1.. _cgroup-v2: 2 3================ 4Control Group v2 5================ 6 7:Date: October, 2015 8:Author: Tejun Heo <tj@kernel.org> 9 10This is the authoritative documentation on the design, interface and 11conventions of cgroup v2. It describes all userland-visible aspects 12of cgroup including core and specific controller behaviors. All 13future changes must be reflected in this document. Documentation for 14v1 is available under :ref:`Documentation/admin-guide/cgroup-v1/index.rst <cgroup-v1>`. 15 16.. CONTENTS 17 18 1. Introduction 19 1-1. Terminology 20 1-2. What is cgroup? 21 2. Basic Operations 22 2-1. Mounting 23 2-2. Organizing Processes and Threads 24 2-2-1. Processes 25 2-2-2. Threads 26 2-3. [Un]populated Notification 27 2-4. Controlling Controllers 28 2-4-1. Enabling and Disabling 29 2-4-2. Top-down Constraint 30 2-4-3. No Internal Process Constraint 31 2-5. Delegation 32 2-5-1. Model of Delegation 33 2-5-2. Delegation Containment 34 2-6. Guidelines 35 2-6-1. Organize Once and Control 36 2-6-2. Avoid Name Collisions 37 3. Resource Distribution Models 38 3-1. Weights 39 3-2. Limits 40 3-3. Protections 41 3-4. Allocations 42 4. Interface Files 43 4-1. Format 44 4-2. Conventions 45 4-3. Core Interface Files 46 5. Controllers 47 5-1. CPU 48 5-1-1. CPU Interface Files 49 5-2. Memory 50 5-2-1. Memory Interface Files 51 5-2-2. Usage Guidelines 52 5-2-3. Memory Ownership 53 5-3. IO 54 5-3-1. IO Interface Files 55 5-3-2. Writeback 56 5-3-3. IO Latency 57 5-3-3-1. How IO Latency Throttling Works 58 5-3-3-2. IO Latency Interface Files 59 5-3-4. IO Priority 60 5-4. PID 61 5-4-1. PID Interface Files 62 5-5. Cpuset 63 5.5-1. Cpuset Interface Files 64 5-6. Device 65 5-7. RDMA 66 5-7-1. RDMA Interface Files 67 5-8. HugeTLB 68 5.8-1. HugeTLB Interface Files 69 5-9. Misc 70 5.9-1 Miscellaneous cgroup Interface Files 71 5.9-2 Migration and Ownership 72 5-10. Others 73 5-10-1. perf_event 74 5-N. Non-normative information 75 5-N-1. CPU controller root cgroup process behaviour 76 5-N-2. IO controller root cgroup process behaviour 77 6. Namespace 78 6-1. Basics 79 6-2. The Root and Views 80 6-3. Migration and setns(2) 81 6-4. Interaction with Other Namespaces 82 P. Information on Kernel Programming 83 P-1. Filesystem Support for Writeback 84 D. Deprecated v1 Core Features 85 R. Issues with v1 and Rationales for v2 86 R-1. Multiple Hierarchies 87 R-2. Thread Granularity 88 R-3. Competition Between Inner Nodes and Threads 89 R-4. Other Interface Issues 90 R-5. Controller Issues and Remedies 91 R-5-1. Memory 92 93 94Introduction 95============ 96 97Terminology 98----------- 99 100"cgroup" stands for "control group" and is never capitalized. The 101singular form is used to designate the whole feature and also as a 102qualifier as in "cgroup controllers". When explicitly referring to 103multiple individual control groups, the plural form "cgroups" is used. 104 105 106What is cgroup? 107--------------- 108 109cgroup is a mechanism to organize processes hierarchically and 110distribute system resources along the hierarchy in a controlled and 111configurable manner. 112 113cgroup is largely composed of two parts - the core and controllers. 114cgroup core is primarily responsible for hierarchically organizing 115processes. A cgroup controller is usually responsible for 116distributing a specific type of system resource along the hierarchy 117although there are utility controllers which serve purposes other than 118resource distribution. 119 120cgroups form a tree structure and every process in the system belongs 121to one and only one cgroup. All threads of a process belong to the 122same cgroup. On creation, all processes are put in the cgroup that 123the parent process belongs to at the time. A process can be migrated 124to another cgroup. Migration of a process doesn't affect already 125existing descendant processes. 126 127Following certain structural constraints, controllers may be enabled or 128disabled selectively on a cgroup. All controller behaviors are 129hierarchical - if a controller is enabled on a cgroup, it affects all 130processes which belong to the cgroups consisting the inclusive 131sub-hierarchy of the cgroup. When a controller is enabled on a nested 132cgroup, it always restricts the resource distribution further. The 133restrictions set closer to the root in the hierarchy can not be 134overridden from further away. 135 136 137Basic Operations 138================ 139 140Mounting 141-------- 142 143Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2 144hierarchy can be mounted with the following mount command:: 145 146 # mount -t cgroup2 none $MOUNT_POINT 147 148cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All 149controllers which support v2 and are not bound to a v1 hierarchy are 150automatically bound to the v2 hierarchy and show up at the root. 151Controllers which are not in active use in the v2 hierarchy can be 152bound to other hierarchies. This allows mixing v2 hierarchy with the 153legacy v1 multiple hierarchies in a fully backward compatible way. 154 155A controller can be moved across hierarchies only after the controller 156is no longer referenced in its current hierarchy. Because per-cgroup 157controller states are destroyed asynchronously and controllers may 158have lingering references, a controller may not show up immediately on 159the v2 hierarchy after the final umount of the previous hierarchy. 160Similarly, a controller should be fully disabled to be moved out of 161the unified hierarchy and it may take some time for the disabled 162controller to become available for other hierarchies; furthermore, due 163to inter-controller dependencies, other controllers may need to be 164disabled too. 165 166While useful for development and manual configurations, moving 167controllers dynamically between the v2 and other hierarchies is 168strongly discouraged for production use. It is recommended to decide 169the hierarchies and controller associations before starting using the 170controllers after system boot. 171 172During transition to v2, system management software might still 173automount the v1 cgroup filesystem and so hijack all controllers 174during boot, before manual intervention is possible. To make testing 175and experimenting easier, the kernel parameter cgroup_no_v1= allows 176disabling controllers in v1 and make them always available in v2. 177 178cgroup v2 currently supports the following mount options. 179 180 nsdelegate 181 Consider cgroup namespaces as delegation boundaries. This 182 option is system wide and can only be set on mount or modified 183 through remount from the init namespace. The mount option is 184 ignored on non-init namespace mounts. Please refer to the 185 Delegation section for details. 186 187 favordynmods 188 Reduce the latencies of dynamic cgroup modifications such as 189 task migrations and controller on/offs at the cost of making 190 hot path operations such as forks and exits more expensive. 191 The static usage pattern of creating a cgroup, enabling 192 controllers, and then seeding it with CLONE_INTO_CGROUP is 193 not affected by this option. 194 195 memory_localevents 196 Only populate memory.events with data for the current cgroup, 197 and not any subtrees. This is legacy behaviour, the default 198 behaviour without this option is to include subtree counts. 199 This option is system wide and can only be set on mount or 200 modified through remount from the init namespace. The mount 201 option is ignored on non-init namespace mounts. 202 203 memory_recursiveprot 204 Recursively apply memory.min and memory.low protection to 205 entire subtrees, without requiring explicit downward 206 propagation into leaf cgroups. This allows protecting entire 207 subtrees from one another, while retaining free competition 208 within those subtrees. This should have been the default 209 behavior but is a mount-option to avoid regressing setups 210 relying on the original semantics (e.g. specifying bogusly 211 high 'bypass' protection values at higher tree levels). 212 213 memory_hugetlb_accounting 214 Count HugeTLB memory usage towards the cgroup's overall 215 memory usage for the memory controller (for the purpose of 216 statistics reporting and memory protetion). This is a new 217 behavior that could regress existing setups, so it must be 218 explicitly opted in with this mount option. 