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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 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. Extended attributes usage
33	5. Questions
34	
35	1. Control Groups
36	=================
37	
38	1.1 What are cgroups ?
39	----------------------
40	
41	Control Groups provide a mechanism for aggregating/partitioning sets of
42	tasks, and all their future children, into hierarchical groups with
43	specialized behaviour.
44	
45	Definitions:
46	
47	A *cgroup* associates a set of tasks with a set of parameters for one
48	or more subsystems.
49	
50	A *subsystem* is a module that makes use of the task grouping
51	facilities provided by cgroups to treat groups of tasks in
52	particular ways. A subsystem is typically a "resource controller" that
53	schedules a resource or applies per-cgroup limits, but it may be
54	anything that wants to act on a group of processes, e.g. a
55	virtualization subsystem.
56	
57	A *hierarchy* is a set of cgroups arranged in a tree, such that
58	every task in the system is in exactly one of the cgroups in the
59	hierarchy, and a set of subsystems; each subsystem has system-specific
60	state attached to each cgroup in the hierarchy.  Each hierarchy has
61	an instance of the cgroup virtual filesystem associated with it.
62	
63	At any one time there may be multiple active hierarchies of task
64	cgroups. Each hierarchy is a partition of all tasks in the system.
65	
66	User-level code may create and destroy cgroups by name in an
67	instance of the cgroup virtual file system, specify and query to
68	which cgroup a task is assigned, and list the task PIDs assigned to
69	a cgroup. Those creations and assignments only affect the hierarchy
70	associated with that instance of the cgroup file system.
71	
72	On their own, the only use for cgroups is for simple job
73	tracking. The intention is that other subsystems hook into the generic
74	cgroup support to provide new attributes for cgroups, such as
75	accounting/limiting the resources which processes in a cgroup can
76	access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allow
77	you to associate a set of CPUs and a set of memory nodes with the
78	tasks in each cgroup.
79	
80	1.2 Why are cgroups needed ?
81	----------------------------
82	
83	There are multiple efforts to provide process aggregations in the
84	Linux kernel, mainly for resource-tracking purposes. Such efforts
85	include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server
86	namespaces. These all require the basic notion of a
87	grouping/partitioning of processes, with newly forked processes ending
88	up in the same group (cgroup) as their parent process.
89	
90	The kernel cgroup patch provides the minimum essential kernel
91	mechanisms required to efficiently implement such groups. It has
92	minimal impact on the system fast paths, and provides hooks for
93	specific subsystems such as cpusets to provide additional behaviour as
94	desired.
95	
96	Multiple hierarchy support is provided to allow for situations where
97	the division of tasks into cgroups is distinctly different for
98	different subsystems - having parallel hierarchies allows each
99	hierarchy to be a natural division of tasks, without having to handle
100	complex combinations of tasks that would be present if several
101	unrelated subsystems needed to be forced into the same tree of
102	cgroups.
103	
104	At one extreme, each resource controller or subsystem could be in a
105	separate hierarchy; at the other extreme, all subsystems
106	would be attached to the same hierarchy.
107	
108	As an example of a scenario (originally proposed by vatsa@in.ibm.com)
109	that can benefit from multiple hierarchies, consider a large
110	university server with various users - students, professors, system
111	tasks etc. The resource planning for this server could be along the
112	following lines:
113	
114	       CPU :          "Top cpuset"
115	                       /       \
116	               CPUSet1         CPUSet2
117	                  |               |
118	               (Professors)    (Students)
119	
120	               In addition (system tasks) are attached to topcpuset (so
121	               that they can run anywhere) with a limit of 20%
122	
123	       Memory : Professors (50%), Students (30%), system (20%)
124	
125	       Disk : Professors (50%), Students (30%), system (20%)
126	
127	       Network : WWW browsing (20%), Network File System (60%), others (20%)
128	                               / \
129	               Professors (15%)  students (5%)
130	
131	Browsers like Firefox/Lynx go into the WWW network class, while (k)nfsd goes
132	into the NFS network class.
133	
134	At the same time Firefox/Lynx will share an appropriate CPU/Memory class
135	depending on who launched it (prof/student).
136	
137	With the ability to classify tasks differently for different resources
138	(by putting those resource subsystems in different hierarchies),
139	the admin can easily set up a script which receives exec notifications
140	and depending on who is launching the browser he can
141	
142	    # echo browser_pid > /sys/fs/cgroup/<restype>/<userclass>/tasks
143	
144	With only a single hierarchy, he now would potentially have to create
145	a separate cgroup for every browser launched and associate it with
146	appropriate network and other resource class.  This may lead to
147	proliferation of such cgroups.
