Based on kernel version 3.12. Page generated on 2013-11-13 21:56 EST.
1 CGROUPS 2 ------- 3 4 Written by Paul Menage <firstname.lastname@example.org> 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 <email@example.com> 11 Modified by Christoph Lameter <firstname.lastname@example.org> 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 email@example.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.