1================ 2Control Group v2 3================ 4 5:Date: October, 2015 6:Author: Tejun Heo <tj@kernel.org> 7 8This is the authoritative documentation on the design, interface and 9conventions of cgroup v2. It describes all userland-visible aspects 10of cgroup including core and specific controller behaviors. All 11future changes must be reflected in this document. Documentation for 12v1 is available under Documentation/admin-guide/cgroup-v1/. 13 14.. CONTENTS 15 16 1. Introduction 17 1-1. Terminology 18 1-2. What is cgroup? 19 2. Basic Operations 20 2-1. Mounting 21 2-2. Organizing Processes and Threads 22 2-2-1. Processes 23 2-2-2. Threads 24 2-3. [Un]populated Notification 25 2-4. Controlling Controllers 26 2-4-1. Enabling and Disabling 27 2-4-2. Top-down Constraint 28 2-4-3. No Internal Process Constraint 29 2-5. Delegation 30 2-5-1. Model of Delegation 31 2-5-2. Delegation Containment 32 2-6. Guidelines 33 2-6-1. Organize Once and Control 34 2-6-2. Avoid Name Collisions 35 3. Resource Distribution Models 36 3-1. Weights 37 3-2. Limits 38 3-3. Protections 39 3-4. Allocations 40 4. Interface Files 41 4-1. Format 42 4-2. Conventions 43 4-3. Core Interface Files 44 5. Controllers 45 5-1. CPU 46 5-1-1. CPU Interface Files 47 5-2. Memory 48 5-2-1. Memory Interface Files 49 5-2-2. Usage Guidelines 50 5-2-3. Memory Ownership 51 5-3. IO 52 5-3-1. IO Interface Files 53 5-3-2. Writeback 54 5-3-3. IO Latency 55 5-3-3-1. How IO Latency Throttling Works 56 5-3-3-2. IO Latency Interface Files 57 5-4. PID 58 5-4-1. PID Interface Files 59 5-5. Cpuset 60 5.5-1. Cpuset Interface Files 61 5-6. Device 62 5-7. RDMA 63 5-7-1. RDMA Interface Files 64 5-8. Misc 65 5-8-1. perf_event 66 5-N. Non-normative information 67 5-N-1. CPU controller root cgroup process behaviour 68 5-N-2. IO controller root cgroup process behaviour 69 6. Namespace 70 6-1. Basics 71 6-2. The Root and Views 72 6-3. Migration and setns(2) 73 6-4. Interaction with Other Namespaces 74 P. Information on Kernel Programming 75 P-1. Filesystem Support for Writeback 76 D. Deprecated v1 Core Features 77 R. Issues with v1 and Rationales for v2 78 R-1. Multiple Hierarchies 79 R-2. Thread Granularity 80 R-3. Competition Between Inner Nodes and Threads 81 R-4. Other Interface Issues 82 R-5. Controller Issues and Remedies 83 R-5-1. Memory 84 85 86Introduction 87============ 88 89Terminology 90----------- 91 92"cgroup" stands for "control group" and is never capitalized. The 93singular form is used to designate the whole feature and also as a 94qualifier as in "cgroup controllers". When explicitly referring to 95multiple individual control groups, the plural form "cgroups" is used. 96 97 98What is cgroup? 99--------------- 100 101cgroup is a mechanism to organize processes hierarchically and 102distribute system resources along the hierarchy in a controlled and 103configurable manner. 104 105cgroup is largely composed of two parts - the core and controllers. 106cgroup core is primarily responsible for hierarchically organizing 107processes. A cgroup controller is usually responsible for 108distributing a specific type of system resource along the hierarchy 109although there are utility controllers which serve purposes other than 110resource distribution. 111 112cgroups form a tree structure and every process in the system belongs 113to one and only one cgroup. All threads of a process belong to the 114same cgroup. On creation, all processes are put in the cgroup that 115the parent process belongs to at the time. A process can be migrated 116to another cgroup. Migration of a process doesn't affect already 117existing descendant processes. 118 119Following certain structural constraints, controllers may be enabled or 120disabled selectively on a cgroup. All controller behaviors are 121hierarchical - if a controller is enabled on a cgroup, it affects all 122processes which belong to the cgroups consisting the inclusive 123sub-hierarchy of the cgroup. When a controller is enabled on a nested 124cgroup, it always restricts the resource distribution further. The 125restrictions set closer to the root in the hierarchy can not be 126overridden from further away. 127 128 129Basic Operations 130================ 131 132Mounting 133-------- 134 135Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2 136hierarchy can be mounted with the following mount command:: 137 138 # mount -t cgroup2 none $MOUNT_POINT 139 140cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All 141controllers which support v2 and are not bound to a v1 hierarchy are 142automatically bound to the v2 hierarchy and show up at the root. 143Controllers which are not in active use in the v2 hierarchy can be 144bound to other hierarchies. This allows mixing v2 hierarchy with the 145legacy v1 multiple hierarchies in a fully backward compatible way. 146 147A controller can be moved across hierarchies only after the controller 148is no longer referenced in its current hierarchy. Because per-cgroup 149controller states are destroyed asynchronously and controllers may 150have lingering references, a controller may not show up immediately on 151the v2 hierarchy after the final umount of the previous hierarchy. 152Similarly, a controller should be fully disabled to be moved out of 153the unified hierarchy and it may take some time for the disabled 154controller to become available for other hierarchies; furthermore, due 155to inter-controller dependencies, other controllers may need to be 156disabled too. 157 158While useful for development and manual configurations, moving 159controllers dynamically between the v2 and other hierarchies is 160strongly discouraged for production use. It is recommended to decide 161the hierarchies and controller associations before starting using the 162controllers after system boot. 163 164During transition to v2, system management software might still 165automount the v1 cgroup filesystem and so hijack all controllers 166during boot, before manual intervention is possible. To make testing 167and experimenting easier, the kernel parameter cgroup_no_v1= allows 168disabling controllers in v1 and make them always available in v2. 169 170cgroup v2 currently supports the following mount options. 171 172 nsdelegate 173 174 Consider cgroup namespaces as delegation boundaries. This 175 option is system wide and can only be set on mount or modified 176 through remount from the init namespace. The mount option is 177 ignored on non-init namespace mounts. Please refer to the 178 Delegation section for details. 179 180 memory_localevents 181 182 Only populate memory.events with data for the current cgroup, 183 and not any subtrees. This is legacy behaviour, the default 184 behaviour without this option is to include subtree counts. 185 This option is system wide and can only be set on mount or 186 modified through remount from the init namespace. The mount 187 option is ignored on non-init namespace mounts. 188 189 190Organizing Processes and Threads 191-------------------------------- 192 193Processes 194~~~~~~~~~ 195 196Initially, only the root cgroup exists to which all processes belong. 197A child cgroup can be created by creating a sub-directory:: 198 199 # mkdir $CGROUP_NAME 200 201A given cgroup may have multiple child cgroups forming a tree 202structure. Each cgroup has a read-writable interface file 203"cgroup.procs". When read, it lists the PIDs of all processes which 204belong to the cgroup one-per-line. The PIDs are not ordered and the 205same PID may show up more than once if the process got moved to 206another cgroup and then back or the PID got recycled while reading. 207 208A process can be migrated into a cgroup by writing its PID to the 209target cgroup's "cgroup.procs" file. Only one process can be migrated 210on a single write(2) call. If a process is composed of multiple 211threads, writing the PID of any thread migrates all threads of the 212process. 213 214When a process forks a child process, the new process is born into the 215cgroup that the forking process belongs to at the time of the 216operation. After exit, a process stays associated with the cgroup 217that it belonged to at the time of exit until it's reaped; however, a 218zombie process does not appear in "cgroup.procs" and thus can't be 219moved to another cgroup. 220 221A cgroup which doesn't have any children or live processes can be 222destroyed by removing the directory. Note that a cgroup which doesn't 223have any children and is associated only with zombie processes is 224considered empty and can be removed:: 225 226 # rmdir $CGROUP_NAME 227 228"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy 229cgroup is in use in the system, this file may contain multiple lines, 230one for each hierarchy. The entry for cgroup v2 is always in the 231format "0::$PATH":: 232 233 # cat /proc/842/cgroup 234 ... 235 0::/test-cgroup/test-cgroup-nested 236 237If the process becomes a zombie and the cgroup it was associated with 238is removed subsequently, " (deleted)" is appended to the path:: 239 240 # cat /proc/842/cgroup 241 ... 242 0::/test-cgroup/test-cgroup-nested (deleted) 243 244 245Threads 246~~~~~~~ 247 248cgroup v2 supports thread granularity for a subset of controllers to 249support use cases requiring hierarchical resource distribution across 250the threads of a group of processes. By default, all threads of a 251process belong to the same cgroup, which also serves as the resource 252domain to host resource consumptions which are not specific to a 253process or thread. The thread mode allows threads to be spread across 254a subtree while still maintaining the common resource domain for them. 255 256Controllers which support thread mode are called threaded controllers. 257The ones which don't are called domain controllers. 258 259Marking a cgroup threaded makes it join the resource domain of its 260parent as a threaded cgroup. The parent may be another threaded 261cgroup whose resource domain is further up in the hierarchy. The root 262of a threaded subtree, that is, the nearest ancestor which is not 263threaded, is called threaded domain or thread root interchangeably and 264serves as the resource domain for the entire subtree. 265 266Inside a threaded subtree, threads of a process can be put in 267different cgroups and are not subject to the no internal process 268constraint - threaded controllers can be enabled on non-leaf cgroups 269whether they have threads in them or not. 270 271As the threaded domain cgroup hosts all the domain resource 272consumptions of the subtree, it is considered to have internal 273resource consumptions whether there are processes in it or not and 274can't have populated child cgroups which aren't threaded. Because the 275root cgroup is not subject to no internal process constraint, it can 276serve both as a threaded domain and a parent to domain cgroups. 277 278The current operation mode or type of the cgroup is shown in the 279"cgroup.type" file which indicates whether the cgroup is a normal 280domain, a domain which is serving as the domain of a threaded subtree, 281or a threaded cgroup. 282 283On creation, a cgroup is always a domain cgroup and can be made 284threaded by writing "threaded" to the "cgroup.type" file. The 285operation is single direction:: 286 287 # echo threaded > cgroup.type 288 289Once threaded, the cgroup can't be made a domain again. To enable the 290thread mode, the following conditions must be met. 291 292- As the cgroup will join the parent's resource domain. The parent 293 must either be a valid (threaded) domain or a threaded cgroup. 294 295- When the parent is an unthreaded domain, it must not have any domain 296 controllers enabled or populated domain children. The root is 297 exempt from this requirement. 298 299Topology-wise, a cgroup can be in an invalid state. Please consider 300the following topology:: 301 302 A (threaded domain) - B (threaded) - C (domain, just created) 303 304C is created as a domain but isn't connected to a parent which can 305host child domains. C can't be used until it is turned into a 306threaded cgroup. "cgroup.type" file will report "domain (invalid)" in 307these cases. Operations which fail due to invalid topology use 308EOPNOTSUPP as the errno. 309 310A domain cgroup is turned into a threaded domain when one of its child 311cgroup becomes threaded or threaded controllers are enabled in the 312"cgroup.subtree_control" file while there are processes in the cgroup. 313A threaded domain reverts to a normal domain when the conditions 314clear. 315 316When read, "cgroup.threads" contains the list of the thread IDs of all 317threads in the cgroup. Except that the operations are per-thread 318instead of per-process, "cgroup.threads" has the same format and 319behaves the same way as "cgroup.procs". While "cgroup.threads" can be 320written to in any cgroup, as it can only move threads inside the same 321threaded domain, its operations are confined inside each threaded 322subtree. 