148	
149	Also let's say that the administrator would like to give enhanced network
150	access temporarily to a student's browser (since it is night and the user
151	wants to do online gaming :))  OR give one of the student's simulation
152	apps enhanced CPU power.
153	
154	With ability to write PIDs directly to resource classes, it's just a
155	matter of:
156	
157	       # echo pid > /sys/fs/cgroup/network/<new_class>/tasks
158	       (after some time)
159	       # echo pid > /sys/fs/cgroup/network/<orig_class>/tasks
160	
161	Without this ability, the administrator would have to split the cgroup into
162	multiple separate ones and then associate the new cgroups with the
163	new resource classes.
164	
165	
166	
167	1.3 How are cgroups implemented ?
168	---------------------------------
169	
170	Control Groups extends the kernel as follows:
171	
172	 - Each task in the system has a reference-counted pointer to a
173	   css_set.
174	
175	 - A css_set contains a set of reference-counted pointers to
176	   cgroup_subsys_state objects, one for each cgroup subsystem
177	   registered in the system. There is no direct link from a task to
178	   the cgroup of which it's a member in each hierarchy, but this
179	   can be determined by following pointers through the
180	   cgroup_subsys_state objects. This is because accessing the
181	   subsystem state is something that's expected to happen frequently
182	   and in performance-critical code, whereas operations that require a
183	   task's actual cgroup assignments (in particular, moving between
184	   cgroups) are less common. A linked list runs through the cg_list
185	   field of each task_struct using the css_set, anchored at
186	   css_set->tasks.
187	
188	 - A cgroup hierarchy filesystem can be mounted for browsing and
189	   manipulation from user space.
190	
191	 - You can list all the tasks (by PID) attached to any cgroup.
192	
193	The implementation of cgroups requires a few, simple hooks
194	into the rest of the kernel, none in performance-critical paths:
195	
196	 - in init/main.c, to initialize the root cgroups and initial
197	   css_set at system boot.
198	
199	 - in fork and exit, to attach and detach a task from its css_set.
200	
201	In addition, a new file system of type "cgroup" may be mounted, to
202	enable browsing and modifying the cgroups presently known to the
203	kernel.  When mounting a cgroup hierarchy, you may specify a
204	comma-separated list of subsystems to mount as the filesystem mount
205	options.  By default, mounting the cgroup filesystem attempts to
206	mount a hierarchy containing all registered subsystems.
207	
208	If an active hierarchy with exactly the same set of subsystems already
209	exists, it will be reused for the new mount. If no existing hierarchy
210	matches, and any of the requested subsystems are in use in an existing
211	hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
212	is activated, associated with the requested subsystems.
213	
214	It's not currently possible to bind a new subsystem to an active
215	cgroup hierarchy, or to unbind a subsystem from an active cgroup
216	hierarchy. This may be possible in future, but is fraught with nasty
217	error-recovery issues.
218	
219	When a cgroup filesystem is unmounted, if there are any
220	child cgroups created below the top-level cgroup, that hierarchy
221	will remain active even though unmounted; if there are no
222	child cgroups then the hierarchy will be deactivated.
223	
224	No new system calls are added for cgroups - all support for
225	querying and modifying cgroups is via this cgroup file system.
226	
227	Each task under /proc has an added file named 'cgroup' displaying,
228	for each active hierarchy, the subsystem names and the cgroup name
229	as the path relative to the root of the cgroup file system.
230	
231	Each cgroup is represented by a directory in the cgroup file system
232	containing the following files describing that cgroup:
233	
234	 - tasks: list of tasks (by PID) attached to that cgroup.  This list
235	   is not guaranteed to be sorted.  Writing a thread ID into this file
236	   moves the thread into this cgroup.
237	 - cgroup.procs: list of thread group IDs in the cgroup.  This list is
238	   not guaranteed to be sorted or free of duplicate TGIDs, and userspace
239	   should sort/uniquify the list if this property is required.
240	   Writing a thread group ID into this file moves all threads in that
241	   group into this cgroup.
242	 - notify_on_release flag: run the release agent on exit?
243	 - release_agent: the path to use for release notifications (this file
244	   exists in the top cgroup only)
245	
246	Other subsystems such as cpusets may add additional files in each
247	cgroup dir.
