About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / cgroups.txt




Custom Search

Based on kernel version 2.6.27. Page generated on 2008-10-13 09:53 EST.

1					CGROUPS
2					-------
3	
4	Written by Paul Menage <menage[AT]google.com> based on Documentation/cpusets[DOT]txt
5	
6	Original copyright statements from cpusets.txt:
7	Portions Copyright (C) 2004 BULL SA.
8	Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
9	Modified by Paul Jackson <pj[AT]sgi[DOT]com>
10	Modified by Christoph Lameter <clameter[AT]sgi[DOT]com>
11	
12	CONTENTS:
13	=========
14	
15	1. Control Groups
16	  1.1 What are cgroups ?
17	  1.2 Why are cgroups needed ?
18	  1.3 How are cgroups implemented ?
19	  1.4 What does notify_on_release do ?
20	  1.5 How do I use cgroups ?
21	2. Usage Examples and Syntax
22	  2.1 Basic Usage
23	  2.2 Attaching processes
24	3. Kernel API
25	  3.1 Overview
26	  3.2 Synchronization
27	  3.3 Subsystem API
28	4. Questions
29	
30	1. Control Groups
31	=================
32	
33	1.1 What are cgroups ?
34	----------------------
35	
36	Control Groups provide a mechanism for aggregating/partitioning sets of
37	tasks, and all their future children, into hierarchical groups with
38	specialized behaviour.
39	
40	Definitions:
41	
42	A *cgroup* associates a set of tasks with a set of parameters for one
43	or more subsystems.
44	
45	A *subsystem* is a module that makes use of the task grouping
46	facilities provided by cgroups to treat groups of tasks in
47	particular ways. A subsystem is typically a "resource controller" that
48	schedules a resource or applies per-cgroup limits, but it may be
49	anything that wants to act on a group of processes, e.g. a
50	virtualization subsystem.
51	
52	A *hierarchy* is a set of cgroups arranged in a tree, such that
53	every task in the system is in exactly one of the cgroups in the
54	hierarchy, and a set of subsystems; each subsystem has system-specific
55	state attached to each cgroup in the hierarchy.  Each hierarchy has
56	an instance of the cgroup virtual filesystem associated with it.
57	
58	At any one time there may be multiple active hierachies of task
59	cgroups. Each hierarchy is a partition of all tasks in the system.
60	
61	User level code may create and destroy cgroups by name in an
62	instance of the cgroup virtual file system, specify and query to
63	which cgroup a task is assigned, and list the task pids assigned to
64	a cgroup. Those creations and assignments only affect the hierarchy
65	associated with that instance of the cgroup file system.
66	
67	On their own, the only use for cgroups is for simple job
68	tracking. The intention is that other subsystems hook into the generic
69	cgroup support to provide new attributes for cgroups, such as
70	accounting/limiting the resources which processes in a cgroup can
71	access. For example, cpusets (see Documentation/cpusets.txt) allows
72	you to associate a set of CPUs and a set of memory nodes with the
73	tasks in each cgroup.
74	
75	1.2 Why are cgroups needed ?
76	----------------------------
77	
78	There are multiple efforts to provide process aggregations in the
79	Linux kernel, mainly for resource tracking purposes. Such efforts
80	include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server
81	namespaces. These all require the basic notion of a
82	grouping/partitioning of processes, with newly forked processes ending
83	in the same group (cgroup) as their parent process.
84	
85	The kernel cgroup patch provides the minimum essential kernel
86	mechanisms required to efficiently implement such groups. It has
87	minimal impact on the system fast paths, and provides hooks for
88	specific subsystems such as cpusets to provide additional behaviour as
89	desired.
90	
91	Multiple hierarchy support is provided to allow for situations where
92	the division of tasks into cgroups is distinctly different for
93	different subsystems - having parallel hierarchies allows each
94	hierarchy to be a natural division of tasks, without having to handle
95	complex combinations of tasks that would be present if several
96	unrelated subsystems needed to be forced into the same tree of
97	cgroups.
98	
99	At one extreme, each resource controller or subsystem could be in a
100	separate hierarchy; at the other extreme, all subsystems
101	would be attached to the same hierarchy.