323 324The threaded domain cgroup serves as the resource domain for the whole 325subtree, and, while the threads can be scattered across the subtree, 326all the processes are considered to be in the threaded domain cgroup. 327"cgroup.procs" in a threaded domain cgroup contains the PIDs of all 328processes in the subtree and is not readable in the subtree proper. 329However, "cgroup.procs" can be written to from anywhere in the subtree 330to migrate all threads of the matching process to the cgroup. 331 332Only threaded controllers can be enabled in a threaded subtree. When 333a threaded controller is enabled inside a threaded subtree, it only 334accounts for and controls resource consumptions associated with the 335threads in the cgroup and its descendants. All consumptions which 336aren't tied to a specific thread belong to the threaded domain cgroup. 337 338Because a threaded subtree is exempt from no internal process 339constraint, a threaded controller must be able to handle competition 340between threads in a non-leaf cgroup and its child cgroups. Each 341threaded controller defines how such competitions are handled. 342 343 344[Un]populated Notification 345-------------------------- 346 347Each non-root cgroup has a "cgroup.events" file which contains 348"populated" field indicating whether the cgroup's sub-hierarchy has 349live processes in it. Its value is 0 if there is no live process in 350the cgroup and its descendants; otherwise, 1. poll and [id]notify 351events are triggered when the value changes. This can be used, for 352example, to start a clean-up operation after all processes of a given 353sub-hierarchy have exited. The populated state updates and 354notifications are recursive. Consider the following sub-hierarchy 355where the numbers in the parentheses represent the numbers of processes 356in each cgroup:: 357 358 A(4) - B(0) - C(1) 359 \ D(0) 360 361A, B and C's "populated" fields would be 1 while D's 0. After the one 362process in C exits, B and C's "populated" fields would flip to "0" and 363file modified events will be generated on the "cgroup.events" files of 364both cgroups. 365 366 367Controlling Controllers 368----------------------- 369 370Enabling and Disabling 371~~~~~~~~~~~~~~~~~~~~~~ 372 373Each cgroup has a "cgroup.controllers" file which lists all 374controllers available for the cgroup to enable:: 375 376 # cat cgroup.controllers 377 cpu io memory 378 379No controller is enabled by default. Controllers can be enabled and 380disabled by writing to the "cgroup.subtree_control" file:: 381 382 # echo "+cpu +memory -io" > cgroup.subtree_control 383 384Only controllers which are listed in "cgroup.controllers" can be 385enabled. When multiple operations are specified as above, either they 386all succeed or fail. If multiple operations on the same controller 387are specified, the last one is effective. 388 389Enabling a controller in a cgroup indicates that the distribution of 390the target resource across its immediate children will be controlled. 391Consider the following sub-hierarchy. The enabled controllers are 392listed in parentheses:: 393 394 A(cpu,memory) - B(memory) - C() 395 \ D() 396 397As A has "cpu" and "memory" enabled, A will control the distribution 398of CPU cycles and memory to its children, in this case, B. As B has 399"memory" enabled but not "CPU", C and D will compete freely on CPU 400cycles but their division of memory available to B will be controlled. 401 402As a controller regulates the distribution of the target resource to 403the cgroup's children, enabling it creates the controller's interface 404files in the child cgroups. In the above example, enabling "cpu" on B 405would create the "cpu." prefixed controller interface files in C and 406D. Likewise, disabling "memory" from B would remove the "memory." 407prefixed controller interface files from C and D. This means that the 408controller interface files - anything which doesn't start with 409"cgroup." are owned by the parent rather than the cgroup itself. 410 411 412Top-down Constraint 413~~~~~~~~~~~~~~~~~~~ 414 415Resources are distributed top-down and a cgroup can further distribute 416a resource only if the resource has been distributed to it from the 417parent. This means that all non-root "cgroup.subtree_control" files 418can only contain controllers which are enabled in the parent's 419"cgroup.subtree_control" file. A controller can be enabled only if 420the parent has the controller enabled and a controller can't be 421disabled if one or more children have it enabled. 422 423 424No Internal Process Constraint 425~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 426 427Non-root cgroups can distribute domain resources to their children 428only when they don't have any processes of their own. In other words, 429only domain cgroups which don't contain any processes can have domain 430controllers enabled in their "cgroup.subtree_control" files. 431 432This guarantees that, when a domain controller is looking at the part 433of the hierarchy which has it enabled, processes are always only on 434the leaves. This rules out situations where child cgroups compete 435against internal processes of the parent. 436 437The root cgroup is exempt from this restriction. Root contains 438processes and anonymous resource consumption which can't be associated 439with any other cgroups and requires special treatment from most 440controllers. How resource consumption in the root cgroup is governed 441is up to each controller (for more information on this topic please 442refer to the Non-normative information section in the Controllers 443chapter). 444 445Note that the restriction doesn't get in the way if there is no 446enabled controller in the cgroup's "cgroup.subtree_control". This is 447important as otherwise it wouldn't be possible to create children of a 448populated cgroup. To control resource distribution of a cgroup, the 449cgroup must create children and transfer all its processes to the 450children before enabling controllers in its "cgroup.subtree_control" 451file. 452 453 454Delegation 455---------- 456 457Model of Delegation 458~~~~~~~~~~~~~~~~~~~ 459 460A cgroup can be delegated in two ways. First, to a less privileged 461user by granting write access of the directory and its "cgroup.procs", 462"cgroup.threads" and "cgroup.subtree_control" files to the user. 463Second, if the "nsdelegate" mount option is set, automatically to a 464cgroup namespace on namespace creation. 465 466Because the resource control interface files in a given directory 467control the distribution of the parent's resources, the delegatee 468shouldn't be allowed to write to them. For the first method, this is 469achieved by not granting access to these files. For the second, the 470kernel rejects writes to all files other than "cgroup.procs" and 471"cgroup.subtree_control" on a namespace root from inside the 472namespace. 473 474The end results are equivalent for both delegation types. Once 475delegated, the user can build sub-hierarchy under the directory, 476organize processes inside it as it sees fit and further distribute the 477resources it received from the parent. The limits and other settings 478of all resource controllers are hierarchical and regardless of what 479happens in the delegated sub-hierarchy, nothing can escape the 480resource restrictions imposed by the parent. 481 482Currently, cgroup doesn't impose any restrictions on the number of 483cgroups in or nesting depth of a delegated sub-hierarchy; however, 484this may be limited explicitly in the future. 485 486 487Delegation Containment 488~~~~~~~~~~~~~~~~~~~~~~ 489 490A delegated sub-hierarchy is contained in the sense that processes 491can't be moved into or out of the sub-hierarchy by the delegatee. 492 493For delegations to a less privileged user, this is achieved by 494requiring the following conditions for a process with a non-root euid 495to migrate a target process into a cgroup by writing its PID to the 496"cgroup.procs" file. 497 498- The writer must have write access to the "cgroup.procs" file. 499 500- The writer must have write access to the "cgroup.procs" file of the 501 common ancestor of the source and destination cgroups. 502 503The above two constraints ensure that while a delegatee may migrate 504processes around freely in the delegated sub-hierarchy it can't pull 505in from or push out to outside the sub-hierarchy. 506 507For an example, let's assume cgroups C0 and C1 have been delegated to 508user U0 who created C00, C01 under C0 and C10 under C1 as follows and 509all processes under C0 and C1 belong to U0:: 510 511 ~~~~~~~~~~~~~ - C0 - C00 512 ~ cgroup ~ \ C01 513 ~ hierarchy ~ 514 ~~~~~~~~~~~~~ - C1 - C10 515 516Let's also say U0 wants to write the PID of a process which is 517currently in C10 into "C00/cgroup.procs". U0 has write access to the 518file; however, the common ancestor of the source cgroup C10 and the 519destination cgroup C00 is above the points of delegation and U0 would 520not have write access to its "cgroup.procs" files and thus the write 521will be denied with -EACCES. 522 523For delegations to namespaces, containment is achieved by requiring 524that both the source and destination cgroups are reachable from the 525namespace of the process which is attempting the migration. If either 526is not reachable, the migration is rejected with -ENOENT. 527 528 529Guidelines 530---------- 531 532Organize Once and Control 533~~~~~~~~~~~~~~~~~~~~~~~~~ 534 535Migrating a process across cgroups is a relatively expensive operation 536and stateful resources such as memory are not moved together with the 537process. This is an explicit design decision as there often exist 538inherent trade-offs between migration and various hot paths in terms 539of synchronization cost. 540 541As such, migrating processes across cgroups frequently as a means to 542apply different resource restrictions is discouraged. A workload 543should be assigned to a cgroup according to the system's logical and 544resource structure once on start-up. Dynamic adjustments to resource 545distribution can be made by changing controller configuration through 546the interface files. 547 548 549Avoid Name Collisions 550~~~~~~~~~~~~~~~~~~~~~ 551 552Interface files for a cgroup and its children cgroups occupy the same 553directory and it is possible to create children cgroups which collide 554with interface files. 555 556All cgroup core interface files are prefixed with "cgroup." and each 557controller's interface files are prefixed with the controller name and 558a dot. A controller's name is composed of lower case alphabets and 559'_'s but never begins with an '_' so it can be used as the prefix 560character for collision avoidance. Also, interface file names won't 561start or end with terms which are often used in categorizing workloads 562such as job, service, slice, unit or workload. 563 564cgroup doesn't do anything to prevent name collisions and it's the 565user's responsibility to avoid them. 566 567 568Resource Distribution Models 569============================ 570 571cgroup controllers implement several resource distribution schemes 572depending on the resource type and expected use cases. This section 573describes major schemes in use along with their expected behaviors. 574 575 576Weights 577------- 578 579A parent's resource is distributed by adding up the weights of all 580active children and giving each the fraction matching the ratio of its 581weight against the sum. As only children which can make use of the 582resource at the moment participate in the distribution, this is 583work-conserving. Due to the dynamic nature, this model is usually 584used for stateless resources. 585 586All weights are in the range [1, 10000] with the default at 100. This 587allows symmetric multiplicative biases in both directions at fine 588enough granularity while staying in the intuitive range. 589 590As long as the weight is in range, all configuration combinations are 591valid and there is no reason to reject configuration changes or 592process migrations. 593 594"cpu.weight" proportionally distributes CPU cycles to active children 595and is an example of this type. 596 597 598Limits 599------ 600 601A child can only consume upto the configured amount of the resource. 602Limits can be over-committed - the sum of the limits of children can 603exceed the amount of resource available to the parent. 604 605Limits are in the range [0, max] and defaults to "max", which is noop. 606 607As limits can be over-committed, all configuration combinations are 608valid and there is no reason to reject configuration changes or 609process migrations. 610 611"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume 612on an IO device and is an example of this type. 613 614 615Protections 616----------- 617 618A cgroup is protected upto the configured amount of the resource 619as long as the usages of all its ancestors are under their 620protected levels. Protections can be hard guarantees or best effort 621soft boundaries. Protections can also be over-committed in which case 622only upto the amount available to the parent is protected among 623children. 