystem name. Should be
516	  no longer than MAX_CGROUP_TYPE_NAMELEN.
517	
518	- early_init: indicate if the subsystem needs early initialization
519	  at system boot.
520	
521	Each cgroup object created by the system has an array of pointers,
522	indexed by subsystem id; this pointer is entirely managed by the
523	subsystem; the generic cgroup code will never touch this pointer.
524	
525	3.2 Synchronization
526	-------------------
527	
528	There is a global mutex, cgroup_mutex, used by the cgroup
529	system. This should be taken by anything that wants to modify a
530	cgroup. It may also be taken to prevent cgroups from being
531	modified, but more specific locks may be more appropriate in that
532	situation.
533	
534	See kernel/cgroup.c for more details.
535	
536	Subsystems can take/release the cgroup_mutex via the functions
537	cgroup_lock()/cgroup_unlock().
538	
539	Accessing a task's cgroup pointer may be done in the following ways:
540	- while holding cgroup_mutex
541	- while holding the task's alloc_lock (via task_lock())
542	- inside an rcu_read_lock() section via rcu_dereference()
543	
544	3.3 Subsystem API
545	-----------------
546	
547	Each subsystem should:
548	
549	- add an entry in linux/cgroup_subsys.h
550	- define a cgroup_subsys object called <name>_subsys
551	
552	If a subsystem can be compiled as a module, it should also have in its
553	module initcall a call to cgroup_load_subsys(), and in its exitcall a
554	call to cgroup_unload_subsys(). It should also set its_subsys.module =
555	THIS_MODULE in its .c file.
556	
557	Each subsystem may export the following methods. The only mandatory
558	methods are create/destroy. Any others that are null are presumed to
559	be successful no-ops.
560	
561	struct cgroup_subsys_state *create(struct cgroup_subsys *ss,
562					   struct cgroup *cgrp)
563	(cgroup_mutex held by caller)
564	
565	Called to create a subsystem state object for a cgroup. The
566	subsystem should allocate its subsystem state object for the passed
567	cgroup, returning a pointer to the new object on success or a
568	negative error code. On success, the subsystem pointer should point to
569	a structure of type cgroup_subsys_state (typically embedded in a
570	larger subsystem-specific object), which will be initialized by the
571	cgroup system. Note that this will be called at initialization to
572	create the root subsystem state for this subsystem; this case can be
573	identified by the passed cgroup object having a NULL parent (since
574	it's the root of the hierarchy) and may be an appropriate place for
575	initialization code.
576	
577	void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
578	(cgroup_mutex held by caller)
579	
580	The cgroup system is about to destroy the passed cgroup; the subsystem
581	should do any necessary cleanup and free its subsystem state
582	object. By the time this method is called, the cgroup has already been
583	unlinked from the file system and from the child list of its parent;
584	cgroup->parent is still valid. (Note - can also be called for a
585	newly-created cgroup if an error occurs after this subsystem's
586	create() method has been called for the new cgroup).
587	
588	int pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp);
589	
590	Called before checking the reference count on each subsystem. This may
591	be useful for subsystems which have some extra references even if
592	there are not tasks in the cgroup. If pre_destroy() returns error code,
593	rmdir() will fail with it. From this behavior, pre_destroy() can be
594	called multiple times against a cgroup.
595	
596	int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
597		       struct task_struct *task)
598	(cgroup_mutex held by caller)
599	
600	Called prior to moving a task into a cgroup; if the subsystem
601	returns an error, this will abort the attach operation.  If a NULL
602	task is passed, then a successful result indicates that *any*
603	unspecified task can be moved into the cgroup. Note that this isn't
604	called on a fork. If this method returns 0 (success) then this should
605	remain valid while the caller holds cgroup_mutex and it is ensured that either
606	attach() or cancel_attach() will be called in future.
607	
608	int can_attach_task(struct cgroup *cgrp, struct task_struct *tsk);
609	(cgroup_mutex held by caller)
610	
611	As can_attach, but for operations that must be run once per task to be
612	attached (possibly many when using cgroup_attach_proc). Called after
613	can_attach.
614	
615	void cancel_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
616		       struct task_struct *task, bool threadgroup)
617	(cgroup_mutex held by caller)
618	
619	Called when a task attach operation has failed after can_attach() has succeeded.
620	A subsystem whose can_attach() has some side-effects should provide this
621	function, so that the subsystem can implement a rollback. If not, not necessary.
622	This will be called only about subsystems whose can_attach() operation have
623	succeeded.
624	
625	void pre_attach(struct cgroup *cgrp);
626	(cgroup_mutex held by caller)
627	
628	For any non-per-thread attachment work that needs to happen before
629	attach_task. Needed by cpuset.
630	
631	void attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
632		    struct cgroup *old_cgrp, struct task_struct *task)
633	(cgroup_mutex held by caller)
634	
635	Called after the task has been attached to the cgroup, to allow any
636	post-attachment activity that requires memory allocations or blocking.
637	
638	void attach_task(struct cgroup *cgrp, struct task_struct *tsk);
639	(cgroup_mutex held by caller)
640	
641	As attach, but for operations that must be run once per task to be attached,
642	like can_attach_task. Called before attach. Currently does not support any
643	subsystem that might need the old_cgrp for every thread in the group.
644	
645	void fork(struct cgroup_subsy *ss, struct task_struct *task)
646	
647	Called when a task is forked into a cgroup.
648	
649	void exit(struct cgroup_subsys *ss, struct task_struct *task)
650	
651	Called during task exit.
652	
653	int populate(struct cgroup_subsys *ss, struct cgroup *cgrp)
654	(cgroup_mutex held by caller)
655	
656	Called after creation of a cgroup to allow a subsystem to populate
657	the cgroup directory with file entries.  The subsystem should make
658	calls to cgroup_add_file() with objects of type cftype (see
659	include/linux/cgroup.h for details).  Note that although this
660	method can return an error code, the error code is currently not
661	always handled well.
662	
663	void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp)
664	(cgroup_mutex held by caller)
665	
666	Called during cgroup_create() to do any parameter
667	initialization which might be required before a task could attach.  For
668	example in cpusets, no task may attach before 'cpus' and 'mems' are set
669	up.
670	
671	void bind(struct cgroup_subsys *ss, struct cgroup *root)
672	(cgroup_mutex and ss->hierarchy_mutex held by caller)
673	
674	Called when a cgroup subsystem is rebound to a different hierarchy
675	and root cgroup. Currently this will only involve movement between
676	the default hierarchy (which never has sub-cgroups) and a hierarchy
677	that is being created/destroyed (and hence has no sub-cgroups).
678	
679	4. Questions
680	============
681	
682	Q: what's up with this '/bin/echo' ?
683	A: bash's builtin 'echo' command does not check calls to write() against
684	   errors. If you use it in the cgroup file system, you won't be
685	   able to tell whether a command succeeded or failed.
686	
687	Q: When I attach processes, only the first of the line gets really attached !
688	A: We can only return one error code per call to write(). So you should also
689	   put only ONE pid.
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