248	
249	New cgroups are created using the mkdir system call or shell
250	command.  The properties of a cgroup, such as its flags, are
251	modified by writing to the appropriate file in that cgroups
252	directory, as listed above.
253	
254	The named hierarchical structure of nested cgroups allows partitioning
255	a large system into nested, dynamically changeable, "soft-partitions".
256	
257	The attachment of each task, automatically inherited at fork by any
258	children of that task, to a cgroup allows organizing the work load
259	on a system into related sets of tasks.  A task may be re-attached to
260	any other cgroup, if allowed by the permissions on the necessary
261	cgroup file system directories.
262	
263	When a task is moved from one cgroup to another, it gets a new
264	css_set pointer - if there's an already existing css_set with the
265	desired collection of cgroups then that group is reused, otherwise a new
266	css_set is allocated. The appropriate existing css_set is located by
267	looking into a hash table.
268	
269	To allow access from a cgroup to the css_sets (and hence tasks)
270	that comprise it, a set of cg_cgroup_link objects form a lattice;
271	each cg_cgroup_link is linked into a list of cg_cgroup_links for
272	a single cgroup on its cgrp_link_list field, and a list of
273	cg_cgroup_links for a single css_set on its cg_link_list.
274	
275	Thus the set of tasks in a cgroup can be listed by iterating over
276	each css_set that references the cgroup, and sub-iterating over
277	each css_set's task set.
278	
279	The use of a Linux virtual file system (vfs) to represent the
280	cgroup hierarchy provides for a familiar permission and name space
281	for cgroups, with a minimum of additional kernel code.
282	
283	1.4 What does notify_on_release do ?
284	------------------------------------
285	
286	If the notify_on_release flag is enabled (1) in a cgroup, then
287	whenever the last task in the cgroup leaves (exits or attaches to
288	some other cgroup) and the last child cgroup of that cgroup
289	is removed, then the kernel runs the command specified by the contents
290	of the "release_agent" file in that hierarchy's root directory,
291	supplying the pathname (relative to the mount point of the cgroup
292	file system) of the abandoned cgroup.  This enables automatic
293	removal of abandoned cgroups.  The default value of
294	notify_on_release in the root cgroup at system boot is disabled
295	(0).  The default value of other cgroups at creation is the current
296	value of their parents' notify_on_release settings. The default value of
297	a cgroup hierarchy's release_agent path is empty.
298	
299	1.5 What does clone_children do ?
300	---------------------------------
301	
302	This flag only affects the cpuset controller. If the clone_children
303	flag is enabled (1) in a cgroup, a new cpuset cgroup will copy its
304	configuration from the parent during initialization.
305	
306	1.6 How do I use cgroups ?
307	--------------------------
308	
309	To start a new job that is to be contained within a cgroup, using
310	the "cpuset" cgroup subsystem, the steps are something like:
311	
312	 1) mount -t tmpfs cgroup_root /sys/fs/cgroup
313	 2) mkdir /sys/fs/cgroup/cpuset
314	 3) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
315	 4) Create the new cgroup by doing mkdir's and write's (or echo's) in
316	    the /sys/fs/cgroup virtual file system.
317	 5) Start a task that will be the "founding father" of the new job.
318	 6) Attach that task to the new cgroup by writing its PID to the
319	    /sys/fs/cgroup/cpuset/tasks file for that cgroup.
320	 7) fork, exec or clone the job tasks from this founding father task.
321	
322	For example, the following sequence of commands will setup a cgroup
323	named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
324	and then start a subshell 'sh' in that cgroup:
325	
326	  mount -t tmpfs cgroup_root /sys/fs/cgroup
327	  mkdir /sys/fs/cgroup/cpuset
328	  mount -t cgroup cpuset -ocpuset /sys/fs/cgroup/cpuset
329	  cd /sys/fs/cgroup/cpuset
330	  mkdir Charlie
331	  cd Charlie
332	  /bin/echo 2-3 > cpuset.cpus
333	  /bin/echo 1 > cpuset.mems
334	  /bin/echo $$ > tasks
335	  sh
336	  # The subshell 'sh' is now running in cgroup Charlie
337	  # The next line should display '/Charlie'
338	  cat /proc/self/cgroup
339	
340	2. Usage Examples and Syntax
341	============================
342	
343	2.1 Basic Usage
344	---------------
345	
346	Creating, modifying, using cgroups can be done through the cgroup
347	virtual filesystem.