624 625Protections are in the range [0, max] and defaults to 0, which is 626noop. 627 628As protections can be over-committed, all configuration combinations 629are valid and there is no reason to reject configuration changes or 630process migrations. 631 632"memory.low" implements best-effort memory protection and is an 633example of this type. 634 635 636Allocations 637----------- 638 639A cgroup is exclusively allocated a certain amount of a finite 640resource. Allocations can't be over-committed - the sum of the 641allocations of children can not exceed the amount of resource 642available to the parent. 643 644Allocations are in the range [0, max] and defaults to 0, which is no 645resource. 646 647As allocations can't be over-committed, some configuration 648combinations are invalid and should be rejected. Also, if the 649resource is mandatory for execution of processes, process migrations 650may be rejected. 651 652"cpu.rt.max" hard-allocates realtime slices and is an example of this 653type. 654 655 656Interface Files 657=============== 658 659Format 660------ 661 662All interface files should be in one of the following formats whenever 663possible:: 664 665 New-line separated values 666 (when only one value can be written at once) 667 668 VAL0\n 669 VAL1\n 670 ... 671 672 Space separated values 673 (when read-only or multiple values can be written at once) 674 675 VAL0 VAL1 ...\n 676 677 Flat keyed 678 679 KEY0 VAL0\n 680 KEY1 VAL1\n 681 ... 682 683 Nested keyed 684 685 KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01... 686 KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11... 687 ... 688 689For a writable file, the format for writing should generally match 690reading; however, controllers may allow omitting later fields or 691implement restricted shortcuts for most common use cases. 692 693For both flat and nested keyed files, only the values for a single key 694can be written at a time. For nested keyed files, the sub key pairs 695may be specified in any order and not all pairs have to be specified. 696 697 698Conventions 699----------- 700 701- Settings for a single feature should be contained in a single file. 702 703- The root cgroup should be exempt from resource control and thus 704 shouldn't have resource control interface files. Also, 705 informational files on the root cgroup which end up showing global 706 information available elsewhere shouldn't exist. 707 708- The default time unit is microseconds. If a different unit is ever 709 used, an explicit unit suffix must be present. 710 711- A parts-per quantity should use a percentage decimal with at least 712 two digit fractional part - e.g. 13.40. 713 714- If a controller implements weight based resource distribution, its 715 interface file should be named "weight" and have the range [1, 716 10000] with 100 as the default. The values are chosen to allow 717 enough and symmetric bias in both directions while keeping it 718 intuitive (the default is 100%). 719 720- If a controller implements an absolute resource guarantee and/or 721 limit, the interface files should be named "min" and "max" 722 respectively. If a controller implements best effort resource 723 guarantee and/or limit, the interface files should be named "low" 724 and "high" respectively. 725 726 In the above four control files, the special token "max" should be 727 used to represent upward infinity for both reading and writing. 728 729- If a setting has a configurable default value and keyed specific 730 overrides, the default entry should be keyed with "default" and 731 appear as the first entry in the file. 732 733 The default value can be updated by writing either "default $VAL" or 734 "$VAL". 735 736 When writing to update a specific override, "default" can be used as 737 the value to indicate removal of the override. Override entries 738 with "default" as the value must not appear when read. 739 740 For example, a setting which is keyed by major:minor device numbers 741 with integer values may look like the following:: 742 743 # cat cgroup-example-interface-file 744 default 150 745 8:0 300 746 747 The default value can be updated by:: 748 749 # echo 125 > cgroup-example-interface-file 750 751 or:: 752 753 # echo "default 125" > cgroup-example-interface-file 754 755 An override can be set by:: 756 757 # echo "8:16 170" > cgroup-example-interface-file 758 759 and cleared by:: 760 761 # echo "8:0 default" > cgroup-example-interface-file 762 # cat cgroup-example-interface-file 763 default 125 764 8:16 170 765 766- For events which are not very high frequency, an interface file 767 "events" should be created which lists event key value pairs. 768 Whenever a notifiable event happens, file modified event should be 769 generated on the file. 770 771 772Core Interface Files 773-------------------- 774 775All cgroup core files are prefixed with "cgroup." 776 777 cgroup.type 778 779 A read-write single value file which exists on non-root 780 cgroups. 781 782 When read, it indicates the current type of the cgroup, which 783 can be one of the following values. 784 785 - "domain" : A normal valid domain cgroup. 786 787 - "domain threaded" : A threaded domain cgroup which is 788 serving as the root of a threaded subtree. 789 790 - "domain invalid" : A cgroup which is in an invalid state. 791 It can't be populated or have controllers enabled. It may 792 be allowed to become a threaded cgroup. 793 794 - "threaded" : A threaded cgroup which is a member of a 795 threaded subtree. 796 797 A cgroup can be turned into a threaded cgroup by writing 798 "threaded" to this file. 799 800 cgroup.procs 801 A read-write new-line separated values file which exists on 802 all cgroups. 803 804 When read, it lists the PIDs of all processes which belong to 805 the cgroup one-per-line. The PIDs are not ordered and the 806 same PID may show up more than once if the process got moved 807 to another cgroup and then back or the PID got recycled while 808 reading. 809 810 A PID can be written to migrate the process associated with 811 the PID to the cgroup. The writer should match all of the 812 following conditions. 813 814 - It must have write access to the "cgroup.procs" file. 815 816 - It must have write access to the "cgroup.procs" file of the 817 common ancestor of the source and destination cgroups. 818 819 When delegating a sub-hierarchy, write access to this file 820 should be granted along with the containing directory. 821 822 In a threaded cgroup, reading this file fails with EOPNOTSUPP 823 as all the processes belong to the thread root. Writing is 824 supported and moves every thread of the process to the cgroup. 825 826 cgroup.threads 827 A read-write new-line separated values file which exists on 828 all cgroups. 829 830 When read, it lists the TIDs of all threads which belong to 831 the cgroup one-per-line. The TIDs are not ordered and the 832 same TID may show up more than once if the thread got moved to 833 another cgroup and then back or the TID got recycled while 834 reading. 835 836 A TID can be written to migrate the thread associated with the 837 TID to the cgroup. The writer should match all of the 838 following conditions. 839 840 - It must have write access to the "cgroup.threads" file. 841 842 - The cgroup that the thread is currently in must be in the 843 same resource domain as the destination cgroup. 844 845 - It must have write access to the "cgroup.procs" file of the 846 common ancestor of the source and destination cgroups. 847 848 When delegating a sub-hierarchy, write access to this file 849 should be granted along with the containing directory. 850 851 cgroup.controllers 852 A read-only space separated values file which exists on all 853 cgroups. 854 855 It shows space separated list of all controllers available to 856 the cgroup. The controllers are not ordered. 857 858 cgroup.subtree_control 859 A read-write space separated values file which exists on all 860 cgroups. Starts out empty. 861 862 When read, it shows space separated list of the controllers 863 which are enabled to control resource distribution from the 864 cgroup to its children. 865 866 Space separated list of controllers prefixed with '+' or '-' 867 can be written to enable or disable controllers. A controller 868 name prefixed with '+' enables the controller and '-' 869 disables. If a controller appears more than once on the list, 870 the last one is effective. When multiple enable and disable 871 operations are specified, either all succeed or all fail. 872 873 cgroup.events 874 A read-only flat-keyed file which exists on non-root cgroups. 875 The following entries are defined. Unless specified 876 otherwise, a value change in this file generates a file 877 modified event. 878 879 populated 880 1 if the cgroup or its descendants contains any live 881 processes; otherwise, 0. 882 frozen 883 1 if the cgroup is frozen; otherwise, 0. 884 885 cgroup.max.descendants 886 A read-write single value files. The default is "max". 887 888 Maximum allowed number of descent cgroups. 889 If the actual number of descendants is equal or larger, 890 an attempt to create a new cgroup in the hierarchy will fail. 891 892 cgroup.max.depth 893 A read-write single value files. The default is "max". 894 895 Maximum allowed descent depth below the current cgroup. 896 If the actual descent depth is equal or larger, 897 an attempt to create a new child cgroup will fail. 898 899 cgroup.stat 900 A read-only flat-keyed file with the following entries: 901 902 nr_descendants 903 Total number of visible descendant cgroups. 904 905 nr_dying_descendants 906 Total number of dying descendant cgroups. A cgroup becomes 907 dying after being deleted by a user. The cgroup will remain 908 in dying state for some time undefined time (which can depend 909 on system load) before being completely destroyed. 910 911 A process can't enter a dying cgroup under any circumstances, 912 a dying cgroup can't revive. 913 914 A dying cgroup can consume system resources not exceeding 915 limits, which were active at the moment of cgroup deletion. 916 917 cgroup.freeze 918 A read-write single value file which exists on non-root cgroups. 919 Allowed values are "0" and "1". The default is "0". 920 921 Writing "1" to the file causes freezing of the cgroup and all 922 descendant cgroups. This means that all belonging processes will 923 be stopped and will not run until the cgroup will be explicitly 924 unfrozen. Freezing of the cgroup may take some time; when this action 925 is completed, the "frozen" value in the cgroup.events control file 926 will be updated to "1" and the corresponding notification will be 927 issued. 928 929 A cgroup can be frozen either by its own settings, or by settings 930 of any ancestor cgroups. If any of ancestor cgroups is frozen, the 931 cgroup will remain frozen. 932 933 Processes in the frozen cgroup can be killed by a fatal signal. 934 They also can enter and leave a frozen cgroup: either by an explicit 935 move by a user, or if freezing of the cgroup races with fork(). 936 If a process is moved to a frozen cgroup, it stops. If a process is 937 moved out of a frozen cgroup, it becomes running. 938 939 Frozen status of a cgroup doesn't affect any cgroup tree operations: 940 it's possible to delete a frozen (and empty) cgroup, as well as 941 create new sub-cgroups. 942 943Controllers 944=========== 945 946CPU 947--- 948 949The "cpu" controllers regulates distribution of CPU cycles. This 950controller implements weight and absolute bandwidth limit models for 951normal scheduling policy and absolute bandwidth allocation model for 952realtime scheduling policy. 953 954In all the above models, cycles distribution is defined only on a temporal 955base and it does not account for the frequency at which tasks are executed. 956The (optional) utilization clamping support allows to hint the schedutil 957cpufreq governor about the minimum desired frequency which should always be 958provided by a CPU, as well as the maximum desired frequency, which should not 959be exceeded by a CPU. 960 961WARNING: cgroup2 doesn't yet support control of realtime processes and 962the cpu controller can only be enabled when all RT processes are in 963the root cgroup. Be aware that system management software may already 964have placed RT processes into nonroot cgroups during the system boot 965process, and these processes may need to be moved to the root cgroup 966before the cpu controller can be enabled. 967 968 969CPU Interface Files 970~~~~~~~~~~~~~~~~~~~ 971 972All time durations are in microseconds. 973 974 cpu.stat 975 A read-only flat-keyed file which exists on non-root cgroups. 