219 220 A few caveats to keep in mind: 221 222 * There is no HugeTLB pool management involved in the memory 223 controller. The pre-allocated pool does not belong to anyone. 224 Specifically, when a new HugeTLB folio is allocated to 225 the pool, it is not accounted for from the perspective of the 226 memory controller. It is only charged to a cgroup when it is 227 actually used (for e.g at page fault time). Host memory 228 overcommit management has to consider this when configuring 229 hard limits. In general, HugeTLB pool management should be 230 done via other mechanisms (such as the HugeTLB controller). 231 * Failure to charge a HugeTLB folio to the memory controller 232 results in SIGBUS. This could happen even if the HugeTLB pool 233 still has pages available (but the cgroup limit is hit and 234 reclaim attempt fails). 235 * Charging HugeTLB memory towards the memory controller affects 236 memory protection and reclaim dynamics. Any userspace tuning 237 (of low, min limits for e.g) needs to take this into account. 238 * HugeTLB pages utilized while this option is not selected 239 will not be tracked by the memory controller (even if cgroup 240 v2 is remounted later on). 241 242 243Organizing Processes and Threads 244-------------------------------- 245 246Processes 247~~~~~~~~~ 248 249Initially, only the root cgroup exists to which all processes belong. 250A child cgroup can be created by creating a sub-directory:: 251 252 # mkdir $CGROUP_NAME 253 254A given cgroup may have multiple child cgroups forming a tree 255structure. Each cgroup has a read-writable interface file 256"cgroup.procs". When read, it lists the PIDs of all processes which 257belong to the cgroup one-per-line. The PIDs are not ordered and the 258same PID may show up more than once if the process got moved to 259another cgroup and then back or the PID got recycled while reading. 260 261A process can be migrated into a cgroup by writing its PID to the 262target cgroup's "cgroup.procs" file. Only one process can be migrated 263on a single write(2) call. If a process is composed of multiple 264threads, writing the PID of any thread migrates all threads of the 265process. 266 267When a process forks a child process, the new process is born into the 268cgroup that the forking process belongs to at the time of the 269operation. After exit, a process stays associated with the cgroup 270that it belonged to at the time of exit until it's reaped; however, a 271zombie process does not appear in "cgroup.procs" and thus can't be 272moved to another cgroup. 273 274A cgroup which doesn't have any children or live processes can be 275destroyed by removing the directory. Note that a cgroup which doesn't 276have any children and is associated only with zombie processes is 277considered empty and can be removed:: 278 279 # rmdir $CGROUP_NAME 280 281"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy 282cgroup is in use in the system, this file may contain multiple lines, 283one for each hierarchy. The entry for cgroup v2 is always in the 284format "0::$PATH":: 285 286 # cat /proc/842/cgroup 287 ... 288 0::/test-cgroup/test-cgroup-nested 289 290If the process becomes a zombie and the cgroup it was associated with 291is removed subsequently, " (deleted)" is appended to the path:: 292 293 # cat /proc/842/cgroup 294 ... 295 0::/test-cgroup/test-cgroup-nested (deleted) 296 297 298Threads 299~~~~~~~ 300 301cgroup v2 supports thread granularity for a subset of controllers to 302support use cases requiring hierarchical resource distribution across 303the threads of a group of processes. By default, all threads of a 304process belong to the same cgroup, which also serves as the resource 305domain to host resource consumptions which are not specific to a 306process or thread. The thread mode allows threads to be spread across 307a subtree while still maintaining the common resource domain for them. 308 309Controllers which support thread mode are called threaded controllers. 310The ones which don't are called domain controllers. 311 312Marking a cgroup threaded makes it join the resource domain of its 313parent as a threaded cgroup. The parent may be another threaded 314cgroup whose resource domain is further up in the hierarchy. The root 315of a threaded subtree, that is, the nearest ancestor which is not 316threaded, is called threaded domain or thread root interchangeably and 317serves as the resource domain for the entire subtree. 318 319Inside a threaded subtree, threads of a process can be put in 320different cgroups and are not subject to the no internal process 321constraint - threaded controllers can be enabled on non-leaf cgroups 322whether they have threads in them or not. 323 324As the threaded domain cgroup hosts all the domain resource 325consumptions of the subtree, it is considered to have internal 326resource consumptions whether there are processes in it or not and 327can't have populated child cgroups which aren't threaded. Because the 328root cgroup is not subject to no internal process constraint, it can 329serve both as a threaded domain and a parent to domain cgroups. 330 331The current operation mode or type of the cgroup is shown in the 332"cgroup.type" file which indicates whether the cgroup is a normal 333domain, a domain which is serving as the domain of a threaded subtree, 334or a threaded cgroup. 335 336On creation, a cgroup is always a domain cgroup and can be made 337threaded by writing "threaded" to the "cgroup.type" file. The 338operation is single direction:: 339 340 # echo threaded > cgroup.type 341 342Once threaded, the cgroup can't be made a domain again. To enable the 343thread mode, the following conditions must be met. 344 345- As the cgroup will join the parent's resource domain. The parent 346 must either be a valid (threaded) domain or a threaded cgroup. 347 348- When the parent is an unthreaded domain, it must not have any domain 349 controllers enabled or populated domain children. The root is 350 exempt from this requirement. 351 352Topology-wise, a cgroup can be in an invalid state. Please consider 353the following topology:: 354 355 A (threaded domain) - B (threaded) - C (domain, just created) 356 357C is created as a domain but isn't connected to a parent which can 358host child domains. C can't be used until it is turned into a 359threaded cgroup. "cgroup.type" file will report "domain (invalid)" in 360these cases. Operations which fail due to invalid topology use 361EOPNOTSUPP as the errno. 362 363A domain cgroup is turned into a threaded domain when one of its child 364cgroup becomes threaded or threaded controllers are enabled in the 365"cgroup.subtree_control" file while there are processes in the cgroup. 366A threaded domain reverts to a normal domain when the conditions 367clear. 368 369When read, "cgroup.threads" contains the list of the thread IDs of all 370threads in the cgroup. Except that the operations are per-thread 371instead of per-process, "cgroup.threads" has the same format and 372behaves the same way as "cgroup.procs". While "cgroup.threads" can be 373written to in any cgroup, as it can only move threads inside the same 374threaded domain, its operations are confined inside each threaded 375subtree. 376 377The threaded domain cgroup serves as the resource domain for the whole 378subtree, and, while the threads can be scattered across the subtree, 379all the processes are considered to be in the threaded domain cgroup. 380"cgroup.procs" in a threaded domain cgroup contains the PIDs of all 381processes in the subtree and is not readable in the subtree proper. 382However, "cgroup.procs" can be written to from anywhere in the subtree 383to migrate all threads of the matching process to the cgroup. 384 385Only threaded controllers can be enabled in a threaded subtree. When 386a threaded controller is enabled inside a threaded subtree, it only 387accounts for and controls resource consumptions associated with the 388threads in the cgroup and its descendants. All consumptions which 389aren't tied to a specific thread belong to the threaded domain cgroup. 390 391Because a threaded subtree is exempt from no internal process 392constraint, a threaded controller must be able to handle competition 393between threads in a non-leaf cgroup and its child cgroups. Each 394threaded controller defines how such competitions are handled. 