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  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/cpuset 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 css_reset(struct cgroup_subsys_state *css)
  603. (cgroup_mutex held by caller)
  604.  
  605. An optional operation which should restore @css's configuration to the
  606. initial state.  This is currently only used on the unified hierarchy
  607. when a subsystem is disabled on a cgroup through
  608. "cgroup.subtree_control" but should remain enabled because other
  609. subsystems depend on it.  cgroup core makes such a css invisible by
  610. removing the associated interface files and invokes this callback so
  611. that the hidden subsystem can return to the initial neutral state.
  612. This prevents unexpected resource control from a hidden css and
  613. ensures that the configuration is in the initial state when it is made
  614. visible again later.
  615.  
  616. void cancel_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
  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. The parameters are identical to can_attach().
  624.  
  625. void attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
  626. (cgroup_mutex held by caller)
  627.  
  628. Called after the task has been attached to the cgroup, to allow any
  629. post-attachment activity that requires memory allocations or blocking.
  630. The parameters are identical to can_attach().
  631.  
  632. void fork(struct task_struct *task)
  633.  
  634. Called when a task is forked into a cgroup.
  635.  
  636. void exit(struct task_struct *task)
  637.  
  638. Called during task exit.
  639.  
  640. void bind(struct cgroup *root)
  641. (cgroup_mutex held by caller)
  642.  
  643. Called when a cgroup subsystem is rebound to a different hierarchy
  644. and root cgroup. Currently this will only involve movement between
  645. the default hierarchy (which never has sub-cgroups) and a hierarchy
  646. that is being created/destroyed (and hence has no sub-cgroups).
  647.  
  648. 4. Extended attribute usage
  649. ===========================
  650.  
  651. cgroup filesystem supports certain types of extended attributes in its
  652. directories and files.  The current supported types are:
  653.         - Trusted (XATTR_TRUSTED)
  654.         - Security (XATTR_SECURITY)
  655.  
  656. Both require CAP_SYS_ADMIN capability to set.
  657.  
  658. Like in tmpfs, the extended attributes in cgroup filesystem are stored
  659. using kernel memory and it's advised to keep the usage at minimum.  This
  660. is the reason why user defined extended attributes are not supported, since
  661. any user can do it and there's no limit in the value size.
  662.  
  663. The current known users for this feature are SELinux to limit cgroup usage
  664. in containers and systemd for assorted meta data like main PID in a cgroup
  665. (systemd creates a cgroup per service).
  666.  
  667. 5. Questions
  668. ============
  669.  
  670. Q: what's up with this '/bin/echo' ?
  671. A: bash's builtin 'echo' command does not check calls to write() against
  672.    errors. If you use it in the cgroup file system, you won't be
  673.    able to tell whether a command succeeded or failed.
  674.  
  675. Q: When I attach processes, only the first of the line gets really attached !
  676. A: We can only return one error code per call to write(). So you should also
  677.    put only ONE PID.
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