976 This file exists whether the controller is enabled or not. 977 978 It always reports the following three stats: 979 980 - usage_usec 981 - user_usec 982 - system_usec 983 984 and the following three when the controller is enabled: 985 986 - nr_periods 987 - nr_throttled 988 - throttled_usec 989 990 cpu.weight 991 A read-write single value file which exists on non-root 992 cgroups. The default is "100". 993 994 The weight in the range [1, 10000]. 995 996 cpu.weight.nice 997 A read-write single value file which exists on non-root 998 cgroups. The default is "0". 999 1000 The nice value is in the range [-20, 19]. 1001 1002 This interface file is an alternative interface for 1003 "cpu.weight" and allows reading and setting weight using the 1004 same values used by nice(2). Because the range is smaller and 1005 granularity is coarser for the nice values, the read value is 1006 the closest approximation of the current weight. 1007 1008 cpu.max 1009 A read-write two value file which exists on non-root cgroups. 1010 The default is "max 100000". 1011 1012 The maximum bandwidth limit. It's in the following format:: 1013 1014 $MAX $PERIOD 1015 1016 which indicates that the group may consume upto $MAX in each 1017 $PERIOD duration. "max" for $MAX indicates no limit. If only 1018 one number is written, $MAX is updated. 1019 1020 cpu.pressure 1021 A read-only nested-key file which exists on non-root cgroups. 1022 1023 Shows pressure stall information for CPU. See 1024 Documentation/accounting/psi.rst for details. 1025 1026 cpu.uclamp.min 1027 A read-write single value file which exists on non-root cgroups. 1028 The default is "0", i.e. no utilization boosting. 1029 1030 The requested minimum utilization (protection) as a percentage 1031 rational number, e.g. 12.34 for 12.34%. 1032 1033 This interface allows reading and setting minimum utilization clamp 1034 values similar to the sched_setattr(2). This minimum utilization 1035 value is used to clamp the task specific minimum utilization clamp. 1036 1037 The requested minimum utilization (protection) is always capped by 1038 the current value for the maximum utilization (limit), i.e. 1039 `cpu.uclamp.max`. 1040 1041 cpu.uclamp.max 1042 A read-write single value file which exists on non-root cgroups. 1043 The default is "max". i.e. no utilization capping 1044 1045 The requested maximum utilization (limit) as a percentage rational 1046 number, e.g. 98.76 for 98.76%. 1047 1048 This interface allows reading and setting maximum utilization clamp 1049 values similar to the sched_setattr(2). This maximum utilization 1050 value is used to clamp the task specific maximum utilization clamp. 1051 1052 1053 1054Memory 1055------ 1056 1057The "memory" controller regulates distribution of memory. Memory is 1058stateful and implements both limit and protection models. Due to the 1059intertwining between memory usage and reclaim pressure and the 1060stateful nature of memory, the distribution model is relatively 1061complex. 1062 1063While not completely water-tight, all major memory usages by a given 1064cgroup are tracked so that the total memory consumption can be 1065accounted and controlled to a reasonable extent. Currently, the 1066following types of memory usages are tracked. 1067 1068- Userland memory - page cache and anonymous memory. 1069 1070- Kernel data structures such as dentries and inodes. 1071 1072- TCP socket buffers. 1073 1074The above list may expand in the future for better coverage. 1075 1076 1077Memory Interface Files 1078~~~~~~~~~~~~~~~~~~~~~~ 1079 1080All memory amounts are in bytes. If a value which is not aligned to 1081PAGE_SIZE is written, the value may be rounded up to the closest 1082PAGE_SIZE multiple when read back. 1083 1084 memory.current 1085 A read-only single value file which exists on non-root 1086 cgroups. 1087 1088 The total amount of memory currently being used by the cgroup 1089 and its descendants. 1090 1091 memory.min 1092 A read-write single value file which exists on non-root 1093 cgroups. The default is "0". 1094 1095 Hard memory protection. If the memory usage of a cgroup 1096 is within its effective min boundary, the cgroup's memory 1097 won't be reclaimed under any conditions. If there is no 1098 unprotected reclaimable memory available, OOM killer 1099 is invoked. Above the effective min boundary (or 1100 effective low boundary if it is higher), pages are reclaimed 1101 proportionally to the overage, reducing reclaim pressure for 1102 smaller overages. 1103 1104 Effective min boundary is limited by memory.min values of 1105 all ancestor cgroups. If there is memory.min overcommitment 1106 (child cgroup or cgroups are requiring more protected memory 1107 than parent will allow), then each child cgroup will get 1108 the part of parent's protection proportional to its 1109 actual memory usage below memory.min. 1110 1111 Putting more memory than generally available under this 1112 protection is discouraged and may lead to constant OOMs. 1113 1114 If a memory cgroup is not populated with processes, 1115 its memory.min is ignored. 1116 1117 memory.low 1118 A read-write single value file which exists on non-root 1119 cgroups. The default is "0". 1120 1121 Best-effort memory protection. If the memory usage of a 1122 cgroup is within its effective low boundary, the cgroup's 1123 memory won't be reclaimed unless there is no reclaimable 1124 memory available in unprotected cgroups. 1125 Above the effective low boundary (or 1126 effective min boundary if it is higher), pages are reclaimed 1127 proportionally to the overage, reducing reclaim pressure for 1128 smaller overages. 1129 1130 Effective low boundary is limited by memory.low values of 1131 all ancestor cgroups. If there is memory.low overcommitment 1132 (child cgroup or cgroups are requiring more protected memory 1133 than parent will allow), then each child cgroup will get 1134 the part of parent's protection proportional to its 1135 actual memory usage below memory.low. 1136 1137 Putting more memory than generally available under this 1138 protection is discouraged. 1139 1140 memory.high 1141 A read-write single value file which exists on non-root 1142 cgroups. The default is "max". 1143 1144 Memory usage throttle limit. This is the main mechanism to 1145 control memory usage of a cgroup. If a cgroup's usage goes 1146 over the high boundary, the processes of the cgroup are 1147 throttled and put under heavy reclaim pressure. 1148 1149 Going over the high limit never invokes the OOM killer and 1150 under extreme conditions the limit may be breached. 1151 1152 memory.max 1153 A read-write single value file which exists on non-root 1154 cgroups. The default is "max". 1155 1156 Memory usage hard limit. This is the final protection 1157 mechanism. If a cgroup's memory usage reaches this limit and 1158 can't be reduced, the OOM killer is invoked in the cgroup. 1159 Under certain circumstances, the usage may go over the limit 1160 temporarily. 1161 1162 This is the ultimate protection mechanism. As long as the 1163 high limit is used and monitored properly, this limit's 1164 utility is limited to providing the final safety net. 1165 1166 memory.oom.group 1167 A read-write single value file which exists on non-root 1168 cgroups. The default value is "0". 1169 1170 Determines whether the cgroup should be treated as 1171 an indivisible workload by the OOM killer. If set, 1172 all tasks belonging to the cgroup or to its descendants 1173 (if the memory cgroup is not a leaf cgroup) are killed 1174 together or not at all. This can be used to avoid 1175 partial kills to guarantee workload integrity. 1176 1177 Tasks with the OOM protection (oom_score_adj set to -1000) 1178 are treated as an exception and are never killed. 1179 1180 If the OOM killer is invoked in a cgroup, it's not going 1181 to kill any tasks outside of this cgroup, regardless 1182 memory.oom.group values of ancestor cgroups. 1183 1184 memory.events 1185 A read-only flat-keyed file which exists on non-root cgroups. 1186 The following entries are defined. Unless specified 1187 otherwise, a value change in this file generates a file 1188 modified event. 1189 1190 Note that all fields in this file are hierarchical and the 1191 file modified event can be generated due to an event down the 1192 hierarchy. For for the local events at the cgroup level see 1193 memory.events.local. 1194 1195 low 1196 The number of times the cgroup is reclaimed due to 1197 high memory pressure even though its usage is under 1198 the low boundary. This usually indicates that the low 1199 boundary is over-committed. 1200 1201 high 1202 The number of times processes of the cgroup are 1203 throttled and routed to perform direct memory reclaim 1204 because the high memory boundary was exceeded. For a 1205 cgroup whose memory usage is capped by the high limit 1206 rather than global memory pressure, this event's 1207 occurrences are expected. 1208 1209 max 1210 The number of times the cgroup's memory usage was 1211 about to go over the max boundary. If direct reclaim 1212 fails to bring it down, the cgroup goes to OOM state. 1213 1214 oom 1215 The number of time the cgroup's memory usage was 1216 reached the limit and allocation was about to fail. 1217 1218 Depending on context result could be invocation of OOM 1219 killer and retrying allocation or failing allocation. 1220 1221 Failed allocation in its turn could be returned into 1222 userspace as -ENOMEM or silently ignored in cases like 1223 disk readahead. For now OOM in memory cgroup kills 1224 tasks iff shortage has happened inside page fault. 1225 1226 This event is not raised if the OOM killer is not 1227 considered as an option, e.g. for failed high-order 1228 allocations. 1229 1230 oom_kill 1231 The number of processes belonging to this cgroup 1232 killed by any kind of OOM killer. 1233 1234 memory.events.local 1235 Similar to memory.events but the fields in the file are local 1236 to the cgroup i.e. not hierarchical. The file modified event 1237 generated on this file reflects only the local events. 1238 1239 memory.stat 1240 A read-only flat-keyed file which exists on non-root cgroups. 1241 1242 This breaks down the cgroup's memory footprint into different 1243 types of memory, type-specific details, and other information 1244 on the state and past events of the memory management system. 1245 1246 All memory amounts are in bytes. 1247 1248 The entries are ordered to be human readable, and new entries 1249 can show up in the middle. Don't rely on items remaining in a 1250 fixed position; use the keys to look up specific values! 1251 1252 anon 1253 Amount of memory used in anonymous mappings such as 1254 brk(), sbrk(), and mmap(MAP_ANONYMOUS) 1255 1256 file 1257 Amount of memory used to cache filesystem data, 1258 including tmpfs and shared memory. 1259 1260 kernel_stack 1261 Amount of memory allocated to kernel stacks. 1262 1263 slab 1264 Amount of memory used for storing in-kernel data 1265 structures. 1266 1267 sock 1268 Amount of memory used in network transmission buffers 1269 1270 shmem 1271 Amount of cached filesystem data that is swap-backed, 1272 such as tmpfs, shm segments, shared anonymous mmap()s 1273 1274 file_mapped 1275 Amount of cached filesystem data mapped with mmap() 1276 1277 file_dirty 1278 Amount of cached filesystem data that was modified but 1279 not yet written back to disk 1280 1281 file_writeback 1282 Amount of cached filesystem data that was modified and 1283 is currently being written back to disk 1284 1285 anon_thp 1286 Amount of memory used in anonymous mappings backed by 1287 transparent hugepages 1288 1289 inactive_anon, active_anon, inactive_file, active_file, unevictable 1290 Amount of memory, swap-backed and filesystem-backed, 1291 on the internal memory management lists used by the 1292 page reclaim algorithm. 1293 1294 As these represent internal list state (eg. shmem pages are on anon 1295 memory management lists), inactive_foo + active_foo may not be equal to 1296 the value for the foo counter, since the foo counter is type-based, not 1297 list-based. 1298 1299 slab_reclaimable 1300 Part of "slab" that might be reclaimed, such as 1301 dentries and inodes. 1302 1303 slab_unreclaimable 1304 Part of "slab" that cannot be reclaimed on memory 1305 pressure. 1306 1307 pgfault 1308 Total number of page faults incurred 1309 1310 pgmajfault 1311 Number of major page faults incurred 1312 1313 workingset_refault 1314 1315 Number of refaults of previously evicted pages 1316 1317 workingset_activate 1318 1319 Number of refaulted pages that were immediately activated 1320 1321 workingset_nodereclaim 1322 1323 Number of times a shadow node has been reclaimed 1324 1325 pgrefill 1326 1327 Amount of scanned pages (in an active LRU list) 1328 1329 pgscan 1330 1331 Amount of scanned pages (in an inactive LRU list) 1332 1333 pgsteal 1334 1335 Amount of reclaimed pages 1336 1337 pgactivate 1338 1339 Amount of pages moved to the active LRU list 1340 1341 pgdeactivate 1342 1343 Amount of pages moved to the inactive LRU list 1344 1345 pglazyfree 1346 1347 Amount of pages postponed to be freed under memory pressure 1348 1349 pglazyfreed 1350 1351 Amount of reclaimed lazyfree pages 1352 1353 thp_fault_alloc 1354 1355 Number of transparent hugepages which were allocated to satisfy 1356 a page fault, including COW faults. This counter is not present 1357 when CONFIG_TRANSPARENT_HUGEPAGE is not set. 1358 1359 thp_collapse_alloc 1360 1361 Number of transparent hugepages which were allocated to allow 1362 collapsing an existing range of pages. This counter is not 1363 present when CONFIG_TRANSPARENT_HUGEPAGE is not set. 1364 1365 memory.swap.current 1366 A read-only single value file which exists on non-root 1367 cgroups. 