395 396Currently, the following controllers are threaded and can be enabled 397in a threaded cgroup:: 398 399- cpu 400- cpuset 401- perf_event 402- pids 403 404[Un]populated Notification 405-------------------------- 406 407Each non-root cgroup has a "cgroup.events" file which contains 408"populated" field indicating whether the cgroup's sub-hierarchy has 409live processes in it. Its value is 0 if there is no live process in 410the cgroup and its descendants; otherwise, 1. poll and [id]notify 411events are triggered when the value changes. This can be used, for 412example, to start a clean-up operation after all processes of a given 413sub-hierarchy have exited. The populated state updates and 414notifications are recursive. Consider the following sub-hierarchy 415where the numbers in the parentheses represent the numbers of processes 416in each cgroup:: 417 418 A(4) - B(0) - C(1) 419 \ D(0) 420 421A, B and C's "populated" fields would be 1 while D's 0. After the one 422process in C exits, B and C's "populated" fields would flip to "0" and 423file modified events will be generated on the "cgroup.events" files of 424both cgroups. 425 426 427Controlling Controllers 428----------------------- 429 430Enabling and Disabling 431~~~~~~~~~~~~~~~~~~~~~~ 432 433Each cgroup has a "cgroup.controllers" file which lists all 434controllers available for the cgroup to enable:: 435 436 # cat cgroup.controllers 437 cpu io memory 438 439No controller is enabled by default. Controllers can be enabled and 440disabled by writing to the "cgroup.subtree_control" file:: 441 442 # echo "+cpu +memory -io" > cgroup.subtree_control 443 444Only controllers which are listed in "cgroup.controllers" can be 445enabled. When multiple operations are specified as above, either they 446all succeed or fail. If multiple operations on the same controller 447are specified, the last one is effective. 448 449Enabling a controller in a cgroup indicates that the distribution of 450the target resource across its immediate children will be controlled. 451Consider the following sub-hierarchy. The enabled controllers are 452listed in parentheses:: 453 454 A(cpu,memory) - B(memory) - C() 455 \ D() 456 457As A has "cpu" and "memory" enabled, A will control the distribution 458of CPU cycles and memory to its children, in this case, B. As B has 459"memory" enabled but not "CPU", C and D will compete freely on CPU 460cycles but their division of memory available to B will be controlled. 461 462As a controller regulates the distribution of the target resource to 463the cgroup's children, enabling it creates the controller's interface 464files in the child cgroups. In the above example, enabling "cpu" on B 465would create the "cpu." prefixed controller interface files in C and 466D. Likewise, disabling "memory" from B would remove the "memory." 467prefixed controller interface files from C and D. This means that the 468controller interface files - anything which doesn't start with 469"cgroup." are owned by the parent rather than the cgroup itself. 470 471 472Top-down Constraint 473~~~~~~~~~~~~~~~~~~~ 474 475Resources are distributed top-down and a cgroup can further distribute 476a resource only if the resource has been distributed to it from the 477parent. This means that all non-root "cgroup.subtree_control" files 478can only contain controllers which are enabled in the parent's 479"cgroup.subtree_control" file. A controller can be enabled only if 480the parent has the controller enabled and a controller can't be 481disabled if one or more children have it enabled. 482 483 484No Internal Process Constraint 485~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 486 487Non-root cgroups can distribute domain resources to their children 488only when they don't have any processes of their own. In other words, 489only domain cgroups which don't contain any processes can have domain 490controllers enabled in their "cgroup.subtree_control" files. 491 492This guarantees that, when a domain controller is looking at the part 493of the hierarchy which has it enabled, processes are always only on 494the leaves. This rules out situations where child cgroups compete 495against internal processes of the parent. 496 497The root cgroup is exempt from this restriction. Root contains 498processes and anonymous resource consumption which can't be associated 499with any other cgroups and requires special treatment from most 500controllers. How resource consumption in the root cgroup is governed 501is up to each controller (for more information on this topic please 502refer to the Non-normative information section in the Controllers 503chapter). 504 505Note that the restriction doesn't get in the way if there is no 506enabled controller in the cgroup's "cgroup.subtree_control". This is 507important as otherwise it wouldn't be possible to create children of a 508populated cgroup. To control resource distribution of a cgroup, the 509cgroup must create children and transfer all its processes to the 510children before enabling controllers in its "cgroup.subtree_control" 511file. 512 513 514Delegation 515---------- 516 517Model of Delegation 518~~~~~~~~~~~~~~~~~~~ 519 520A cgroup can be delegated in two ways. First, to a less privileged 521user by granting write access of the directory and its "cgroup.procs", 522"cgroup.threads" and "cgroup.subtree_control" files to the user. 523Second, if the "nsdelegate" mount option is set, automatically to a 524cgroup namespace on namespace creation. 525 526Because the resource control interface files in a given directory 527control the distribution of the parent's resources, the delegatee 528shouldn't be allowed to write to them. For the first method, this is 529achieved by not granting access to these files. For the second, the 530kernel rejects writes to all files other than "cgroup.procs" and 531"cgroup.subtree_control" on a namespace root from inside the 532namespace. 533 534The end results are equivalent for both delegation types. Once 535delegated, the user can build sub-hierarchy under the directory, 536organize processes inside it as it sees fit and further distribute the 537resources it received from the parent. The limits and other settings 538of all resource controllers are hierarchical and regardless of what 539happens in the delegated sub-hierarchy, nothing can escape the 540resource restrictions imposed by the parent. 541 542Currently, cgroup doesn't impose any restrictions on the number of 543cgroups in or nesting depth of a delegated sub-hierarchy; however, 544this may be limited explicitly in the future. 545 546 547Delegation Containment 548~~~~~~~~~~~~~~~~~~~~~~ 549 550A delegated sub-hierarchy is contained in the sense that processes 551can't be moved into or out of the sub-hierarchy by the delegatee. 552 553For delegations to a less privileged user, this is achieved by 554requiring the following conditions for a process with a non-root euid 555to migrate a target process into a cgroup by writing its PID to the 556"cgroup.procs" file. 557 558- The writer must have write access to the "cgroup.procs" file. 559 560- The writer must have write access to the "cgroup.procs" file of the 561 common ancestor of the source and destination cgroups. 562 563The above two constraints ensure that while a delegatee may migrate 564processes around freely in the delegated sub-hierarchy it can't pull 565in from or push out to outside the sub-hierarchy. 566 567For an example, let's assume cgroups C0 and C1 have been delegated to 568user U0 who created C00, C01 under C0 and C10 under C1 as follows and 569all processes under C0 and C1 belong to U0:: 570 571 ~~~~~~~~~~~~~ - C0 - C00 572 ~ cgroup ~ \ C01 573 ~ hierarchy ~ 574 ~~~~~~~~~~~~~ - C1 - C10 575 576Let's also say U0 wants to write the PID of a process which is 577currently in C10 into "C00/cgroup.procs". U0 has write access to the 578file; however, the common ancestor of the source cgroup C10 and the 579destination cgroup C00 is above the points of delegation and U0 would 580not have write access to its "cgroup.procs" files and thus the write 581will be denied with -EACCES. 582 583For delegations to namespaces, containment is achieved by requiring 584that both the source and destination cgroups are reachable from the 585namespace of the process which is attempting the migration. If either 586is not reachable, the migration is rejected with -ENOENT. 