348	
349	To mount a cgroup hierarchy with all available subsystems, type:
350	# mount -t cgroup xxx /sys/fs/cgroup
351	
352	The "xxx" is not interpreted by the cgroup code, but will appear in
353	/proc/mounts so may be any useful identifying string that you like.
354	
355	Note: Some subsystems do not work without some user input first.  For instance,
356	if cpusets are enabled the user will have to populate the cpus and mems files
357	for each new cgroup created before that group can be used.
358	
359	As explained in section `1.2 Why are cgroups needed?' you should create
360	different hierarchies of cgroups for each single resource or group of
361	resources you want to control. Therefore, you should mount a tmpfs on
362	/sys/fs/cgroup and create directories for each cgroup resource or resource
363	group.
364	
365	# mount -t tmpfs cgroup_root /sys/fs/cgroup
366	# mkdir /sys/fs/cgroup/rg1
367	
368	To mount a cgroup hierarchy with just the cpuset and memory
369	subsystems, type:
370	# mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1
371	
372	While remounting cgroups is currently supported, it is not recommend
373	to use it. Remounting allows changing bound subsystems and
374	release_agent. Rebinding is hardly useful as it only works when the
375	hierarchy is empty and release_agent itself should be replaced with
376	conventional fsnotify. The support for remounting will be removed in
377	the future.
378	
379	To Specify a hierarchy's release_agent:
380	# mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \
381	  xxx /sys/fs/cgroup/rg1
382	
383	Note that specifying 'release_agent' more than once will return failure.
384	
385	Note that changing the set of subsystems is currently only supported
386	when the hierarchy consists of a single (root) cgroup. Supporting
387	the ability to arbitrarily bind/unbind subsystems from an existing
388	cgroup hierarchy is intended to be implemented in the future.
389	
390	Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
391	tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
392	is the cgroup that holds the whole system.
393	
394	If you want to change the value of release_agent:
395	# echo "/sbin/new_release_agent" > /sys/fs/cgroup/rg1/release_agent
396	
397	It can also be changed via remount.
398	
399	If you want to create a new cgroup under /sys/fs/cgroup/rg1:
400	# cd /sys/fs/cgroup/rg1
401	# mkdir my_cgroup
402	
403	Now you want to do something with this cgroup.
404	# cd my_cgroup
405	
406	In this directory you can find several files:
407	# ls
408	cgroup.procs notify_on_release tasks
409	(plus whatever files added by the attached subsystems)
410	
411	Now attach your shell to this cgroup:
412	# /bin/echo $$ > tasks
413	
414	You can also create cgroups inside your cgroup by using mkdir in this
415	directory.
416	# mkdir my_sub_cs
417	
418	To remove a cgroup, just use rmdir:
419	# rmdir my_sub_cs
420	
421	This will fail if the cgroup is in use (has cgroups inside, or
422	has processes attached, or is held alive by other subsystem-specific
423	reference).
424	
425	2.2 Attaching processes
426	-----------------------
427	
428	# /bin/echo PID > tasks
429	
430	Note that it is PID, not PIDs. You can only attach ONE task at a time.
431	If you have several tasks to attach, you have to do it one after another:
432	
433	# /bin/echo PID1 > tasks
434	# /bin/echo PID2 > tasks
435		...
436	# /bin/echo PIDn > tasks
437	
438	You can attach the current shell task by echoing 0:
439	
440	# echo 0 > tasks
441	
442	You can use the cgroup.procs file instead of the tasks file to move all
443	threads in a threadgroup at once. Echoing the PID of any task in a
444	threadgroup to cgroup.procs causes all tasks in that threadgroup to be
445	attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
446	in the writing task's threadgroup.
447	
448	Note: Since every task is always a member of exactly one cgroup in each
449	mounted hierarchy, to remove a task from its current cgroup you must
450	move it into a new cgroup (possibly the root cgroup) by writing to the
451	new cgroup's tasks file.
452	
453	Note: Due to some restrictions enforced by some cgroup subsystems, moving
454	a process to another cgroup can fail.
455	
456	2.3 Mounting hierarchies by name
457	--------------------------------
458	
459	Passing the name=<x> option when mounting a cgroups hierarchy
460	associates the given name with the hierarchy.  This can be used when
461	mounting a pre-existing hierarchy, in order to refer to it by name
462	rather than by its set of active subsystems.  Each hierarchy is either
463	nameless, or has a unique name.