1368 1369 The total amount of swap currently being used by the cgroup 1370 and its descendants. 1371 1372 memory.swap.max 1373 A read-write single value file which exists on non-root 1374 cgroups. The default is "max". 1375 1376 Swap usage hard limit. If a cgroup's swap usage reaches this 1377 limit, anonymous memory of the cgroup will not be swapped out. 1378 1379 memory.swap.events 1380 A read-only flat-keyed file which exists on non-root cgroups. 1381 The following entries are defined. Unless specified 1382 otherwise, a value change in this file generates a file 1383 modified event. 1384 1385 max 1386 The number of times the cgroup's swap usage was about 1387 to go over the max boundary and swap allocation 1388 failed. 1389 1390 fail 1391 The number of times swap allocation failed either 1392 because of running out of swap system-wide or max 1393 limit. 1394 1395 When reduced under the current usage, the existing swap 1396 entries are reclaimed gradually and the swap usage may stay 1397 higher than the limit for an extended period of time. This 1398 reduces the impact on the workload and memory management. 1399 1400 memory.pressure 1401 A read-only nested-key file which exists on non-root cgroups. 1402 1403 Shows pressure stall information for memory. See 1404 Documentation/accounting/psi.rst for details. 1405 1406 1407Usage Guidelines 1408~~~~~~~~~~~~~~~~ 1409 1410"memory.high" is the main mechanism to control memory usage. 1411Over-committing on high limit (sum of high limits > available memory) 1412and letting global memory pressure to distribute memory according to 1413usage is a viable strategy. 1414 1415Because breach of the high limit doesn't trigger the OOM killer but 1416throttles the offending cgroup, a management agent has ample 1417opportunities to monitor and take appropriate actions such as granting 1418more memory or terminating the workload. 1419 1420Determining whether a cgroup has enough memory is not trivial as 1421memory usage doesn't indicate whether the workload can benefit from 1422more memory. For example, a workload which writes data received from 1423network to a file can use all available memory but can also operate as 1424performant with a small amount of memory. A measure of memory 1425pressure - how much the workload is being impacted due to lack of 1426memory - is necessary to determine whether a workload needs more 1427memory; unfortunately, memory pressure monitoring mechanism isn't 1428implemented yet. 1429 1430 1431Memory Ownership 1432~~~~~~~~~~~~~~~~ 1433 1434A memory area is charged to the cgroup which instantiated it and stays 1435charged to the cgroup until the area is released. Migrating a process 1436to a different cgroup doesn't move the memory usages that it 1437instantiated while in the previous cgroup to the new cgroup. 1438 1439A memory area may be used by processes belonging to different cgroups. 1440To which cgroup the area will be charged is in-deterministic; however, 1441over time, the memory area is likely to end up in a cgroup which has 1442enough memory allowance to avoid high reclaim pressure. 1443 1444If a cgroup sweeps a considerable amount of memory which is expected 1445to be accessed repeatedly by other cgroups, it may make sense to use 1446POSIX_FADV_DONTNEED to relinquish the ownership of memory areas 1447belonging to the affected files to ensure correct memory ownership. 1448 1449 1450IO 1451-- 1452 1453The "io" controller regulates the distribution of IO resources. This 1454controller implements both weight based and absolute bandwidth or IOPS 1455limit distribution; however, weight based distribution is available 1456only if cfq-iosched is in use and neither scheme is available for 1457blk-mq devices. 1458 1459 1460IO Interface Files 1461~~~~~~~~~~~~~~~~~~ 1462 1463 io.stat 1464 A read-only nested-keyed file which exists on non-root 1465 cgroups. 1466 1467 Lines are keyed by $MAJ:$MIN device numbers and not ordered. 1468 The following nested keys are defined. 1469 1470 ====== ===================== 1471 rbytes Bytes read 1472 wbytes Bytes written 1473 rios Number of read IOs 1474 wios Number of write IOs 1475 dbytes Bytes discarded 1476 dios Number of discard IOs 1477 ====== ===================== 1478 1479 An example read output follows: 1480 1481 8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0 1482 8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021 1483 1484 io.cost.qos 1485 A read-write nested-keyed file with exists only on the root 1486 cgroup. 1487 1488 This file configures the Quality of Service of the IO cost 1489 model based controller (CONFIG_BLK_CGROUP_IOCOST) which 1490 currently implements "io.weight" proportional control. Lines 1491 are keyed by $MAJ:$MIN device numbers and not ordered. The 1492 line for a given device is populated on the first write for 1493 the device on "io.cost.qos" or "io.cost.model". The following 1494 nested keys are defined. 1495 1496 ====== ===================================== 1497 enable Weight-based control enable 1498 ctrl "auto" or "user" 1499 rpct Read latency percentile [0, 100] 1500 rlat Read latency threshold 1501 wpct Write latency percentile [0, 100] 1502 wlat Write latency threshold 1503 min Minimum scaling percentage [1, 10000] 1504 max Maximum scaling percentage [1, 10000] 1505 ====== ===================================== 1506 1507 The controller is disabled by default and can be enabled by 1508 setting "enable" to 1. "rpct" and "wpct" parameters default 1509 to zero and the controller uses internal device saturation 1510 state to adjust the overall IO rate between "min" and "max". 1511 1512 When a better control quality is needed, latency QoS 1513 parameters can be configured. For example:: 1514 1515 8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0 1516 1517 shows that on sdb, the controller is enabled, will consider 1518 the device saturated if the 95th percentile of read completion 1519 latencies is above 75ms or write 150ms, and adjust the overall 1520 IO issue rate between 50% and 150% accordingly. 1521 1522 The lower the saturation point, the better the latency QoS at 1523 the cost of aggregate bandwidth. The narrower the allowed 1524 adjustment range between "min" and "max", the more conformant 1525 to the cost model the IO behavior. Note that the IO issue 1526 base rate may be far off from 100% and setting "min" and "max" 1527 blindly can lead to a significant loss of device capacity or 1528 control quality. "min" and "max" are useful for regulating 1529 devices which show wide temporary behavior changes - e.g. a 1530 ssd which accepts writes at the line speed for a while and 1531 then completely stalls for multiple seconds. 1532 1533 When "ctrl" is "auto", the parameters are controlled by the 1534 kernel and may change automatically. Setting "ctrl" to "user" 1535 or setting any of the percentile and latency parameters puts 1536 it into "user" mode and disables the automatic changes. The 1537 automatic mode can be restored by setting "ctrl" to "auto". 1538 1539 io.cost.model 1540 A read-write nested-keyed file with exists only on the root 1541 cgroup. 1542 1543 This file configures the cost model of the IO cost model based 1544 controller (CONFIG_BLK_CGROUP_IOCOST) which currently 1545 implements "io.weight" proportional control. Lines are keyed 1546 by $MAJ:$MIN device numbers and not ordered. The line for a 1547 given device is populated on the first write for the device on 1548 "io.cost.qos" or "io.cost.model". The following nested keys 1549 are defined. 1550 1551 ===== ================================ 1552 ctrl "auto" or "user" 1553 model The cost model in use - "linear" 1554 ===== ================================ 1555 1556 When "ctrl" is "auto", the kernel may change all parameters 1557 dynamically. When "ctrl" is set to "user" or any other 1558 parameters are written to, "ctrl" become "user" and the 1559 automatic changes are disabled. 1560 1561 When "model" is "linear", the following model parameters are 1562 defined. 1563 1564 ============= ======================================== 1565 [r|w]bps The maximum sequential IO throughput 1566 [r|w]seqiops The maximum 4k sequential IOs per second 1567 [r|w]randiops The maximum 4k random IOs per second 1568 ============= ======================================== 1569 1570 From the above, the builtin linear model determines the base 1571 costs of a sequential and random IO and the cost coefficient 1572 for the IO size. While simple, this model can cover most 1573 common device classes acceptably. 1574 1575 The IO cost model isn't expected to be accurate in absolute 1576 sense and is scaled to the device behavior dynamically. 1577 1578 If needed, tools/cgroup/iocost_coef_gen.py can be used to 1579 generate device-specific coefficients. 1580 1581 io.weight 1582 A read-write flat-keyed file which exists on non-root cgroups. 1583 The default is "default 100". 1584 1585 The first line is the default weight applied to devices 1586 without specific override. The rest are overrides keyed by 1587 $MAJ:$MIN device numbers and not ordered. The weights are in 1588 the range [1, 10000] and specifies the relative amount IO time 1589 the cgroup can use in relation to its siblings. 1590 1591 The default weight can be updated by writing either "default 1592 $WEIGHT" or simply "$WEIGHT". Overrides can be set by writing 1593 "$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default". 1594 1595 An example read output follows:: 1596 1597 default 100 1598 8:16 200 1599 8:0 50 1600 1601 io.max 1602 A read-write nested-keyed file which exists on non-root 1603 cgroups. 1604 1605 BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN 1606 device numbers and not ordered. The following nested keys are 1607 defined. 1608 1609 ===== ================================== 1610 rbps Max read bytes per second 1611 wbps Max write bytes per second 1612 riops Max read IO operations per second 1613 wiops Max write IO operations per second 1614 ===== ================================== 1615 1616 When writing, any number of nested key-value pairs can be 1617 specified in any order. "max" can be specified as the value 1618 to remove a specific limit. If the same key is specified 1619 multiple times, the outcome is undefined. 1620 1621 BPS and IOPS are measured in each IO direction and IOs are 1622 delayed if limit is reached. Temporary bursts are allowed. 1623 1624 Setting read limit at 2M BPS and write at 120 IOPS for 8:16:: 1625 1626 echo "8:16 rbps=2097152 wiops=120" > io.max 1627 1628 Reading returns the following:: 1629 1630 8:16 rbps=2097152 wbps=max riops=max wiops=120 1631 1632 Write IOPS limit can be removed by writing the following:: 1633 1634 echo "8:16 wiops=max" > io.max 1635 1636 Reading now returns the following:: 1637 1638 8:16 rbps=2097152 wbps=max riops=max wiops=max 1639 1640 io.pressure 1641 A read-only nested-key file which exists on non-root cgroups. 1642 1643 Shows pressure stall information for IO. See 1644 Documentation/accounting/psi.rst for details. 1645 1646 1647Writeback 1648~~~~~~~~~ 1649 1650Page cache is dirtied through buffered writes and shared mmaps and 1651written asynchronously to the backing filesystem by the writeback 1652mechanism. Writeback sits between the memory and IO domains and 1653regulates the proportion of dirty memory by balancing dirtying and 1654write IOs. 1655 1656The io controller, in conjunction with the memory controller, 1657implements control of page cache writeback IOs. The memory controller 1658defines the memory domain that dirty memory ratio is calculated and 1659maintained for and the io controller defines the io domain which 1660writes out dirty pages for the memory domain. Both system-wide and 1661per-cgroup dirty memory states are examined and the more restrictive 1662of the two is enforced. 1663 1664cgroup writeback requires explicit support from the underlying 1665filesystem. Currently, cgroup writeback is implemented on ext2, ext4 1666and btrfs. On other filesystems, all writeback IOs are attributed to 1667the root cgroup. 1668 1669There are inherent differences in memory and writeback management 1670which affects how cgroup ownership is tracked. Memory is tracked per 1671page while writeback per inode. For the purpose of writeback, an 1672inode is assigned to a cgroup and all IO requests to write dirty pages 1673from the inode are attributed to that cgroup. 