102	
103	As an example of a scenario (originally proposed by vatsa[AT]in.ibm[DOT]com)
104	that can benefit from multiple hierarchies, consider a large
105	university server with various users - students, professors, system
106	tasks etc. The resource planning for this server could be along the
107	following lines:
108	
109	       CPU :           Top cpuset
110	                       /       \
111	               CPUSet1         CPUSet2
112	                  |              |
113	               (Profs)         (Students)
114	
115	               In addition (system tasks) are attached to topcpuset (so
116	               that they can run anywhere) with a limit of 20%
117	
118	       Memory : Professors (50%), students (30%), system (20%)
119	
120	       Disk : Prof (50%), students (30%), system (20%)
121	
122	       Network : WWW browsing (20%), Network File System (60%), others (20%)
123	                               / \
124	                       Prof (15%) students (5%)
125	
126	Browsers like firefox/lynx go into the WWW network class, while (k)nfsd go
127	into NFS network class.
128	
129	At the same time firefox/lynx will share an appropriate CPU/Memory class
130	depending on who launched it (prof/student).
131	
132	With the ability to classify tasks differently for different resources
133	(by putting those resource subsystems in different hierarchies) then
134	the admin can easily set up a script which receives exec notifications
135	and depending on who is launching the browser he can
136	
137	       # echo browser_pid > /mnt/<restype>/<userclass>/tasks
138	
139	With only a single hierarchy, he now would potentially have to create
140	a separate cgroup for every browser launched and associate it with
141	approp network and other resource class.  This may lead to
142	proliferation of such cgroups.
143	
144	Also lets say that the administrator would like to give enhanced network
145	access temporarily to a student's browser (since it is night and the user
146	wants to do online gaming :))  OR give one of the students simulation
147	apps enhanced CPU power,
148	
149	With ability to write pids directly to resource classes, it's just a
150	matter of :
151	
152	       # echo pid > /mnt/network/<new_class>/tasks
153	       (after some time)
154	       # echo pid > /mnt/network/<orig_class>/tasks
155	
156	Without this ability, he would have to split the cgroup into
157	multiple separate ones and then associate the new cgroups with the
158	new resource classes.
159	
160	
161	
162	1.3 How are cgroups implemented ?
163	---------------------------------
164	
165	Control Groups extends the kernel as follows:
166	
167	 - Each task in the system has a reference-counted pointer to a
168	   css_set.
169	
170	 - A css_set contains a set of reference-counted pointers to
171	   cgroup_subsys_state objects, one for each cgroup subsystem
172	   registered in the system. There is no direct link from a task to
173	   the cgroup of which it's a member in each hierarchy, but this
174	   can be determined by following pointers through the
175	   cgroup_subsys_state objects. This is because accessing the
176	   subsystem state is something that's expected to happen frequently
177	   and in performance-critical code, whereas operations that require a
178	   task's actual cgroup assignments (in particular, moving between
179	   cgroups) are less common. A linked list runs through the cg_list
180	   field of each task_struct using the css_set, anchored at
181	   css_set->tasks.
182	
183	 - A cgroup hierarchy filesystem can be mounted  for browsing and
184	   manipulation from user space.
185	
186	 - You can list all the tasks (by pid) attached to any cgroup.
187	
188	The implementation of cgroups requires a few, simple hooks
189	into the rest of the kernel, none in performance critical paths:
190	
191	 - in init/main.c, to initialize the root cgroups and initial
192	   css_set at system boot.
193	
194	 - in fork and exit, to attach and detach a task from its css_set.
195	
196	In addition a new file system, of type "cgroup" may be mounted, to
197	enable browsing and modifying the cgroups presently known to the
198	kernel.  When mounting a cgroup hierarchy, you may specify a
199	comma-separated list of subsystems to mount as the filesystem mount
200	options.  By default, mounting the cgroup filesystem attempts to
201	mount a hierarchy containing all registered subsystems.
202	
203	If an active hierarchy with exactly the same set of subsystems already
204	exists, it will be reused for the new mount. If no existing hierarchy
205	matches, and any of the requested subsystems are in use in an existing
206	hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
207	is activated, associated with the requested subsystems.
208	
209	It's not currently possible to bind a new subsystem to an active
210	cgroup hierarchy, or to unbind a subsystem from an active cgroup
211	hierarchy. This may be possible in future, but is fraught with nasty
212	error-recovery issues.
213	
214	When a cgroup filesystem is unmounted, if there are any
215	child cgroups created below the top-level cgroup, that hierarchy
216	will remain active even though unmounted; if there are no
217	child cgroups then the hierarchy will be deactivated.