464	
465	The name should match [\w.-]+
466	
467	When passing a name=<x> option for a new hierarchy, you need to
468	specify subsystems manually; the legacy behaviour of mounting all
469	subsystems when none are explicitly specified is not supported when
470	you give a subsystem a name.
471	
472	The name of the subsystem appears as part of the hierarchy description
473	in /proc/mounts and /proc/<pid>/cgroups.
474	
475	2.4 Notification API
476	--------------------
477	
478	There is mechanism which allows to get notifications about changing
479	status of a cgroup.
480	
481	To register a new notification handler you need to:
482	 - create a file descriptor for event notification using eventfd(2);
483	 - open a control file to be monitored (e.g. memory.usage_in_bytes);
484	 - write "<event_fd> <control_fd> <args>" to cgroup.event_control.
485	   Interpretation of args is defined by control file implementation;
486	
487	eventfd will be woken up by control file implementation or when the
488	cgroup is removed.
489	
490	To unregister a notification handler just close eventfd.
491	
492	NOTE: Support of notifications should be implemented for the control
493	file. See documentation for the subsystem.
494	
495	3. Kernel API
496	=============
497	
498	3.1 Overview
499	------------
500	
501	Each kernel subsystem that wants to hook into the generic cgroup
502	system needs to create a cgroup_subsys object. This contains
503	various methods, which are callbacks from the cgroup system, along
504	with a subsystem ID which will be assigned by the cgroup system.
505	
506	Other fields in the cgroup_subsys object include:
507	
508	- subsys_id: a unique array index for the subsystem, indicating which
509	  entry in cgroup->subsys[] this subsystem should be managing.
510	
511	- name: should be initialized to a unique subsystem name. Should be
512	  no longer than MAX_CGROUP_TYPE_NAMELEN.
513	
514	- early_init: indicate if the subsystem needs early initialization
515	  at system boot.
516	
517	Each cgroup object created by the system has an array of pointers,
518	indexed by subsystem ID; this pointer is entirely managed by the
519	subsystem; the generic cgroup code will never touch this pointer.
520	
521	3.2 Synchronization
522	-------------------
523	
524	There is a global mutex, cgroup_mutex, used by the cgroup
525	system. This should be taken by anything that wants to modify a
526	cgroup. It may also be taken to prevent cgroups from being
527	modified, but more specific locks may be more appropriate in that
528	situation.
529	
530	See kernel/cgroup.c for more details.
531	
532	Subsystems can take/release the cgroup_mutex via the functions
533	cgroup_lock()/cgroup_unlock().
534	
535	Accessing a task's cgroup pointer may be done in the following ways:
536	- while holding cgroup_mutex
537	- while holding the task's alloc_lock (via task_lock())
538	- inside an rcu_read_lock() section via rcu_dereference()
539	
540	3.3 Subsystem API
541	-----------------
542	
543	Each subsystem should:
544	
545	- add an entry in linux/cgroup_subsys.h
546	- define a cgroup_subsys object called <name>_subsys
547	
548	If a subsystem can be compiled as a module, it should also have in its
549	module initcall a call to cgroup_load_subsys(), and in its exitcall a
550	call to cgroup_unload_subsys(). It should also set its_subsys.module =
551	THIS_MODULE in its .c file.
552	
553	Each subsystem may export the following methods. The only mandatory
554	methods are css_alloc/free. Any others that are null are presumed to
555	be successful no-ops.
556	
557	struct cgroup_subsys_state *css_alloc(struct cgroup *cgrp)
558	(cgroup_mutex held by caller)
559	
560	Called to allocate a subsystem state object for a cgroup. The
561	subsystem should allocate its subsystem state object for the passed
562	cgroup, returning a pointer to the new object on success or a
563	ERR_PTR() value. On success, the subsystem pointer should point to
564	a structure of type cgroup_subsys_state (typically embedded in a
565	larger subsystem-specific object), which will be initialized by the
566	cgroup system. Note that this will be called at initialization to
567	create the root subsystem state for this subsystem; this case can be
568	identified by the passed cgroup object having a NULL parent (since
569	it's the root of the hierarchy) and may be an appropriate place for
570	initialization code.