587 588 589Guidelines 590---------- 591 592Organize Once and Control 593~~~~~~~~~~~~~~~~~~~~~~~~~ 594 595Migrating a process across cgroups is a relatively expensive operation 596and stateful resources such as memory are not moved together with the 597process. This is an explicit design decision as there often exist 598inherent trade-offs between migration and various hot paths in terms 599of synchronization cost. 600 601As such, migrating processes across cgroups frequently as a means to 602apply different resource restrictions is discouraged. A workload 603should be assigned to a cgroup according to the system's logical and 604resource structure once on start-up. Dynamic adjustments to resource 605distribution can be made by changing controller configuration through 606the interface files. 607 608 609Avoid Name Collisions 610~~~~~~~~~~~~~~~~~~~~~ 611 612Interface files for a cgroup and its children cgroups occupy the same 613directory and it is possible to create children cgroups which collide 614with interface files. 615 616All cgroup core interface files are prefixed with "cgroup." and each 617controller's interface files are prefixed with the controller name and 618a dot. A controller's name is composed of lower case alphabets and 619'_'s but never begins with an '_' so it can be used as the prefix 620character for collision avoidance. Also, interface file names won't 621start or end with terms which are often used in categorizing workloads 622such as job, service, slice, unit or workload. 623 624cgroup doesn't do anything to prevent name collisions and it's the 625user's responsibility to avoid them. 626 627 628Resource Distribution Models 629============================ 630 631cgroup controllers implement several resource distribution schemes 632depending on the resource type and expected use cases. This section 633describes major schemes in use along with their expected behaviors. 634 635 636Weights 637------- 638 639A parent's resource is distributed by adding up the weights of all 640active children and giving each the fraction matching the ratio of its 641weight against the sum. As only children which can make use of the 642resource at the moment participate in the distribution, this is 643work-conserving. Due to the dynamic nature, this model is usually 644used for stateless resources. 645 646All weights are in the range [1, 10000] with the default at 100. This 647allows symmetric multiplicative biases in both directions at fine 648enough granularity while staying in the intuitive range. 649 650As long as the weight is in range, all configuration combinations are 651valid and there is no reason to reject configuration changes or 652process migrations. 653 654"cpu.weight" proportionally distributes CPU cycles to active children 655and is an example of this type. 656 657 658.. _cgroupv2-limits-distributor: 659 660Limits 661------ 662 663A child can only consume up to the configured amount of the resource. 664Limits can be over-committed - the sum of the limits of children can 665exceed the amount of resource available to the parent. 666 667Limits are in the range [0, max] and defaults to "max", which is noop. 668 669As limits can be over-committed, all configuration combinations are 670valid and there is no reason to reject configuration changes or 671process migrations. 672 673"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume 674on an IO device and is an example of this type. 675 676.. _cgroupv2-protections-distributor: 677 678Protections 679----------- 680 681A cgroup is protected up to the configured amount of the resource 682as long as the usages of all its ancestors are under their 683protected levels. Protections can be hard guarantees or best effort 684soft boundaries. Protections can also be over-committed in which case 685only up to the amount available to the parent is protected among 686children. 687 688Protections are in the range [0, max] and defaults to 0, which is 689noop. 690 691As protections can be over-committed, all configuration combinations 692are valid and there is no reason to reject configuration changes or 693process migrations. 694 695"memory.low" implements best-effort memory protection and is an 696example of this type. 697 698 699Allocations 700----------- 701 702A cgroup is exclusively allocated a certain amount of a finite 703resource. Allocations can't be over-committed - the sum of the 704allocations of children can not exceed the amount of resource 705available to the parent. 706 707Allocations are in the range [0, max] and defaults to 0, which is no 708resource. 709 710As allocations can't be over-committed, some configuration 711combinations are invalid and should be rejected. Also, if the 712resource is mandatory for execution of processes, process migrations 713may be rejected. 714 715"cpu.rt.max" hard-allocates realtime slices and is an example of this 716type. 717 718 719Interface Files 720=============== 721 722Format 723------ 724 725All interface files should be in one of the following formats whenever 726possible:: 727 728 New-line separated values 729 (when only one value can be written at once) 730 731 VAL0\n 732 VAL1\n 733 ... 734 735 Space separated values 736 (when read-only or multiple values can be written at once) 737 738 VAL0 VAL1 ...\n 739 740 Flat keyed 741 742 KEY0 VAL0\n 743 KEY1 VAL1\n 744 ... 745 746 Nested keyed 747 748 KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01... 749 KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11... 750 ... 751 752For a writable file, the format for writing should generally match 753reading; however, controllers may allow omitting later fields or 754implement restricted shortcuts for most common use cases. 755 756For both flat and nested keyed files, only the values for a single key 757can be written at a time. For nested keyed files, the sub key pairs 758may be specified in any order and not all pairs have to be specified. 759 760 761Conventions 762----------- 763 764- Settings for a single feature should be contained in a single file. 765 766- The root cgroup should be exempt from resource control and thus 767 shouldn't have resource control interface files. 768 769- The default time unit is microseconds. If a different unit is ever 770 used, an explicit unit suffix must be present. 771 772- A parts-per quantity should use a percentage decimal with at least 773 two digit fractional part - e.g. 13.40. 774 775- If a controller implements weight based resource distribution, its 776 interface file should be named "weight" and have the range [1, 777 10000] with 100 as the default. The values are chosen to allow 778 enough and symmetric bias in both directions while keeping it 779 intuitive (the default is 100%). 780 781- If a controller implements an absolute resource guarantee and/or 782 limit, the interface files should be named "min" and "max" 783 respectively. If a controller implements best effort resource 784 guarantee and/or limit, the interface files should be named "low" 785 and "high" respectively. 786 787 In the above four control files, the special token "max" should be 788 used to represent upward infinity for both reading and writing. 789 790- If a setting has a configurable default value and keyed specific 791 overrides, the default entry should be keyed with "default" and 792 appear as the first entry in the file. 793 794 The default value can be updated by writing either "default $VAL" or 795 "$VAL". 796 797 When writing to update a specific override, "default" can be used as 798 the value to indicate removal of the override. Override entries 799 with "default" as the value must not appear when read. 800 801 For example, a setting which is keyed by major:minor device numbers 802 with integer values may look like the following:: 803 804 # cat cgroup-example-interface-file 805 default 150 806 8:0 300 807 808 The default value can be updated by:: 809 810 # echo 125 > cgroup-example-interface-file 811 812 or:: 813 814 # echo "default 125" > cgroup-example-interface-file 815 816 An override can be set by:: 817 818 # echo "8:16 170" > cgroup-example-interface-file 819 820 and cleared by:: 821 822 # echo "8:0 default" > cgroup-example-interface-file 823 # cat cgroup-example-interface-file 824 default 125 825 8:16 170 826 827- For events which are not very high frequency, an interface file 828 "events" should be created which lists event key value pairs. 