218	
219	No new system calls are added for cgroups - all support for
220	querying and modifying cgroups is via this cgroup file system.
221	
222	Each task under /proc has an added file named 'cgroup' displaying,
223	for each active hierarchy, the subsystem names and the cgroup name
224	as the path relative to the root of the cgroup file system.
225	
226	Each cgroup is represented by a directory in the cgroup file system
227	containing the following files describing that cgroup:
228	
229	 - tasks: list of tasks (by pid) attached to that cgroup
230	 - releasable flag: cgroup currently removeable?
231	 - 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)
234	
235	Other subsystems such as cpusets may add additional files in each
236	cgroup dir.
237	
238	New cgroups are created using the mkdir system call or shell
239	command.  The properties of a cgroup, such as its flags, are
240	modified by writing to the appropriate file in that cgroups
241	directory, as listed above.
242	
243	The named hierarchical structure of nested cgroups allows partitioning
244	a large system into nested, dynamically changeable, "soft-partitions".
245	
246	The attachment of each task, automatically inherited at fork by any
247	children of that task, to a cgroup allows organizing the work load
248	on a system into related sets of tasks.  A task may be re-attached to
249	any other cgroup, if allowed by the permissions on the necessary
250	cgroup file system directories.
251	
252	When a task is moved from one cgroup to another, it gets a new
253	css_set pointer - if there's an already existing css_set with the
254	desired collection of cgroups then that group is reused, else a new
255	css_set is allocated. Note that the current implementation uses a
256	linear search to locate an appropriate existing css_set, so isn't
257	very efficient. A future version will use a hash table for better
258	performance.
259	
260	To allow access from a cgroup to the css_sets (and hence tasks)
261	that comprise it, a set of cg_cgroup_link objects form a lattice;
262	each cg_cgroup_link is linked into a list of cg_cgroup_links for
263	a single cgroup on its cgrp_link_list field, and a list of
264	cg_cgroup_links for a single css_set on its cg_link_list.
265	
266	Thus the set of tasks in a cgroup can be listed by iterating over
267	each css_set that references the cgroup, and sub-iterating over
268	each css_set's task set.
269	
270	The use of a Linux virtual file system (vfs) to represent the
271	cgroup hierarchy provides for a familiar permission and name space
272	for cgroups, with a minimum of additional kernel code.
273	
274	1.4 What does notify_on_release do ?
275	------------------------------------
276	
277	If the notify_on_release flag is enabled (1) in a cgroup, then
278	whenever the last task in the cgroup leaves (exits or attaches to
279	some other cgroup) and the last child cgroup of that cgroup
280	is removed, then the kernel runs the command specified by the contents
281	of the "release_agent" file in that hierarchy's root directory,
282	supplying the pathname (relative to the mount point of the cgroup
283	file system) of the abandoned cgroup.  This enables automatic
284	removal of abandoned cgroups.  The default value of
285	notify_on_release in the root cgroup at system boot is disabled
286	(0).  The default value of other cgroups at creation is the current
287	value of their parents notify_on_release setting. The default value of
288	a cgroup hierarchy's release_agent path is empty.
289	
290	1.5 How do I use cgroups ?
291	--------------------------
292	
293	To start a new job that is to be contained within a cgroup, using
294	the "cpuset" cgroup subsystem, the steps are something like:
295	
296	 1) mkdir /dev/cgroup
297	 2) mount -t cgroup -ocpuset cpuset /dev/cgroup
298	 3) Create the new cgroup by doing mkdir's and write's (or echo's) in
299	    the /dev/cgroup virtual file system.
300	 4) Start a task that will be the "founding father" of the new job.
301	 5) Attach that task to the new cgroup by writing its pid to the
302	    /dev/cgroup tasks file for that cgroup.
303	 6) fork, exec or clone the job tasks from this founding father task.
304	
305	For example, the following sequence of commands will setup a cgroup
306	named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
307	and then start a subshell 'sh' in that cgroup:
308	
309	  mount -t cgroup cpuset -ocpuset /dev/cgroup
310	  cd /dev/cgroup
311	  mkdir Charlie
312	  cd Charlie
313	  /bin/echo 2-3 > cpuset.cpus
314	  /bin/echo 1 > cpuset.mems
315	  /bin/echo $$ > tasks
316	  sh
317	  # The subshell 'sh' is now running in cgroup Charlie
318	  # The next line should display '/Charlie'
319	  cat /proc/self/cgroup
320	
321	2. Usage Examples and Syntax
322	============================
323	
324	2.1 Basic Usage
325	---------------
326	
327	Creating, modifying, using the cgroups can be done through the cgroup
328	virtual filesystem.