1674 1675As cgroup ownership for memory is tracked per page, there can be pages 1676which are associated with different cgroups than the one the inode is 1677associated with. These are called foreign pages. The writeback 1678constantly keeps track of foreign pages and, if a particular foreign 1679cgroup becomes the majority over a certain period of time, switches 1680the ownership of the inode to that cgroup. 1681 1682While this model is enough for most use cases where a given inode is 1683mostly dirtied by a single cgroup even when the main writing cgroup 1684changes over time, use cases where multiple cgroups write to a single 1685inode simultaneously are not supported well. In such circumstances, a 1686significant portion of IOs are likely to be attributed incorrectly. 1687As memory controller assigns page ownership on the first use and 1688doesn't update it until the page is released, even if writeback 1689strictly follows page ownership, multiple cgroups dirtying overlapping 1690areas wouldn't work as expected. It's recommended to avoid such usage 1691patterns. 1692 1693The sysctl knobs which affect writeback behavior are applied to cgroup 1694writeback as follows. 1695 1696 vm.dirty_background_ratio, vm.dirty_ratio 1697 These ratios apply the same to cgroup writeback with the 1698 amount of available memory capped by limits imposed by the 1699 memory controller and system-wide clean memory. 1700 1701 vm.dirty_background_bytes, vm.dirty_bytes 1702 For cgroup writeback, this is calculated into ratio against 1703 total available memory and applied the same way as 1704 vm.dirty[_background]_ratio. 1705 1706 1707IO Latency 1708~~~~~~~~~~ 1709 1710This is a cgroup v2 controller for IO workload protection. You provide a group 1711with a latency target, and if the average latency exceeds that target the 1712controller will throttle any peers that have a lower latency target than the 1713protected workload. 1714 1715The limits are only applied at the peer level in the hierarchy. This means that 1716in the diagram below, only groups A, B, and C will influence each other, and 1717groups D and F will influence each other. Group G will influence nobody:: 1718 1719 [root] 1720 / | \ 1721 A B C 1722 / \ | 1723 D F G 1724 1725 1726So the ideal way to configure this is to set io.latency in groups A, B, and C. 1727Generally you do not want to set a value lower than the latency your device 1728supports. Experiment to find the value that works best for your workload. 1729Start at higher than the expected latency for your device and watch the 1730avg_lat value in io.stat for your workload group to get an idea of the 1731latency you see during normal operation. Use the avg_lat value as a basis for 1732your real setting, setting at 10-15% higher than the value in io.stat. 1733 1734How IO Latency Throttling Works 1735~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1736 1737io.latency is work conserving; so as long as everybody is meeting their latency 1738target the controller doesn't do anything. Once a group starts missing its 1739target it begins throttling any peer group that has a higher target than itself. 1740This throttling takes 2 forms: 1741 1742- Queue depth throttling. This is the number of outstanding IO's a group is 1743 allowed to have. We will clamp down relatively quickly, starting at no limit 1744 and going all the way down to 1 IO at a time. 1745 1746- Artificial delay induction. There are certain types of IO that cannot be 1747 throttled without possibly adversely affecting higher priority groups. This 1748 includes swapping and metadata IO. These types of IO are allowed to occur 1749 normally, however they are "charged" to the originating group. If the 1750 originating group is being throttled you will see the use_delay and delay 1751 fields in io.stat increase. The delay value is how many microseconds that are 1752 being added to any process that runs in this group. Because this number can 1753 grow quite large if there is a lot of swapping or metadata IO occurring we 1754 limit the individual delay events to 1 second at a time. 1755 1756Once the victimized group starts meeting its latency target again it will start 1757unthrottling any peer groups that were throttled previously. If the victimized 1758group simply stops doing IO the global counter will unthrottle appropriately. 1759 1760IO Latency Interface Files 1761~~~~~~~~~~~~~~~~~~~~~~~~~~ 1762 1763 io.latency 1764 This takes a similar format as the other controllers. 1765 1766 "MAJOR:MINOR target=<target time in microseconds" 1767 1768 io.stat 1769 If the controller is enabled you will see extra stats in io.stat in 1770 addition to the normal ones. 1771 1772 depth 1773 This is the current queue depth for the group. 1774 1775 avg_lat 1776 This is an exponential moving average with a decay rate of 1/exp 1777 bound by the sampling interval. The decay rate interval can be 1778 calculated by multiplying the win value in io.stat by the 1779 corresponding number of samples based on the win value. 1780 1781 win 1782 The sampling window size in milliseconds. This is the minimum 1783 duration of time between evaluation events. Windows only elapse 1784 with IO activity. Idle periods extend the most recent window. 1785 1786PID 1787--- 1788 1789The process number controller is used to allow a cgroup to stop any 1790new tasks from being fork()'d or clone()'d after a specified limit is 1791reached. 1792 1793The number of tasks in a cgroup can be exhausted in ways which other 1794controllers cannot prevent, thus warranting its own controller. For 1795example, a fork bomb is likely to exhaust the number of tasks before 1796hitting memory restrictions. 1797 1798Note that PIDs used in this controller refer to TIDs, process IDs as 1799used by the kernel. 1800 1801 1802PID Interface Files 1803~~~~~~~~~~~~~~~~~~~ 1804 1805 pids.max 1806 A read-write single value file which exists on non-root 1807 cgroups. The default is "max". 1808 1809 Hard limit of number of processes. 1810 1811 pids.current 1812 A read-only single value file which exists on all cgroups. 1813 1814 The number of processes currently in the cgroup and its 1815 descendants. 1816 1817Organisational operations are not blocked by cgroup policies, so it is 1818possible to have pids.current > pids.max. This can be done by either 1819setting the limit to be smaller than pids.current, or attaching enough 1820processes to the cgroup such that pids.current is larger than 1821pids.max. However, it is not possible to violate a cgroup PID policy 1822through fork() or clone(). These will return -EAGAIN if the creation 1823of a new process would cause a cgroup policy to be violated. 1824 1825 1826Cpuset 1827------ 1828 1829The "cpuset" controller provides a mechanism for constraining 1830the CPU and memory node placement of tasks to only the resources 1831specified in the cpuset interface files in a task's current cgroup. 1832This is especially valuable on large NUMA systems where placing jobs 1833on properly sized subsets of the systems with careful processor and 1834memory placement to reduce cross-node memory access and contention 1835can improve overall system performance. 1836 1837The "cpuset" controller is hierarchical. That means the controller 1838cannot use CPUs or memory nodes not allowed in its parent. 1839 1840 1841Cpuset Interface Files 1842~~~~~~~~~~~~~~~~~~~~~~ 1843 1844 cpuset.cpus 1845 A read-write multiple values file which exists on non-root 1846 cpuset-enabled cgroups. 1847 1848 It lists the requested CPUs to be used by tasks within this 1849 cgroup. The actual list of CPUs to be granted, however, is 1850 subjected to constraints imposed by its parent and can differ 1851 from the requested CPUs. 1852 1853 The CPU numbers are comma-separated numbers or ranges. 1854 For example: 1855 1856 # cat cpuset.cpus 1857 0-4,6,8-10 1858 1859 An empty value indicates that the cgroup is using the same 1860 setting as the nearest cgroup ancestor with a non-empty 1861 "cpuset.cpus" or all the available CPUs if none is found. 1862 1863 The value of "cpuset.cpus" stays constant until the next update 1864 and won't be affected by any CPU hotplug events. 1865 1866 cpuset.cpus.effective 1867 A read-only multiple values file which exists on all 1868 cpuset-enabled cgroups. 1869 1870 It lists the onlined CPUs that are actually granted to this 1871 cgroup by its parent. These CPUs are allowed to be used by 1872 tasks within the current cgroup. 1873 1874 If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows 1875 all the CPUs from the parent cgroup that can be available to 1876 be used by this cgroup. Otherwise, it should be a subset of 1877 "cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus" 1878 can be granted. In this case, it will be treated just like an 1879 empty "cpuset.cpus". 1880 1881 Its value will be affected by CPU hotplug events. 1882 1883 cpuset.mems 1884 A read-write multiple values file which exists on non-root 1885 cpuset-enabled cgroups. 1886 1887 It lists the requested memory nodes to be used by tasks within 1888 this cgroup. The actual list of memory nodes granted, however, 1889 is subjected to constraints imposed by its parent and can differ 1890 from the requested memory nodes. 1891 1892 The memory node numbers are comma-separated numbers or ranges. 1893 For example: 1894 1895 # cat cpuset.mems 1896 0-1,3 1897 1898 An empty value indicates that the cgroup is using the same 1899 setting as the nearest cgroup ancestor with a non-empty 1900 "cpuset.mems" or all the available memory nodes if none 1901 is found. 1902 1903 The value of "cpuset.mems" stays constant until the next update 1904 and won't be affected by any memory nodes hotplug events. 1905 1906 cpuset.mems.effective 1907 A read-only multiple values file which exists on all 1908 cpuset-enabled cgroups. 1909 1910 It lists the onlined memory nodes that are actually granted to 1911 this cgroup by its parent. These memory nodes are allowed to 1912 be used by tasks within the current cgroup. 1913 1914 If "cpuset.mems" is empty, it shows all the memory nodes from the 1915 parent cgroup that will be available to be used by this cgroup. 1916 Otherwise, it should be a subset of "cpuset.mems" unless none of 1917 the memory nodes listed in "cpuset.mems" can be granted. In this 1918 case, it will be treated just like an empty "cpuset.mems". 1919 1920 Its value will be affected by memory nodes hotplug events. 1921 1922 cpuset.cpus.partition 1923 A read-write single value file which exists on non-root 1924 cpuset-enabled cgroups. This flag is owned by the parent cgroup 1925 and is not delegatable. 1926 1927 It accepts only the following input values when written to. 1928 1929 "root" - a partition root 1930 "member" - a non-root member of a partition 1931 1932 When set to be a partition root, the current cgroup is the 1933 root of a new partition or scheduling domain that comprises 1934 itself and all its descendants except those that are separate 1935 partition roots themselves and their descendants. The root 1936 cgroup is always a partition root. 1937 1938 There are constraints on where a partition root can be set. 1939 It can only be set in a cgroup if all the following conditions 1940 are true. 1941 1942 1) The "cpuset.cpus" is not empty and the list of CPUs are 1943 exclusive, i.e. they are not shared by any of its siblings. 1944 2) The parent cgroup is a partition root. 1945 3) The "cpuset.cpus" is also a proper subset of the parent's 1946 "cpuset.cpus.effective". 1947 4) There is no child cgroups with cpuset enabled. This is for 1948 eliminating corner cases that have to be handled if such a 1949 condition is allowed. 1950 1951 Setting it to partition root will take the CPUs away from the 1952 effective CPUs of the parent cgroup. Once it is set, this 1953 file cannot be reverted back to "member" if there are any child 1954 cgroups with cpuset enabled. 1955 1956 A parent partition cannot distribute all its CPUs to its 1957 child partitions. There must be at least one cpu left in the 1958 parent partition. 1959 1960 Once becoming a partition root, changes to "cpuset.cpus" is 1961 generally allowed as long as the first condition above is true, 1962 the change will not take away all the CPUs from the parent 1963 partition and the new "cpuset.cpus" value is a superset of its 1964 children's "cpuset.cpus" values. 1965 1966 Sometimes, external factors like changes to ancestors' 1967 "cpuset.cpus" or cpu hotplug can cause the state of the partition 1968 root to change. On read, the "cpuset.sched.partition" file 1969 can show the following values. 1970 1971 "member" Non-root member of a partition 1972 "root" Partition root 1973 "root invalid" Invalid partition root 1974 1975 It is a partition root if the first 2 partition root conditions 1976 above are true and at least one CPU from "cpuset.cpus" is 1977 granted by the parent cgroup. 