829 Whenever a notifiable event happens, file modified event should be 830 generated on the file. 831 832 833Core Interface Files 834-------------------- 835 836All cgroup core files are prefixed with "cgroup." 837 838 cgroup.type 839 A read-write single value file which exists on non-root 840 cgroups. 841 842 When read, it indicates the current type of the cgroup, which 843 can be one of the following values. 844 845 - "domain" : A normal valid domain cgroup. 846 847 - "domain threaded" : A threaded domain cgroup which is 848 serving as the root of a threaded subtree. 849 850 - "domain invalid" : A cgroup which is in an invalid state. 851 It can't be populated or have controllers enabled. It may 852 be allowed to become a threaded cgroup. 853 854 - "threaded" : A threaded cgroup which is a member of a 855 threaded subtree. 856 857 A cgroup can be turned into a threaded cgroup by writing 858 "threaded" to this file. 859 860 cgroup.procs 861 A read-write new-line separated values file which exists on 862 all cgroups. 863 864 When read, it lists the PIDs of all processes which belong to 865 the cgroup one-per-line. The PIDs are not ordered and the 866 same PID may show up more than once if the process got moved 867 to another cgroup and then back or the PID got recycled while 868 reading. 869 870 A PID can be written to migrate the process associated with 871 the PID to the cgroup. The writer should match all of the 872 following conditions. 873 874 - It must have write access to the "cgroup.procs" file. 875 876 - It must have write access to the "cgroup.procs" file of the 877 common ancestor of the source and destination cgroups. 878 879 When delegating a sub-hierarchy, write access to this file 880 should be granted along with the containing directory. 881 882 In a threaded cgroup, reading this file fails with EOPNOTSUPP 883 as all the processes belong to the thread root. Writing is 884 supported and moves every thread of the process to the cgroup. 885 886 cgroup.threads 887 A read-write new-line separated values file which exists on 888 all cgroups. 889 890 When read, it lists the TIDs of all threads which belong to 891 the cgroup one-per-line. The TIDs are not ordered and the 892 same TID may show up more than once if the thread got moved to 893 another cgroup and then back or the TID got recycled while 894 reading. 895 896 A TID can be written to migrate the thread associated with the 897 TID to the cgroup. The writer should match all of the 898 following conditions. 899 900 - It must have write access to the "cgroup.threads" file. 901 902 - The cgroup that the thread is currently in must be in the 903 same resource domain as the destination cgroup. 904 905 - It must have write access to the "cgroup.procs" file of the 906 common ancestor of the source and destination cgroups. 907 908 When delegating a sub-hierarchy, write access to this file 909 should be granted along with the containing directory. 910 911 cgroup.controllers 912 A read-only space separated values file which exists on all 913 cgroups. 914 915 It shows space separated list of all controllers available to 916 the cgroup. The controllers are not ordered. 917 918 cgroup.subtree_control 919 A read-write space separated values file which exists on all 920 cgroups. Starts out empty. 921 922 When read, it shows space separated list of the controllers 923 which are enabled to control resource distribution from the 924 cgroup to its children. 925 926 Space separated list of controllers prefixed with '+' or '-' 927 can be written to enable or disable controllers. A controller 928 name prefixed with '+' enables the controller and '-' 929 disables. If a controller appears more than once on the list, 930 the last one is effective. When multiple enable and disable 931 operations are specified, either all succeed or all fail. 932 933 cgroup.events 934 A read-only flat-keyed file which exists on non-root cgroups. 935 The following entries are defined. Unless specified 936 otherwise, a value change in this file generates a file 937 modified event. 938 939 populated 940 1 if the cgroup or its descendants contains any live 941 processes; otherwise, 0. 942 frozen 943 1 if the cgroup is frozen; otherwise, 0. 944 945 cgroup.max.descendants 946 A read-write single value files. The default is "max". 947 948 Maximum allowed number of descent cgroups. 949 If the actual number of descendants is equal or larger, 950 an attempt to create a new cgroup in the hierarchy will fail. 951 952 cgroup.max.depth 953 A read-write single value files. The default is "max". 954 955 Maximum allowed descent depth below the current cgroup. 956 If the actual descent depth is equal or larger, 957 an attempt to create a new child cgroup will fail. 958 959 cgroup.stat 960 A read-only flat-keyed file with the following entries: 961 962 nr_descendants 963 Total number of visible descendant cgroups. 964 965 nr_dying_descendants 966 Total number of dying descendant cgroups. A cgroup becomes 967 dying after being deleted by a user. The cgroup will remain 968 in dying state for some time undefined time (which can depend 969 on system load) before being completely destroyed. 970 971 A process can't enter a dying cgroup under any circumstances, 972 a dying cgroup can't revive. 973 974 A dying cgroup can consume system resources not exceeding 975 limits, which were active at the moment of cgroup deletion. 976 977 cgroup.freeze 978 A read-write single value file which exists on non-root cgroups. 979 Allowed values are "0" and "1". The default is "0". 980 981 Writing "1" to the file causes freezing of the cgroup and all 982 descendant cgroups. This means that all belonging processes will 983 be stopped and will not run until the cgroup will be explicitly 984 unfrozen. Freezing of the cgroup may take some time; when this action 985 is completed, the "frozen" value in the cgroup.events control file 986 will be updated to "1" and the corresponding notification will be 987 issued. 988 989 A cgroup can be frozen either by its own settings, or by settings 990 of any ancestor cgroups. If any of ancestor cgroups is frozen, the 991 cgroup will remain frozen. 992 993 Processes in the frozen cgroup can be killed by a fatal signal. 994 They also can enter and leave a frozen cgroup: either by an explicit 995 move by a user, or if freezing of the cgroup races with fork(). 996 If a process is moved to a frozen cgroup, it stops. If a process is 997 moved out of a frozen cgroup, it becomes running. 998 999 Frozen status of a cgroup doesn't affect any cgroup tree operations: 1000 it's possible to delete a frozen (and empty) cgroup, as well as 1001 create new sub-cgroups. 1002 1003 cgroup.kill 1004 A write-only single value file which exists in non-root cgroups. 1005 The only allowed value is "1". 1006 1007 Writing "1" to the file causes the cgroup and all descendant cgroups to 1008 be killed. This means that all processes located in the affected cgroup 1009 tree will be killed via SIGKILL. 1010 1011 Killing a cgroup tree will deal with concurrent forks appropriately and 1012 is protected against migrations. 1013 1014 In a threaded cgroup, writing this file fails with EOPNOTSUPP as 1015 killing cgroups is a process directed operation, i.e. it affects 1016 the whole thread-group. 1017 1018 cgroup.pressure 1019 A read-write single value file that allowed values are "0" and "1". 1020 The default is "1". 1021 1022 Writing "0" to the file will disable the cgroup PSI accounting. 1023 Writing "1" to the file will re-enable the cgroup PSI accounting. 1024 1025 This control attribute is not hierarchical, so disable or enable PSI 1026 accounting in a cgroup does not affect PSI accounting in descendants 1027 and doesn't need pass enablement via ancestors from root. 1028 1029 The reason this control attribute exists is that PSI accounts stalls for 1030 each cgroup separately and aggregates it at each level of the hierarchy. 1031 This may cause non-negligible overhead for some workloads when under 1032 deep level of the hierarchy, in which case this control attribute can 1033 be used to disable PSI accounting in the non-leaf cgroups. 1034 1035 irq.pressure 1036 A read-write nested-keyed file. 1037 1038 Shows pressure stall information for IRQ/SOFTIRQ. See 1039 :ref:`Documentation/accounting/psi.rst <psi>` for details. 