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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 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	3. Kernel API
28	  3.1 Overview
29	  3.2 Synchronization
30	  3.3 Subsystem API
31	4. Extended attributes usage
32	5. 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) allow
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	up 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 goes
131	into the 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),
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 let's 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 student's 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, the administrator 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 thread group IDs in the cgroup.  This list is
237	   not 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, otherwise 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 settings. 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	This flag only affects the cpuset controller. If the clone_children
302	flag is enabled (1) in a cgroup, a new cpuset cgroup will copy its
303	configuration from the parent during initialization.
304	
305	1.6 How do I use cgroups ?
306	--------------------------
307	
308	To start a new job that is to be contained within a cgroup, using
309	the "cpuset" cgroup subsystem, the steps are something like:
310	
311	 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
315	    the /sys/fs/cgroup virtual file system.
316	 5) Start a task that will be the "founding father" of the new job.
317	 6) Attach that task to the new cgroup by writing its PID to the
318	    /sys/fs/cgroup/cpuset/tasks file for that cgroup.
319	 7) fork, exec or clone the job tasks from this founding father task.
320	
321	For example, the following sequence of commands will setup a cgroup
322	named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
323	and then start a subshell 'sh' in that cgroup:
324	
325	  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
329	  mkdir Charlie
330	  cd Charlie
331	  /bin/echo 2-3 > cpuset.cpus
332	  /bin/echo 1 > cpuset.mems
333	  /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	
339	2. Usage Examples and Syntax
340	============================
341	
342	2.1 Basic Usage
343	---------------
344	
345	Creating, modifying, using cgroups can be done through the cgroup
346	virtual filesystem.
347	
348	To mount a cgroup hierarchy with all available subsystems, type:
349	# mount -t cgroup xxx /sys/fs/cgroup
350	
351	The "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	
354	Note: Some subsystems do not work without some user input first.  For instance,
355	if cpusets are enabled the user will have to populate the cpus and mems files
356	for each new cgroup created before that group can be used.
357	
358	As explained in section `1.2 Why are cgroups needed?' you should create
359	different hierarchies of cgroups for each single resource or group of
360	resources you want to control. Therefore, you should mount a tmpfs on
361	/sys/fs/cgroup and create directories for each cgroup resource or resource
362	group.
363	
364	# mount -t tmpfs cgroup_root /sys/fs/cgroup
365	# mkdir /sys/fs/cgroup/rg1
366	
367	To mount a cgroup hierarchy with just the cpuset and memory
368	subsystems, type:
369	# mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1
370	
371	While remounting cgroups is currently supported, it is not recommend
372	to use it. Remounting allows changing bound subsystems and
373	release_agent. Rebinding is hardly useful as it only works when the
374	hierarchy is empty and release_agent itself should be replaced with
375	conventional fsnotify. The support for remounting will be removed in
376	the future.
377	
378	To Specify a hierarchy's release_agent:
379	# mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \
380	  xxx /sys/fs/cgroup/rg1
381	
382	Note that specifying 'release_agent' more than once will return failure.
383	
384	Note that changing the set of subsystems is currently only supported
385	when the hierarchy consists of a single (root) cgroup. Supporting
386	the ability to arbitrarily bind/unbind subsystems from an existing
387	cgroup hierarchy is intended to be implemented in the future.
388	
389	Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
390	tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
391	is the cgroup that holds the whole system.
392	
393	If you want to change the value of release_agent:
394	# echo "/sbin/new_release_agent" > /sys/fs/cgroup/rg1/release_agent
395	
396	It can also be changed via remount.
397	
398	If you want to create a new cgroup under /sys/fs/cgroup/rg1:
399	# cd /sys/fs/cgroup/rg1
400	# mkdir my_cgroup
401	
402	Now you want to do something with this cgroup.
403	# cd my_cgroup
404	
405	In this directory you can find several files:
406	# ls
407	cgroup.procs notify_on_release tasks
408	(plus whatever files added by the attached subsystems)
409	
410	Now attach your shell to this cgroup:
411	# /bin/echo $$ > tasks
412	
413	You can also create cgroups inside your cgroup by using mkdir in this
414	directory.
415	# mkdir my_sub_cs
416	
417	To remove a cgroup, just use rmdir:
418	# rmdir my_sub_cs
419	
420	This will fail if the cgroup is in use (has cgroups inside, or
421	has processes attached, or is held alive by other subsystem-specific
422	reference).