1978 1979 A partition root can become invalid if none of CPUs requested 1980 in "cpuset.cpus" can be granted by the parent cgroup or the 1981 parent cgroup is no longer a partition root itself. In this 1982 case, it is not a real partition even though the restriction 1983 of the first partition root condition above will still apply. 1984 The cpu affinity of all the tasks in the cgroup will then be 1985 associated with CPUs in the nearest ancestor partition. 1986 1987 An invalid partition root can be transitioned back to a 1988 real partition root if at least one of the requested CPUs 1989 can now be granted by its parent. In this case, the cpu 1990 affinity of all the tasks in the formerly invalid partition 1991 will be associated to the CPUs of the newly formed partition. 1992 Changing the partition state of an invalid partition root to 1993 "member" is always allowed even if child cpusets are present. 1994 1995 1996Device controller 1997----------------- 1998 1999Device controller manages access to device files. It includes both 2000creation of new device files (using mknod), and access to the 2001existing device files. 2002 2003Cgroup v2 device controller has no interface files and is implemented 2004on top of cgroup BPF. To control access to device files, a user may 2005create bpf programs of the BPF_CGROUP_DEVICE type and attach them 2006to cgroups. On an attempt to access a device file, corresponding 2007BPF programs will be executed, and depending on the return value 2008the attempt will succeed or fail with -EPERM. 2009 2010A BPF_CGROUP_DEVICE program takes a pointer to the bpf_cgroup_dev_ctx 2011structure, which describes the device access attempt: access type 2012(mknod/read/write) and device (type, major and minor numbers). 2013If the program returns 0, the attempt fails with -EPERM, otherwise 2014it succeeds. 2015 2016An example of BPF_CGROUP_DEVICE program may be found in the kernel 2017source tree in the tools/testing/selftests/bpf/dev_cgroup.c file. 2018 2019 2020RDMA 2021---- 2022 2023The "rdma" controller regulates the distribution and accounting of 2024of RDMA resources. 2025 2026RDMA Interface Files 2027~~~~~~~~~~~~~~~~~~~~ 2028 2029 rdma.max 2030 A readwrite nested-keyed file that exists for all the cgroups 2031 except root that describes current configured resource limit 2032 for a RDMA/IB device. 2033 2034 Lines are keyed by device name and are not ordered. 2035 Each line contains space separated resource name and its configured 2036 limit that can be distributed. 2037 2038 The following nested keys are defined. 2039 2040 ========== ============================= 2041 hca_handle Maximum number of HCA Handles 2042 hca_object Maximum number of HCA Objects 2043 ========== ============================= 2044 2045 An example for mlx4 and ocrdma device follows:: 2046 2047 mlx4_0 hca_handle=2 hca_object=2000 2048 ocrdma1 hca_handle=3 hca_object=max 2049 2050 rdma.current 2051 A read-only file that describes current resource usage. 2052 It exists for all the cgroup except root. 2053 2054 An example for mlx4 and ocrdma device follows:: 2055 2056 mlx4_0 hca_handle=1 hca_object=20 2057 ocrdma1 hca_handle=1 hca_object=23 2058 2059 2060Misc 2061---- 2062 2063perf_event 2064~~~~~~~~~~ 2065 2066perf_event controller, if not mounted on a legacy hierarchy, is 2067automatically enabled on the v2 hierarchy so that perf events can 2068always be filtered by cgroup v2 path. The controller can still be 2069moved to a legacy hierarchy after v2 hierarchy is populated. 2070 2071 2072Non-normative information 2073------------------------- 2074 2075This section contains information that isn't considered to be a part of 2076the stable kernel API and so is subject to change. 2077 2078 2079CPU controller root cgroup process behaviour 2080~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2081 2082When distributing CPU cycles in the root cgroup each thread in this 2083cgroup is treated as if it was hosted in a separate child cgroup of the 2084root cgroup. This child cgroup weight is dependent on its thread nice 2085level. 2086 2087For details of this mapping see sched_prio_to_weight array in 2088kernel/sched/core.c file (values from this array should be scaled 2089appropriately so the neutral - nice 0 - value is 100 instead of 1024). 2090 2091 2092IO controller root cgroup process behaviour 2093~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2094 2095Root cgroup processes are hosted in an implicit leaf child node. 2096When distributing IO resources this implicit child node is taken into 2097account as if it was a normal child cgroup of the root cgroup with a 2098weight value of 200. 2099 2100 2101Namespace 2102========= 2103 2104Basics 2105------ 2106 2107cgroup namespace provides a mechanism to virtualize the view of the 2108"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone 2109flag can be used with clone(2) and unshare(2) to create a new cgroup 2110namespace. The process running inside the cgroup namespace will have 2111its "/proc/$PID/cgroup" output restricted to cgroupns root. The 2112cgroupns root is the cgroup of the process at the time of creation of 2113the cgroup namespace. 2114 2115Without cgroup namespace, the "/proc/$PID/cgroup" file shows the 2116complete path of the cgroup of a process. In a container setup where 2117a set of cgroups and namespaces are intended to isolate processes the 2118"/proc/$PID/cgroup" file may leak potential system level information 2119to the isolated processes. For Example:: 2120 2121 # cat /proc/self/cgroup 2122 0::/batchjobs/container_id1 2123 2124The path '/batchjobs/container_id1' can be considered as system-data 2125and undesirable to expose to the isolated processes. cgroup namespace 2126can be used to restrict visibility of this path. For example, before 2127creating a cgroup namespace, one would see:: 2128 2129 # ls -l /proc/self/ns/cgroup 2130 lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835] 2131 # cat /proc/self/cgroup 2132 0::/batchjobs/container_id1 2133 2134After unsharing a new namespace, the view changes:: 2135 2136 # ls -l /proc/self/ns/cgroup 2137 lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183] 2138 # cat /proc/self/cgroup 2139 0::/ 2140 2141When some thread from a multi-threaded process unshares its cgroup 2142namespace, the new cgroupns gets applied to the entire process (all 2143the threads). This is natural for the v2 hierarchy; however, for the 2144legacy hierarchies, this may be unexpected. 2145 2146A cgroup namespace is alive as long as there are processes inside or 2147mounts pinning it. When the last usage goes away, the cgroup 2148namespace is destroyed. The cgroupns root and the actual cgroups 2149remain. 2150 2151 2152The Root and Views 2153------------------ 2154 2155The 'cgroupns root' for a cgroup namespace is the cgroup in which the 2156process calling unshare(2) is running. For example, if a process in 2157/batchjobs/container_id1 cgroup calls unshare, cgroup 2158/batchjobs/container_id1 becomes the cgroupns root. For the 2159init_cgroup_ns, this is the real root ('/') cgroup. 2160 2161The cgroupns root cgroup does not change even if the namespace creator 2162process later moves to a different cgroup:: 2163 2164 # ~/unshare -c # unshare cgroupns in some cgroup 2165 # cat /proc/self/cgroup 2166 0::/ 2167 # mkdir sub_cgrp_1 2168 # echo 0 > sub_cgrp_1/cgroup.procs 2169 # cat /proc/self/cgroup 2170 0::/sub_cgrp_1 2171 2172Each process gets its namespace-specific view of "/proc/$PID/cgroup" 2173 2174Processes running inside the cgroup namespace will be able to see 2175cgroup paths (in /proc/self/cgroup) only inside their root cgroup. 2176From within an unshared cgroupns:: 2177 2178 # sleep 100000 & 2179 [1] 7353 2180 # echo 7353 > sub_cgrp_1/cgroup.procs 2181 # cat /proc/7353/cgroup 2182 0::/sub_cgrp_1 2183 2184From the initial cgroup namespace, the real cgroup path will be 2185visible:: 2186 2187 $ cat /proc/7353/cgroup 2188 0::/batchjobs/container_id1/sub_cgrp_1 2189 2190From a sibling cgroup namespace (that is, a namespace rooted at a 2191different cgroup), the cgroup path relative to its own cgroup 2192namespace root will be shown. For instance, if PID 7353's cgroup 2193namespace root is at '/batchjobs/container_id2', then it will see:: 2194 2195 # cat /proc/7353/cgroup 2196 0::/../container_id2/sub_cgrp_1 2197 2198Note that the relative path always starts with '/' to indicate that 2199its relative to the cgroup namespace root of the caller. 2200 2201 2202Migration and setns(2) 2203---------------------- 2204 2205Processes inside a cgroup namespace can move into and out of the 2206namespace root if they have proper access to external cgroups. For 2207example, from inside a namespace with cgroupns root at 2208/batchjobs/container_id1, and assuming that the global hierarchy is 2209still accessible inside cgroupns:: 2210 2211 # cat /proc/7353/cgroup 2212 0::/sub_cgrp_1 2213 # echo 7353 > batchjobs/container_id2/cgroup.procs 2214 # cat /proc/7353/cgroup 2215 0::/../container_id2 2216 2217Note that this kind of setup is not encouraged. A task inside cgroup 2218namespace should only be exposed to its own cgroupns hierarchy. 2219 2220setns(2) to another cgroup namespace is allowed when: 2221 2222(a) the process has CAP_SYS_ADMIN against its current user namespace 2223(b) the process has CAP_SYS_ADMIN against the target cgroup 2224 namespace's userns 2225 2226No implicit cgroup changes happen with attaching to another cgroup 2227namespace. It is expected that the someone moves the attaching 2228process under the target cgroup namespace root. 2229 2230 2231Interaction with Other Namespaces 2232--------------------------------- 2233 2234Namespace specific cgroup hierarchy can be mounted by a process 2235running inside a non-init cgroup namespace:: 2236 2237 # mount -t cgroup2 none $MOUNT_POINT 2238 2239This will mount the unified cgroup hierarchy with cgroupns root as the 2240filesystem root. The process needs CAP_SYS_ADMIN against its user and 2241mount namespaces. 2242 2243The virtualization of /proc/self/cgroup file combined with restricting 2244the view of cgroup hierarchy by namespace-private cgroupfs mount 2245provides a properly isolated cgroup view inside the container. 2246 2247 2248Information on Kernel Programming 2249================================= 2250 2251This section contains kernel programming information in the areas 2252where interacting with cgroup is necessary. cgroup core and 2253controllers are not covered. 2254 2255 2256Filesystem Support for Writeback 2257-------------------------------- 2258 2259A filesystem can support cgroup writeback by updating 2260address_space_operations->writepage[s]() to annotate bio's using the 2261following two functions. 2262 2263 wbc_init_bio(@wbc, @bio) 2264 Should be called for each bio carrying writeback data and 2265 associates the bio with the inode's owner cgroup and the 2266 corresponding request queue. This must be called after 2267 a queue (device) has been associated with the bio and 2268 before submission. 2269 2270 wbc_account_cgroup_owner(@wbc, @page, @bytes) 2271 Should be called for each data segment being written out. 2272 While this function doesn't care exactly when it's called 2273 during the writeback session, it's the easiest and most 2274 natural to call it as data segments are added to a bio. 2275 2276With writeback bio's annotated, cgroup support can be enabled per 2277super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for 2278selective disabling of cgroup writeback support which is helpful when 2279certain filesystem features, e.g. journaled data mode, are 2280incompatible. 2281 2282wbc_init_bio() binds the specified bio to its cgroup. Depending on 2283the configuration, the bio may be executed at a lower priority and if 2284the writeback session is holding shared resources, e.g. a journal 2285entry, may lead to priority inversion. There is no one easy solution 2286for the problem. Filesystems can try to work around specific problem 2287cases by skipping wbc_init_bio() and using bio_associate_blkg() 2288directly. 2289 2290 2291Deprecated v1 Core Features 2292=========================== 2293 2294- Multiple hierarchies including named ones are not supported. 2295 2296- All v1 mount options are not supported. 2297 2298- The "tasks" file is removed and "cgroup.procs" is not sorted. 2299 2300- "cgroup.clone_children" is removed. 2301 2302- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file 2303 at the root instead. 2304 2305 2306Issues with v1 and Rationales for v2 2307==================================== 2308 2309Multiple Hierarchies 2310-------------------- 2311 2312cgroup v1 allowed an arbitrary number of hierarchies and each 2313hierarchy could host any number of controllers. While this seemed to 2314provide a high level of flexibility, it wasn't useful in practice. 