329	
330	To mount a cgroup hierarchy will all available subsystems, type:
331	# mount -t cgroup xxx /dev/cgroup
332	
333	The "xxx" is not interpreted by the cgroup code, but will appear in
334	/proc/mounts so may be any useful identifying string that you like.
335	
336	To mount a cgroup hierarchy with just the cpuset and numtasks
337	subsystems, type:
338	# mount -t cgroup -o cpuset,numtasks hier1 /dev/cgroup
339	
340	To change the set of subsystems bound to a mounted hierarchy, just
341	remount with different options:
342	
343	# mount -o remount,cpuset,ns  /dev/cgroup
344	
345	Note that changing the set of subsystems is currently only supported
346	when the hierarchy consists of a single (root) cgroup. Supporting
347	the ability to arbitrarily bind/unbind subsystems from an existing
348	cgroup hierarchy is intended to be implemented in the future.
349	
350	Then under /dev/cgroup you can find a tree that corresponds to the
351	tree of the cgroups in the system. For instance, /dev/cgroup
352	is the cgroup that holds the whole system.
353	
354	If you want to create a new cgroup under /dev/cgroup:
355	# cd /dev/cgroup
356	# mkdir my_cgroup
357	
358	Now you want to do something with this cgroup.
359	# cd my_cgroup
360	
361	In this directory you can find several files:
362	# ls
363	notify_on_release releasable tasks
364	(plus whatever files added by the attached subsystems)
365	
366	Now attach your shell to this cgroup:
367	# /bin/echo $$ > tasks
368	
369	You can also create cgroups inside your cgroup by using mkdir in this
370	directory.
371	# mkdir my_sub_cs
372	
373	To remove a cgroup, just use rmdir:
374	# rmdir my_sub_cs
375	
376	This will fail if the cgroup is in use (has cgroups inside, or
377	has processes attached, or is held alive by other subsystem-specific
378	reference).
379	
380	2.2 Attaching processes
381	-----------------------
382	
383	# /bin/echo PID > tasks
384	
385	Note that it is PID, not PIDs. You can only attach ONE task at a time.
386	If you have several tasks to attach, you have to do it one after another:
387	
388	# /bin/echo PID1 > tasks
389	# /bin/echo PID2 > tasks
390		...
391	# /bin/echo PIDn > tasks
392	
393	You can attach the current shell task by echoing 0:
394	
395	# echo 0 > tasks
396	
397	3. Kernel API
398	=============
399	
400	3.1 Overview
401	------------
402	
403	Each kernel subsystem that wants to hook into the generic cgroup
404	system needs to create a cgroup_subsys object. This contains
405	various methods, which are callbacks from the cgroup system, along
406	with a subsystem id which will be assigned by the cgroup system.
407	
408	Other fields in the cgroup_subsys object include:
409	
410	- subsys_id: a unique array index for the subsystem, indicating which
411	  entry in cgroup->subsys[] this subsystem should be managing.
412	
413	- name: should be initialized to a unique subsystem name. Should be
414	  no longer than MAX_CGROUP_TYPE_NAMELEN.
415	
416	- early_init: indicate if the subsystem needs early initialization
417	  at system boot.
418	
419	Each cgroup object created by the system has an array of pointers,
420	indexed by subsystem id; this pointer is entirely managed by the
421	subsystem; the generic cgroup code will never touch this pointer.
422	
423	3.2 Synchronization
424	-------------------
425	
426	There is a global mutex, cgroup_mutex, used by the cgroup
427	system. This should be taken by anything that wants to modify a
428	cgroup. It may also be taken to prevent cgroups from being
429	modified, but more specific locks may be more appropriate in that
430	situation.
431	
432	See kernel/cgroup.c for more details.
433	
434	Subsystems can take/release the cgroup_mutex via the functions
435	cgroup_lock()/cgroup_unlock().