1040 1041Controllers 1042=========== 1043 1044.. _cgroup-v2-cpu: 1045 1046CPU 1047--- 1048 1049The "cpu" controllers regulates distribution of CPU cycles. This 1050controller implements weight and absolute bandwidth limit models for 1051normal scheduling policy and absolute bandwidth allocation model for 1052realtime scheduling policy. 1053 1054In all the above models, cycles distribution is defined only on a temporal 1055base and it does not account for the frequency at which tasks are executed. 1056The (optional) utilization clamping support allows to hint the schedutil 1057cpufreq governor about the minimum desired frequency which should always be 1058provided by a CPU, as well as the maximum desired frequency, which should not 1059be exceeded by a CPU. 1060 1061WARNING: cgroup2 doesn't yet support control of realtime processes. For 1062a kernel built with the CONFIG_RT_GROUP_SCHED option enabled for group 1063scheduling of realtime processes, the cpu controller can only be enabled 1064when all RT processes are in the root cgroup. This limitation does 1065not apply if CONFIG_RT_GROUP_SCHED is disabled. Be aware that system 1066management software may already have placed RT processes into nonroot 1067cgroups during the system boot process, and these processes may need 1068to be moved to the root cgroup before the cpu controller can be enabled 1069with a CONFIG_RT_GROUP_SCHED enabled kernel. 1070 1071 1072CPU Interface Files 1073~~~~~~~~~~~~~~~~~~~ 1074 1075All time durations are in microseconds. 1076 1077 cpu.stat 1078 A read-only flat-keyed file. 1079 This file exists whether the controller is enabled or not. 1080 1081 It always reports the following three stats: 1082 1083 - usage_usec 1084 - user_usec 1085 - system_usec 1086 1087 and the following five when the controller is enabled: 1088 1089 - nr_periods 1090 - nr_throttled 1091 - throttled_usec 1092 - nr_bursts 1093 - burst_usec 1094 1095 cpu.weight 1096 A read-write single value file which exists on non-root 1097 cgroups. The default is "100". 1098 1099 For non idle groups (cpu.idle = 0), the weight is in the 1100 range [1, 10000]. 1101 1102 If the cgroup has been configured to be SCHED_IDLE (cpu.idle = 1), 1103 then the weight will show as a 0. 1104 1105 cpu.weight.nice 1106 A read-write single value file which exists on non-root 1107 cgroups. The default is "0". 1108 1109 The nice value is in the range [-20, 19]. 1110 1111 This interface file is an alternative interface for 1112 "cpu.weight" and allows reading and setting weight using the 1113 same values used by nice(2). Because the range is smaller and 1114 granularity is coarser for the nice values, the read value is 1115 the closest approximation of the current weight. 1116 1117 cpu.max 1118 A read-write two value file which exists on non-root cgroups. 1119 The default is "max 100000". 1120 1121 The maximum bandwidth limit. It's in the following format:: 1122 1123 $MAX $PERIOD 1124 1125 which indicates that the group may consume up to $MAX in each 1126 $PERIOD duration. "max" for $MAX indicates no limit. If only 1127 one number is written, $MAX is updated. 1128 1129 cpu.max.burst 1130 A read-write single value file which exists on non-root 1131 cgroups. The default is "0". 1132 1133 The burst in the range [0, $MAX]. 1134 1135 cpu.pressure 1136 A read-write nested-keyed file. 1137 1138 Shows pressure stall information for CPU. See 1139 :ref:`Documentation/accounting/psi.rst <psi>` for details. 1140 1141 cpu.uclamp.min 1142 A read-write single value file which exists on non-root cgroups. 1143 The default is "0", i.e. no utilization boosting. 1144 1145 The requested minimum utilization (protection) as a percentage 1146 rational number, e.g. 12.34 for 12.34%. 1147 1148 This interface allows reading and setting minimum utilization clamp 1149 values similar to the sched_setattr(2). This minimum utilization 1150 value is used to clamp the task specific minimum utilization clamp. 1151 1152 The requested minimum utilization (protection) is always capped by 1153 the current value for the maximum utilization (limit), i.e. 1154 `cpu.uclamp.max`. 1155 1156 cpu.uclamp.max 1157 A read-write single value file which exists on non-root cgroups. 1158 The default is "max". i.e. no utilization capping 1159 1160 The requested maximum utilization (limit) as a percentage rational 1161 number, e.g. 98.76 for 98.76%. 1162 1163 This interface allows reading and setting maximum utilization clamp 1164 values similar to the sched_setattr(2). This maximum utilization 1165 value is used to clamp the task specific maximum utilization clamp. 1166 1167 cpu.idle 1168 A read-write single value file which exists on non-root cgroups. 1169 The default is 0. 1170 1171 This is the cgroup analog of the per-task SCHED_IDLE sched policy. 1172 Setting this value to a 1 will make the scheduling policy of the 1173 cgroup SCHED_IDLE. The threads inside the cgroup will retain their 1174 own relative priorities, but the cgroup itself will be treated as 1175 very low priority relative to its peers. 1176 1177 1178 1179Memory 1180------ 1181 1182The "memory" controller regulates distribution of memory. Memory is 1183stateful and implements both limit and protection models. Due to the 1184intertwining between memory usage and reclaim pressure and the 1185stateful nature of memory, the distribution model is relatively 1186complex. 1187 1188While not completely water-tight, all major memory usages by a given 1189cgroup are tracked so that the total memory consumption can be 1190accounted and controlled to a reasonable extent. Currently, the 1191following types of memory usages are tracked. 1192 1193- Userland memory - page cache and anonymous memory. 1194 1195- Kernel data structures such as dentries and inodes. 1196 1197- TCP socket buffers. 1198 1199The above list may expand in the future for better coverage. 1200 1201 1202Memory Interface Files 1203~~~~~~~~~~~~~~~~~~~~~~ 1204 1205All memory amounts are in bytes. If a value which is not aligned to 1206PAGE_SIZE is written, the value may be rounded up to the closest 1207PAGE_SIZE multiple when read back. 1208 1209 memory.current 1210 A read-only single value file which exists on non-root 1211 cgroups. 1212 1213 The total amount of memory currently being used by the cgroup 1214 and its descendants. 1215 1216 memory.min 1217 A read-write single value file which exists on non-root 1218 cgroups. The default is "0". 1219 1220 Hard memory protection. If the memory usage of a cgroup 1221 is within its effective min boundary, the cgroup's memory 1222 won't be reclaimed under any conditions. If there is no 1223 unprotected reclaimable memory available, OOM killer 1224 is invoked. Above the effective min boundary (or 1225 effective low boundary if it is higher), pages are reclaimed 1226 proportionally to the overage, reducing reclaim pressure for 1227 smaller overages. 1228 1229 Effective min boundary is limited by memory.min values of 1230 all ancestor cgroups. If there is memory.min overcommitment 1231 (child cgroup or cgroups are requiring more protected memory 1232 than parent will allow), then each child cgroup will get 1233 the part of parent's protection proportional to its 1234 actual memory usage below memory.min. 1235 1236 Putting more memory than generally available under this 1237 protection is discouraged and may lead to constant OOMs. 1238 1239 If a memory cgroup is not populated with processes, 1240 its memory.min is ignored. 1241 1242 memory.low 1243 A read-write single value file which exists on non-root 1244 cgroups. The default is "0". 1245 1246 Best-effort memory protection. If the memory usage of a 1247 cgroup is within its effective low boundary, the cgroup's 1248 memory won't be reclaimed unless there is no reclaimable 1249 memory available in unprotected cgroups. 1250 Above the effective low boundary (or 1251 effective min boundary if it is higher), pages are reclaimed 1252 proportionally to the overage, reducing reclaim pressure for 1253 smaller overages. 1254 1255 Effective low boundary is limited by memory.low values of 1256 all ancestor cgroups. If there is memory.low overcommitment 1257 (child cgroup or cgroups are requiring more protected memory 1258 than parent will allow), then each child cgroup will get 1259 the part of parent's protection proportional to its 1260 actual memory usage below memory.low. 1261 1262 Putting more memory than generally available under this 1263 protection is discouraged. 