423	
424	2.2 Attaching processes
425	-----------------------
426	
427	# /bin/echo PID > tasks
428	
429	Note that it is PID, not PIDs. You can only attach ONE task at a time.
430	If 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	
437	You can attach the current shell task by echoing 0:
438	
439	# echo 0 > tasks
440	
441	You can use the cgroup.procs file instead of the tasks file to move all
442	threads in a threadgroup at once. Echoing the PID of any task in a
443	threadgroup to cgroup.procs causes all tasks in that threadgroup to be
444	attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
445	in the writing task's threadgroup.
446	
447	Note: Since every task is always a member of exactly one cgroup in each
448	mounted hierarchy, to remove a task from its current cgroup you must
449	move it into a new cgroup (possibly the root cgroup) by writing to the
450	new cgroup's tasks file.
451	
452	Note: Due to some restrictions enforced by some cgroup subsystems, moving
453	a process to another cgroup can fail.
454	
455	2.3 Mounting hierarchies by name
456	--------------------------------
457	
458	Passing the name=<x> option when mounting a cgroups hierarchy
459	associates the given name with the hierarchy.  This can be used when
460	mounting a pre-existing hierarchy, in order to refer to it by name
461	rather than by its set of active subsystems.  Each hierarchy is either
462	nameless, or has a unique name.
463	
464	The name should match [\w.-]+
465	
466	When passing a name=<x> option for a new hierarchy, you need to
467	specify subsystems manually; the legacy behaviour of mounting all
468	subsystems when none are explicitly specified is not supported when
469	you give a subsystem a name.
470	
471	The name of the subsystem appears as part of the hierarchy description
472	in /proc/mounts and /proc/<pid>/cgroups.
473	
474	
475	3. Kernel API
476	=============
477	
478	3.1 Overview
479	------------
480	
481	Each kernel subsystem that wants to hook into the generic cgroup
482	system needs to create a cgroup_subsys object. This contains
483	various methods, which are callbacks from the cgroup system, along
484	with a subsystem ID which will be assigned by the cgroup system.
485	
486	Other fields in the cgroup_subsys object include:
487	
488	- subsys_id: a unique array index for the subsystem, indicating which
489	  entry in cgroup->subsys[] this subsystem should be managing.
490	
491	- name: should be initialized to a unique subsystem name. Should be
492	  no longer than MAX_CGROUP_TYPE_NAMELEN.
493	
494	- early_init: indicate if the subsystem needs early initialization
495	  at system boot.
496	
497	Each cgroup object created by the system has an array of pointers,
498	indexed by subsystem ID; this pointer is entirely managed by the
499	subsystem; the generic cgroup code will never touch this pointer.
500	
501	3.2 Synchronization
502	-------------------
503	
504	There is a global mutex, cgroup_mutex, used by the cgroup
505	system. This should be taken by anything that wants to modify a
506	cgroup. It may also be taken to prevent cgroups from being
507	modified, but more specific locks may be more appropriate in that
508	situation.
509	
510	See kernel/cgroup.c for more details.
511	
512	Subsystems can take/release the cgroup_mutex via the functions
513	cgroup_lock()/cgroup_unlock().
514	
515	Accessing 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	
520	3.3 Subsystem API
521	-----------------
522	
523	Each subsystem should:
524	
525	- add an entry in linux/cgroup_subsys.h
526	- define a cgroup_subsys object called <name>_subsys
527	
528	If a subsystem can be compiled as a module, it should also have in its
529	module initcall a call to cgroup_load_subsys(), and in its exitcall a
530	call to cgroup_unload_subsys(). It should also set its_subsys.module =
531	THIS_MODULE in its .c file.
532	
533	Each subsystem may export the following methods. The only mandatory
534	methods are css_alloc/free. Any others that are null are presumed to
535	be successful no-ops.
536	
537	struct cgroup_subsys_state *css_alloc(struct cgroup *cgrp)
538	(cgroup_mutex held by caller)
539	
540	Called to allocate a subsystem state object for a cgroup. The
541	subsystem should allocate its subsystem state object for the passed
542	cgroup, returning a pointer to the new object on success or a
543	ERR_PTR() value. On success, the subsystem pointer should point to
544	a structure of type cgroup_subsys_state (typically embedded in a
545	larger subsystem-specific object), which will be initialized by the
546	cgroup system. Note that this will be called at initialization to
547	create the root subsystem state for this subsystem; this case can be
548	identified by the passed cgroup object having a NULL parent (since
549	it's the root of the hierarchy) and may be an appropriate place for
550	initialization code.