436	
437	Accessing a task's cgroup pointer may be done in the following ways:
438	- while holding cgroup_mutex
439	- while holding the task's alloc_lock (via task_lock())
440	- inside an rcu_read_lock() section via rcu_dereference()
441	
442	3.3 Subsystem API
443	-----------------
444	
445	Each subsystem should:
446	
447	- add an entry in linux/cgroup_subsys.h
448	- define a cgroup_subsys object called <name>_subsys
449	
450	Each subsystem may export the following methods. The only mandatory
451	methods are create/destroy. Any others that are null are presumed to
452	be successful no-ops.
453	
454	struct cgroup_subsys_state *create(struct cgroup_subsys *ss,
455					   struct cgroup *cgrp)
456	(cgroup_mutex held by caller)
457	
458	Called to create a subsystem state object for a cgroup. The
459	subsystem should allocate its subsystem state object for the passed
460	cgroup, returning a pointer to the new object on success or a
461	negative error code. On success, the subsystem pointer should point to
462	a structure of type cgroup_subsys_state (typically embedded in a
463	larger subsystem-specific object), which will be initialized by the
464	cgroup system. Note that this will be called at initialization to
465	create the root subsystem state for this subsystem; this case can be
466	identified by the passed cgroup object having a NULL parent (since
467	it's the root of the hierarchy) and may be an appropriate place for
468	initialization code.
469	
470	void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
471	(cgroup_mutex held by caller)
472	
473	The cgroup system is about to destroy the passed cgroup; the subsystem
474	should do any necessary cleanup and free its subsystem state
475	object. By the time this method is called, the cgroup has already been
476	unlinked from the file system and from the child list of its parent;
477	cgroup->parent is still valid. (Note - can also be called for a
478	newly-created cgroup if an error occurs after this subsystem's
479	create() method has been called for the new cgroup).
480	
481	void pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp);
482	(cgroup_mutex held by caller)
483	
484	Called before checking the reference count on each subsystem. This may
485	be useful for subsystems which have some extra references even if
486	there are not tasks in the cgroup.
487	
488	int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
489		       struct task_struct *task)
490	(cgroup_mutex held by caller)
491	
492	Called prior to moving a task into a cgroup; if the subsystem
493	returns an error, this will abort the attach operation.  If a NULL
494	task is passed, then a successful result indicates that *any*
495	unspecified task can be moved into the cgroup. Note that this isn't
496	called on a fork. If this method returns 0 (success) then this should
497	remain valid while the caller holds cgroup_mutex.
498	
499	void attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
500		    struct cgroup *old_cgrp, struct task_struct *task)
501	
502	Called after the task has been attached to the cgroup, to allow any
503	post-attachment activity that requires memory allocations or blocking.
504	
505	void fork(struct cgroup_subsy *ss, struct task_struct *task)
506	
507	Called when a task is forked into a cgroup.
508	
509	void exit(struct cgroup_subsys *ss, struct task_struct *task)
510	
511	Called during task exit.
512	
513	int populate(struct cgroup_subsys *ss, struct cgroup *cgrp)
514	
515	Called after creation of a cgroup to allow a subsystem to populate
516	the cgroup directory with file entries.  The subsystem should make
517	calls to cgroup_add_file() with objects of type cftype (see
518	include/linux/cgroup.h for details).  Note that although this
519	method can return an error code, the error code is currently not
520	always handled well.
521	
522	void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp)
523	
524	Called at the end of cgroup_clone() to do any paramater
525	initialization which might be required before a task could attach.  For
526	example in cpusets, no task may attach before 'cpus' and 'mems' are set
527	up.
528	
529	void bind(struct cgroup_subsys *ss, struct cgroup *root)
530	(cgroup_mutex held by caller)
531	
532	Called when a cgroup subsystem is rebound to a different hierarchy
533	and root cgroup. Currently this will only involve movement between
534	the default hierarchy (which never has sub-cgroups) and a hierarchy
535	that is being created/destroyed (and hence has no sub-cgroups).
536	
537	4. Questions
538	============
539	
540	Q: what's up with this '/bin/echo' ?
541	A: bash's builtin 'echo' command does not check calls to write() against
542	   errors. If you use it in the cgroup file system, you won't be
543	   able to tell whether a command succeeded or failed.
544	
545	Q: When I attach processes, only the first of the line gets really attached !
546	A: We can only return one error code per call to write(). So you should also
547	   put only ONE pid.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.