1264 1265 memory.high 1266 A read-write single value file which exists on non-root 1267 cgroups. The default is "max". 1268 1269 Memory usage throttle limit. If a cgroup's usage goes 1270 over the high boundary, the processes of the cgroup are 1271 throttled and put under heavy reclaim pressure. 1272 1273 Going over the high limit never invokes the OOM killer and 1274 under extreme conditions the limit may be breached. The high 1275 limit should be used in scenarios where an external process 1276 monitors the limited cgroup to alleviate heavy reclaim 1277 pressure. 1278 1279 memory.max 1280 A read-write single value file which exists on non-root 1281 cgroups. The default is "max". 1282 1283 Memory usage hard limit. This is the main mechanism to limit 1284 memory usage of a cgroup. If a cgroup's memory usage reaches 1285 this limit and can't be reduced, the OOM killer is invoked in 1286 the cgroup. Under certain circumstances, the usage may go 1287 over the limit temporarily. 1288 1289 In default configuration regular 0-order allocations always 1290 succeed unless OOM killer chooses current task as a victim. 1291 1292 Some kinds of allocations don't invoke the OOM killer. 1293 Caller could retry them differently, return into userspace 1294 as -ENOMEM or silently ignore in cases like disk readahead. 1295 1296 memory.reclaim 1297 A write-only nested-keyed file which exists for all cgroups. 1298 1299 This is a simple interface to trigger memory reclaim in the 1300 target cgroup. 1301 1302 This file accepts a single key, the number of bytes to reclaim. 1303 No nested keys are currently supported. 1304 1305 Example:: 1306 1307 echo "1G" > memory.reclaim 1308 1309 The interface can be later extended with nested keys to 1310 configure the reclaim behavior. For example, specify the 1311 type of memory to reclaim from (anon, file, ..). 1312 1313 Please note that the kernel can over or under reclaim from 1314 the target cgroup. If less bytes are reclaimed than the 1315 specified amount, -EAGAIN is returned. 1316 1317 Please note that the proactive reclaim (triggered by this 1318 interface) is not meant to indicate memory pressure on the 1319 memory cgroup. Therefore socket memory balancing triggered by 1320 the memory reclaim normally is not exercised in this case. 1321 This means that the networking layer will not adapt based on 1322 reclaim induced by memory.reclaim. 1323 1324 memory.peak 1325 A read-only single value file which exists on non-root 1326 cgroups. 1327 1328 The max memory usage recorded for the cgroup and its 1329 descendants since the creation of the cgroup. 1330 1331 memory.oom.group 1332 A read-write single value file which exists on non-root 1333 cgroups. The default value is "0". 1334 1335 Determines whether the cgroup should be treated as 1336 an indivisible workload by the OOM killer. If set, 1337 all tasks belonging to the cgroup or to its descendants 1338 (if the memory cgroup is not a leaf cgroup) are killed 1339 together or not at all. This can be used to avoid 1340 partial kills to guarantee workload integrity. 1341 1342 Tasks with the OOM protection (oom_score_adj set to -1000) 1343 are treated as an exception and are never killed. 1344 1345 If the OOM killer is invoked in a cgroup, it's not going 1346 to kill any tasks outside of this cgroup, regardless 1347 memory.oom.group values of ancestor cgroups. 1348 1349 memory.events 1350 A read-only flat-keyed file which exists on non-root cgroups. 1351 The following entries are defined. Unless specified 1352 otherwise, a value change in this file generates a file 1353 modified event. 1354 1355 Note that all fields in this file are hierarchical and the 1356 file modified event can be generated due to an event down the 1357 hierarchy. For the local events at the cgroup level see 1358 memory.events.local. 1359 1360 low 1361 The number of times the cgroup is reclaimed due to 1362 high memory pressure even though its usage is under 1363 the low boundary. This usually indicates that the low 1364 boundary is over-committed. 1365 1366 high 1367 The number of times processes of the cgroup are 1368 throttled and routed to perform direct memory reclaim 1369 because the high memory boundary was exceeded. For a 1370 cgroup whose memory usage is capped by the high limit 1371 rather than global memory pressure, this event's 1372 occurrences are expected. 1373 1374 max 1375 The number of times the cgroup's memory usage was 1376 about to go over the max boundary. If direct reclaim 1377 fails to bring it down, the cgroup goes to OOM state. 1378 1379 oom 1380 The number of time the cgroup's memory usage was 1381 reached the limit and allocation was about to fail. 1382 1383 This event is not raised if the OOM killer is not 1384 considered as an option, e.g. for failed high-order 1385 allocations or if caller asked to not retry attempts. 1386 1387 oom_kill 1388 The number of processes belonging to this cgroup 1389 killed by any kind of OOM killer. 1390 1391 oom_group_kill 1392 The number of times a group OOM has occurred. 1393 1394 memory.events.local 1395 Similar to memory.events but the fields in the file are local 1396 to the cgroup i.e. not hierarchical. The file modified event 1397 generated on this file reflects only the local events. 1398 1399 memory.stat 1400 A read-only flat-keyed file which exists on non-root cgroups. 1401 1402 This breaks down the cgroup's memory footprint into different 1403 types of memory, type-specific details, and other information 1404 on the state and past events of the memory management system. 1405 1406 All memory amounts are in bytes. 1407 1408 The entries are ordered to be human readable, and new entries 1409 can show up in the middle. Don't rely on items remaining in a 1410 fixed position; use the keys to look up specific values! 1411 1412 If the entry has no per-node counter (or not show in the 1413 memory.numa_stat). We use 'npn' (non-per-node) as the tag 1414 to indicate that it will not show in the memory.numa_stat. 1415 1416 anon 1417 Amount of memory used in anonymous mappings such as 1418 brk(), sbrk(), and mmap(MAP_ANONYMOUS) 1419 1420 file 1421 Amount of memory used to cache filesystem data, 1422 including tmpfs and shared memory. 1423 1424 kernel (npn) 1425 Amount of total kernel memory, including 1426 (kernel_stack, pagetables, percpu, vmalloc, slab) in 1427 addition to other kernel memory use cases. 1428 1429 kernel_stack 1430 Amount of memory allocated to kernel stacks. 1431 1432 pagetables 1433 Amount of memory allocated for page tables. 1434 1435 sec_pagetables 1436 Amount of memory allocated for secondary page tables, 1437 this currently includes KVM mmu allocations on x86 1438 and arm64 and IOMMU page tables. 1439 1440 percpu (npn) 1441 Amount of memory used for storing per-cpu kernel 1442 data structures. 1443 1444 sock (npn) 1445 Amount of memory used in network transmission buffers 1446 1447 vmalloc (npn) 1448 Amount of memory used for vmap backed memory. 1449 1450 shmem 1451 Amount of cached filesystem data that is swap-backed, 1452 such as tmpfs, shm segments, shared anonymous mmap()s 1453 1454 zswap 1455 Amount of memory consumed by the zswap compression backend. 1456 1457 zswapped 1458 Amount of application memory swapped out to zswap. 1459 1460 file_mapped 1461 Amount of cached filesystem data mapped with mmap() 1462 1463 file_dirty 1464 Amount of cached filesystem data that was modified but 1465 not yet written back to disk 1466 1467 file_writeback 1468 Amount of cached filesystem data that was modified and 1469 is currently being written back to disk 1470 1471 swapcached 1472 Amount of swap cached in memory. The swapcache is accounted 1473 against both memory and swap usage. 1474 1475 anon_thp 1476 Amount of memory used in anonymous mappings backed by 1477 transparent hugepages 1478 1479 file_thp 1480 Amount of cached filesystem data backed by transparent 1481 hugepages 1482 1483 shmem_thp 1484 Amount of shm, tmpfs, shared anonymous mmap()s backed by 1485 transparent hugepages 1486 1487 inactive_anon, active_anon, inactive_file, active_file, unevictable 1488 Amount of memory, swap-backed and filesystem-backed, 1489 on the internal memory management lists used by the 1490 page reclaim algorithm. 