551	
552	int css_online(struct cgroup *cgrp)
553	(cgroup_mutex held by caller)
554	
555	Called after @cgrp successfully completed all allocations and made
556	visible to cgroup_for_each_child/descendant_*() iterators. The
557	subsystem may choose to fail creation by returning -errno. This
558	callback can be used to implement reliable state sharing and
559	propagation along the hierarchy. See the comment on
560	cgroup_for_each_descendant_pre() for details.
561	
562	void css_offline(struct cgroup *cgrp);
563	(cgroup_mutex held by caller)
564	
565	This is the counterpart of css_online() and called iff css_online()
566	has succeeded on @cgrp. This signifies the beginning of the end of
567	@cgrp. @cgrp is being removed and the subsystem should start dropping
568	all references it's holding on @cgrp. When all references are dropped,
569	cgroup removal will proceed to the next step - css_free(). After this
570	callback, @cgrp should be considered dead to the subsystem.
571	
572	void css_free(struct cgroup *cgrp)
573	(cgroup_mutex held by caller)
574	
575	The cgroup system is about to free @cgrp; the subsystem should free
576	its subsystem state object. By the time this method is called, @cgrp
577	is completely unused; @cgrp->parent is still valid. (Note - can also
578	be called for a newly-created cgroup if an error occurs after this
579	subsystem's create() method has been called for the new cgroup).
580	
581	int can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
582	(cgroup_mutex held by caller)
583	
584	Called prior to moving one or more tasks into a cgroup; if the
585	subsystem returns an error, this will abort the attach operation.
586	@tset contains the tasks to be attached and is guaranteed to have at
587	least one task in it.
588	
589	If 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	
595	Each @tset entry also contains the task's old cgroup and tasks which
596	aren't switching cgroup can be skipped easily using the
597	cgroup_taskset_for_each() iterator. Note that this isn't called on a
598	fork. If this method returns 0 (success) then this should remain valid
599	while the caller holds cgroup_mutex and it is ensured that either
600	attach() or cancel_attach() will be called in future.
601	
602	void cancel_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
603	(cgroup_mutex held by caller)
604	
605	Called when a task attach operation has failed after can_attach() has succeeded.
606	A subsystem whose can_attach() has some side-effects should provide this
607	function, so that the subsystem can implement a rollback. If not, not necessary.
608	This will be called only about subsystems whose can_attach() operation have
609	succeeded. The parameters are identical to can_attach().
610	
611	void attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
612	(cgroup_mutex held by caller)
613	
614	Called after the task has been attached to the cgroup, to allow any
615	post-attachment activity that requires memory allocations or blocking.
616	The parameters are identical to can_attach().
617	
618	void fork(struct task_struct *task)
619	
620	Called when a task is forked into a cgroup.
621	
622	void exit(struct task_struct *task)
623	
624	Called during task exit.
625	
626	void bind(struct cgroup *root)
627	(cgroup_mutex held by caller)
628	
629	Called when a cgroup subsystem is rebound to a different hierarchy
630	and root cgroup. Currently this will only involve movement between
631	the default hierarchy (which never has sub-cgroups) and a hierarchy
632	that is being created/destroyed (and hence has no sub-cgroups).
633	
634	4. Extended attribute usage
635	===========================
636	
637	cgroup filesystem supports certain types of extended attributes in its
638	directories and files.  The current supported types are:
639		- Trusted (XATTR_TRUSTED)
640		- Security (XATTR_SECURITY)
641	
642	Both require CAP_SYS_ADMIN capability to set.
643	
644	Like in tmpfs, the extended attributes in cgroup filesystem are stored
645	using kernel memory and it's advised to keep the usage at minimum.  This
646	is the reason why user defined extended attributes are not supported, since
647	any user can do it and there's no limit in the value size.
648	
649	The current known users for this feature are SELinux to limit cgroup usage
650	in containers and systemd for assorted meta data like main PID in a cgroup
651	(systemd creates a cgroup per service).
652	
653	5. Questions
654	============
655	
656	Q: what's up with this '/bin/echo' ?
657	A: bash's builtin 'echo' command does not check calls to write() against
658	   errors. If you use it in the cgroup file system, you won't be
659	   able to tell whether a command succeeded or failed.
660	
661	Q: When I attach processes, only the first of the line gets really attached !
662	A: We can only return one error code per call to write(). So you should also
663	   put only ONE PID.
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