2315 2316For example, as there is only one instance of each controller, utility 2317type controllers such as freezer which can be useful in all 2318hierarchies could only be used in one. The issue is exacerbated by 2319the fact that controllers couldn't be moved to another hierarchy once 2320hierarchies were populated. Another issue was that all controllers 2321bound to a hierarchy were forced to have exactly the same view of the 2322hierarchy. It wasn't possible to vary the granularity depending on 2323the specific controller. 2324 2325In practice, these issues heavily limited which controllers could be 2326put on the same hierarchy and most configurations resorted to putting 2327each controller on its own hierarchy. Only closely related ones, such 2328as the cpu and cpuacct controllers, made sense to be put on the same 2329hierarchy. This often meant that userland ended up managing multiple 2330similar hierarchies repeating the same steps on each hierarchy 2331whenever a hierarchy management operation was necessary. 2332 2333Furthermore, support for multiple hierarchies came at a steep cost. 2334It greatly complicated cgroup core implementation but more importantly 2335the support for multiple hierarchies restricted how cgroup could be 2336used in general and what controllers was able to do. 2337 2338There was no limit on how many hierarchies there might be, which meant 2339that a thread's cgroup membership couldn't be described in finite 2340length. The key might contain any number of entries and was unlimited 2341in length, which made it highly awkward to manipulate and led to 2342addition of controllers which existed only to identify membership, 2343which in turn exacerbated the original problem of proliferating number 2344of hierarchies. 2345 2346Also, as a controller couldn't have any expectation regarding the 2347topologies of hierarchies other controllers might be on, each 2348controller had to assume that all other controllers were attached to 2349completely orthogonal hierarchies. This made it impossible, or at 2350least very cumbersome, for controllers to cooperate with each other. 2351 2352In most use cases, putting controllers on hierarchies which are 2353completely orthogonal to each other isn't necessary. What usually is 2354called for is the ability to have differing levels of granularity 2355depending on the specific controller. In other words, hierarchy may 2356be collapsed from leaf towards root when viewed from specific 2357controllers. For example, a given configuration might not care about 2358how memory is distributed beyond a certain level while still wanting 2359to control how CPU cycles are distributed. 2360 2361 2362Thread Granularity 2363------------------ 2364 2365cgroup v1 allowed threads of a process to belong to different cgroups. 2366This didn't make sense for some controllers and those controllers 2367ended up implementing different ways to ignore such situations but 2368much more importantly it blurred the line between API exposed to 2369individual applications and system management interface. 2370 2371Generally, in-process knowledge is available only to the process 2372itself; thus, unlike service-level organization of processes, 2373categorizing threads of a process requires active participation from 2374the application which owns the target process. 2375 2376cgroup v1 had an ambiguously defined delegation model which got abused 2377in combination with thread granularity. cgroups were delegated to 2378individual applications so that they can create and manage their own 2379sub-hierarchies and control resource distributions along them. This 2380effectively raised cgroup to the status of a syscall-like API exposed 2381to lay programs. 2382 2383First of all, cgroup has a fundamentally inadequate interface to be 2384exposed this way. For a process to access its own knobs, it has to 2385extract the path on the target hierarchy from /proc/self/cgroup, 2386construct the path by appending the name of the knob to the path, open 2387and then read and/or write to it. This is not only extremely clunky 2388and unusual but also inherently racy. There is no conventional way to 2389define transaction across the required steps and nothing can guarantee 2390that the process would actually be operating on its own sub-hierarchy. 2391 2392cgroup controllers implemented a number of knobs which would never be 2393accepted as public APIs because they were just adding control knobs to 2394system-management pseudo filesystem. cgroup ended up with interface 2395knobs which were not properly abstracted or refined and directly 2396revealed kernel internal details. These knobs got exposed to 2397individual applications through the ill-defined delegation mechanism 2398effectively abusing cgroup as a shortcut to implementing public APIs 2399without going through the required scrutiny. 2400 2401This was painful for both userland and kernel. Userland ended up with 2402misbehaving and poorly abstracted interfaces and kernel exposing and 2403locked into constructs inadvertently. 2404 2405 2406Competition Between Inner Nodes and Threads 2407------------------------------------------- 2408 2409cgroup v1 allowed threads to be in any cgroups which created an 2410interesting problem where threads belonging to a parent cgroup and its 2411children cgroups competed for resources. This was nasty as two 2412different types of entities competed and there was no obvious way to 2413settle it. Different controllers did different things. 2414 2415The cpu controller considered threads and cgroups as equivalents and 2416mapped nice levels to cgroup weights. This worked for some cases but 2417fell flat when children wanted to be allocated specific ratios of CPU 2418cycles and the number of internal threads fluctuated - the ratios 2419constantly changed as the number of competing entities fluctuated. 2420There also were other issues. The mapping from nice level to weight 2421wasn't obvious or universal, and there were various other knobs which 2422simply weren't available for threads. 2423 2424The io controller implicitly created a hidden leaf node for each 2425cgroup to host the threads. The hidden leaf had its own copies of all 2426the knobs with ``leaf_`` prefixed. While this allowed equivalent 2427control over internal threads, it was with serious drawbacks. It 2428always added an extra layer of nesting which wouldn't be necessary 2429otherwise, made the interface messy and significantly complicated the 2430implementation. 2431 2432The memory controller didn't have a way to control what happened 2433between internal tasks and child cgroups and the behavior was not 2434clearly defined. There were attempts to add ad-hoc behaviors and 2435knobs to tailor the behavior to specific workloads which would have 2436led to problems extremely difficult to resolve in the long term. 2437 2438Multiple controllers struggled with internal tasks and came up with 2439different ways to deal with it; unfortunately, all the approaches were 2440severely flawed and, furthermore, the widely different behaviors 2441made cgroup as a whole highly inconsistent. 2442 2443This clearly is a problem which needs to be addressed from cgroup core 2444in a uniform way. 2445 2446 2447Other Interface Issues 2448---------------------- 2449 2450cgroup v1 grew without oversight and developed a large number of 2451idiosyncrasies and inconsistencies. One issue on the cgroup core side 2452was how an empty cgroup was notified - a userland helper binary was 2453forked and executed for each event. The event delivery wasn't 2454recursive or delegatable. The limitations of the mechanism also led 2455to in-kernel event delivery filtering mechanism further complicating 2456the interface. 2457 2458Controller interfaces were problematic too. An extreme example is 2459controllers completely ignoring hierarchical organization and treating 2460all cgroups as if they were all located directly under the root 2461cgroup. Some controllers exposed a large amount of inconsistent 2462implementation details to userland. 2463 2464There also was no consistency across controllers. When a new cgroup 2465was created, some controllers defaulted to not imposing extra 2466restrictions while others disallowed any resource usage until 2467explicitly configured. Configuration knobs for the same type of 2468control used widely differing naming schemes and formats. Statistics 2469and information knobs were named arbitrarily and used different 2470formats and units even in the same controller. 2471 2472cgroup v2 establishes common conventions where appropriate and updates 2473controllers so that they expose minimal and consistent interfaces. 2474 2475 2476Controller Issues and Remedies 2477------------------------------ 2478 2479Memory 2480~~~~~~ 2481 2482The original lower boundary, the soft limit, is defined as a limit 2483that is per default unset. As a result, the set of cgroups that 2484global reclaim prefers is opt-in, rather than opt-out. The costs for 2485optimizing these mostly negative lookups are so high that the 2486implementation, despite its enormous size, does not even provide the 2487basic desirable behavior. First off, the soft limit has no 2488hierarchical meaning. All configured groups are organized in a global 2489rbtree and treated like equal peers, regardless where they are located 2490in the hierarchy. This makes subtree delegation impossible. Second, 2491the soft limit reclaim pass is so aggressive that it not just 2492introduces high allocation latencies into the system, but also impacts 2493system performance due to overreclaim, to the point where the feature 2494becomes self-defeating. 2495 2496The memory.low boundary on the other hand is a top-down allocated 2497reserve. A cgroup enjoys reclaim protection when it's within its 2498effective low, which makes delegation of subtrees possible. It also 2499enjoys having reclaim pressure proportional to its overage when 2500above its effective low. 2501 2502The original high boundary, the hard limit, is defined as a strict 2503limit that can not budge, even if the OOM killer has to be called. 2504But this generally goes against the goal of making the most out of the 2505available memory. The memory consumption of workloads varies during 2506runtime, and that requires users to overcommit. But doing that with a 2507strict upper limit requires either a fairly accurate prediction of the 2508working set size or adding slack to the limit. Since working set size 2509estimation is hard and error prone, and getting it wrong results in 2510OOM kills, most users tend to err on the side of a looser limit and 2511end up wasting precious resources. 2512 2513The memory.high boundary on the other hand can be set much more 2514conservatively. When hit, it throttles allocations by forcing them 2515into direct reclaim to work off the excess, but it never invokes the 2516OOM killer. As a result, a high boundary that is chosen too 2517aggressively will not terminate the processes, but instead it will 2518lead to gradual performance degradation. The user can monitor this 2519and make corrections until the minimal memory footprint that still 2520gives acceptable performance is found. 2521 2522In extreme cases, with many concurrent allocations and a complete 2523breakdown of reclaim progress within the group, the high boundary can 2524be exceeded. But even then it's mostly better to satisfy the 2525allocation from the slack available in other groups or the rest of the 2526system than killing the group. Otherwise, memory.max is there to 2527limit this type of spillover and ultimately contain buggy or even 2528malicious applications. 2529 2530Setting the original memory.limit_in_bytes below the current usage was 2531subject to a race condition, where concurrent charges could cause the 2532limit setting to fail. memory.max on the other hand will first set the 2533limit to prevent new charges, and then reclaim and OOM kill until the 2534new limit is met - or the task writing to memory.max is killed. 2535 2536The combined memory+swap accounting and limiting is replaced by real 2537control over swap space. 2538 2539The main argument for a combined memory+swap facility in the original 2540cgroup design was that global or parental pressure would always be 2541able to swap all anonymous memory of a child group, regardless of the 2542child's own (possibly untrusted) configuration. However, untrusted 2543groups can sabotage swapping by other means - such as referencing its 2544anonymous memory in a tight loop - and an admin can not assume full 2545swappability when overcommitting untrusted jobs. 2546 2547For trusted jobs, on the other hand, a combined counter is not an 2548intuitive userspace interface, and it flies in the face of the idea 2549that cgroup controllers should account and limit specific physical 2550resources. Swap space is a resource like all others in the system, 2551and that's why unified hierarchy allows distributing it separately. 2552