571	
572	int css_online(struct cgroup *cgrp)
573	(cgroup_mutex held by caller)
574	
575	Called after @cgrp successfully completed all allocations and made
576	visible to cgroup_for_each_child/descendant_*() iterators. The
577	subsystem may choose to fail creation by returning -errno. This
578	callback can be used to implement reliable state sharing and
579	propagation along the hierarchy. See the comment on
580	cgroup_for_each_descendant_pre() for details.
581	
582	void css_offline(struct cgroup *cgrp);
583	(cgroup_mutex held by caller)
584	
585	This is the counterpart of css_online() and called iff css_online()
586	has succeeded on @cgrp. This signifies the beginning of the end of
587	@cgrp. @cgrp is being removed and the subsystem should start dropping
588	all references it's holding on @cgrp. When all references are dropped,
589	cgroup removal will proceed to the next step - css_free(). After this
590	callback, @cgrp should be considered dead to the subsystem.
591	
592	void css_free(struct cgroup *cgrp)
593	(cgroup_mutex held by caller)
594	
595	The cgroup system is about to free @cgrp; the subsystem should free
596	its subsystem state object. By the time this method is called, @cgrp
597	is completely unused; @cgrp->parent is still valid. (Note - can also
598	be called for a newly-created cgroup if an error occurs after this
599	subsystem's create() method has been called for the new cgroup).
600	
601	int can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
602	(cgroup_mutex held by caller)
603	
604	Called prior to moving one or more tasks into a cgroup; if the
605	subsystem returns an error, this will abort the attach operation.
606	@tset contains the tasks to be attached and is guaranteed to have at
607	least one task in it.
608	
609	If there are multiple tasks in the taskset, then:
610	  - it's guaranteed that all are from the same thread group
611	  - @tset contains all tasks from the thread group whether or not
612	    they're switching cgroups
613	  - the first task is the leader
614	
615	Each @tset entry also contains the task's old cgroup and tasks which
616	aren't switching cgroup can be skipped easily using the
617	cgroup_taskset_for_each() iterator. Note that this isn't called on a
618	fork. If this method returns 0 (success) then this should remain valid
619	while the caller holds cgroup_mutex and it is ensured that either
620	attach() or cancel_attach() will be called in future.
621	
622	void cancel_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
623	(cgroup_mutex held by caller)
624	
625	Called when a task attach operation has failed after can_attach() has succeeded.
626	A subsystem whose can_attach() has some side-effects should provide this
627	function, so that the subsystem can implement a rollback. If not, not necessary.
628	This will be called only about subsystems whose can_attach() operation have
629	succeeded. The parameters are identical to can_attach().
630	
631	void attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
632	(cgroup_mutex held by caller)
633	
634	Called after the task has been attached to the cgroup, to allow any
635	post-attachment activity that requires memory allocations or blocking.
636	The parameters are identical to can_attach().
637	
638	void fork(struct task_struct *task)
639	
640	Called when a task is forked into a cgroup.
641	
642	void exit(struct task_struct *task)
643	
644	Called during task exit.
645	
646	void bind(struct cgroup *root)
647	(cgroup_mutex held by caller)
648	
649	Called when a cgroup subsystem is rebound to a different hierarchy
650	and root cgroup. Currently this will only involve movement between
651	the default hierarchy (which never has sub-cgroups) and a hierarchy
652	that is being created/destroyed (and hence has no sub-cgroups).
653	
654	4. Extended attribute usage
655	===========================
656	
657	cgroup filesystem supports certain types of extended attributes in its
658	directories and files.  The current supported types are:
659		- Trusted (XATTR_TRUSTED)
660		- Security (XATTR_SECURITY)
661	
662	Both require CAP_SYS_ADMIN capability to set.
663	
664	Like in tmpfs, the extended attributes in cgroup filesystem are stored
665	using kernel memory and it's advised to keep the usage at minimum.  This
666	is the reason why user defined extended attributes are not supported, since
667	any user can do it and there's no limit in the value size.
668	
669	The current known users for this feature are SELinux to limit cgroup usage
670	in containers and systemd for assorted meta data like main PID in a cgroup
671	(systemd creates a cgroup per service).
672	
673	5. Questions
674	============
675	
676	Q: what's up with this '/bin/echo' ?
677	A: bash's builtin 'echo' command does not check calls to write() against
678	   errors. If you use it in the cgroup file system, you won't be
679	   able to tell whether a command succeeded or failed.
680	
681	Q: When I attach processes, only the first of the line gets really attached !
682	A: We can only return one error code per call to write(). So you should also
683	   put only ONE PID.
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