1491 1492 As these represent internal list state (eg. shmem pages are on anon 1493 memory management lists), inactive_foo + active_foo may not be equal to 1494 the value for the foo counter, since the foo counter is type-based, not 1495 list-based. 1496 1497 slab_reclaimable 1498 Part of "slab" that might be reclaimed, such as 1499 dentries and inodes. 1500 1501 slab_unreclaimable 1502 Part of "slab" that cannot be reclaimed on memory 1503 pressure. 1504 1505 slab (npn) 1506 Amount of memory used for storing in-kernel data 1507 structures. 1508 1509 workingset_refault_anon 1510 Number of refaults of previously evicted anonymous pages. 1511 1512 workingset_refault_file 1513 Number of refaults of previously evicted file pages. 1514 1515 workingset_activate_anon 1516 Number of refaulted anonymous pages that were immediately 1517 activated. 1518 1519 workingset_activate_file 1520 Number of refaulted file pages that were immediately activated. 1521 1522 workingset_restore_anon 1523 Number of restored anonymous pages which have been detected as 1524 an active workingset before they got reclaimed. 1525 1526 workingset_restore_file 1527 Number of restored file pages which have been detected as an 1528 active workingset before they got reclaimed. 1529 1530 workingset_nodereclaim 1531 Number of times a shadow node has been reclaimed 1532 1533 pgscan (npn) 1534 Amount of scanned pages (in an inactive LRU list) 1535 1536 pgsteal (npn) 1537 Amount of reclaimed pages 1538 1539 pgscan_kswapd (npn) 1540 Amount of scanned pages by kswapd (in an inactive LRU list) 1541 1542 pgscan_direct (npn) 1543 Amount of scanned pages directly (in an inactive LRU list) 1544 1545 pgscan_khugepaged (npn) 1546 Amount of scanned pages by khugepaged (in an inactive LRU list) 1547 1548 pgsteal_kswapd (npn) 1549 Amount of reclaimed pages by kswapd 1550 1551 pgsteal_direct (npn) 1552 Amount of reclaimed pages directly 1553 1554 pgsteal_khugepaged (npn) 1555 Amount of reclaimed pages by khugepaged 1556 1557 pgfault (npn) 1558 Total number of page faults incurred 1559 1560 pgmajfault (npn) 1561 Number of major page faults incurred 1562 1563 pgrefill (npn) 1564 Amount of scanned pages (in an active LRU list) 1565 1566 pgactivate (npn) 1567 Amount of pages moved to the active LRU list 1568 1569 pgdeactivate (npn) 1570 Amount of pages moved to the inactive LRU list 1571 1572 pglazyfree (npn) 1573 Amount of pages postponed to be freed under memory pressure 1574 1575 pglazyfreed (npn) 1576 Amount of reclaimed lazyfree pages 1577 1578 zswpin 1579 Number of pages moved in to memory from zswap. 1580 1581 zswpout 1582 Number of pages moved out of memory to zswap. 1583 1584 zswpwb 1585 Number of pages written from zswap to swap. 1586 1587 thp_fault_alloc (npn) 1588 Number of transparent hugepages which were allocated to satisfy 1589 a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE 1590 is not set. 1591 1592 thp_collapse_alloc (npn) 1593 Number of transparent hugepages which were allocated to allow 1594 collapsing an existing range of pages. This counter is not 1595 present when CONFIG_TRANSPARENT_HUGEPAGE is not set. 1596 1597 thp_swpout (npn) 1598 Number of transparent hugepages which are swapout in one piece 1599 without splitting. 1600 1601 thp_swpout_fallback (npn) 1602 Number of transparent hugepages which were split before swapout. 1603 Usually because failed to allocate some continuous swap space 1604 for the huge page. 1605 1606 memory.numa_stat 1607 A read-only nested-keyed file which exists on non-root cgroups. 1608 1609 This breaks down the cgroup's memory footprint into different 1610 types of memory, type-specific details, and other information 1611 per node on the state of the memory management system. 1612 1613 This is useful for providing visibility into the NUMA locality 1614 information within an memcg since the pages are allowed to be 1615 allocated from any physical node. One of the use case is evaluating 1616 application performance by combining this information with the 1617 application's CPU allocation. 1618 1619 All memory amounts are in bytes. 1620 1621 The output format of memory.numa_stat is:: 1622 1623 type N0=<bytes in node 0> N1=<bytes in node 1> ... 1624 1625 The entries are ordered to be human readable, and new entries 1626 can show up in the middle. Don't rely on items remaining in a 1627 fixed position; use the keys to look up specific values! 1628 1629 The entries can refer to the memory.stat. 1630 1631 memory.swap.current 1632 A read-only single value file which exists on non-root 1633 cgroups. 1634 1635 The total amount of swap currently being used by the cgroup 1636 and its descendants. 1637 1638 memory.swap.high 1639 A read-write single value file which exists on non-root 1640 cgroups. The default is "max". 1641 1642 Swap usage throttle limit. If a cgroup's swap usage exceeds 1643 this limit, all its further allocations will be throttled to 1644 allow userspace to implement custom out-of-memory procedures. 1645 1646 This limit marks a point of no return for the cgroup. It is NOT 1647 designed to manage the amount of swapping a workload does 1648 during regular operation. Compare to memory.swap.max, which 1649 prohibits swapping past a set amount, but lets the cgroup 1650 continue unimpeded as long as other memory can be reclaimed. 1651 1652 Healthy workloads are not expected to reach this limit. 1653 1654 memory.swap.peak 1655 A read-only single value file which exists on non-root 1656 cgroups. 1657 1658 The max swap usage recorded for the cgroup and its 1659 descendants since the creation of the cgroup. 1660 1661 memory.swap.max 1662 A read-write single value file which exists on non-root 1663 cgroups. The default is "max". 1664 1665 Swap usage hard limit. If a cgroup's swap usage reaches this 1666 limit, anonymous memory of the cgroup will not be swapped out. 1667 1668 memory.swap.events 1669 A read-only flat-keyed file which exists on non-root cgroups. 1670 The following entries are defined. Unless specified 1671 otherwise, a value change in this file generates a file 1672 modified event. 1673 1674 high 1675 The number of times the cgroup's swap usage was over 1676 the high threshold. 1677 1678 max 1679 The number of times the cgroup's swap usage was about 1680 to go over the max boundary and swap allocation 1681 failed. 1682 1683 fail 1684 The number of times swap allocation failed either 1685 because of running out of swap system-wide or max 1686 limit. 1687 1688 When reduced under the current usage, the existing swap 1689 entries are reclaimed gradually and the swap usage may stay 1690 higher than the limit for an extended period of time. This 1691 reduces the impact on the workload and memory management. 1692 1693 memory.zswap.current 1694 A read-only single value file which exists on non-root 1695 cgroups. 1696 1697 The total amount of memory consumed by the zswap compression 1698 backend. 1699 1700 memory.zswap.max 1701 A read-write single value file which exists on non-root 1702 cgroups. The default is "max". 1703 1704 Zswap usage hard limit. If a cgroup's zswap pool reaches this 1705 limit, it will refuse to take any more stores before existing 1706 entries fault back in or are written out to disk. 1707 1708 memory.zswap.writeback 1709 A read-write single value file. The default value is "1". The 1710 initial value of the root cgroup is 1, and when a new cgroup is 1711 created, it inherits the current value of its parent. 1712 1713 When this is set to 0, all swapping attempts to swapping devices 1714 are disabled. This included both zswap writebacks, and swapping due 1715 to zswap store failures. If the zswap store failures are recurring 1716 (for e.g if the pages are incompressible), users can observe 1717 reclaim inefficiency after disabling writeback (because the same 1718 pages might be rejected again and again). 1719 1720 Note that this is subtly different from setting memory.swap.max to 1721 0, as it still allows for pages to be written to the zswap pool. 1722 1723 memory.pressure 1724 A read-only nested-keyed file. 1725 1726 Shows pressure stall information for memory. See 1727 :ref:`Documentation/accounting/psi.rst <psi>` for details. 1728 1729 1730Usage Guidelines 1731~~~~~~~~~~~~~~~~ 1732 1733"memory.high" is the main mechanism to control memory usage. 1734Over-committing on high limit (sum of high limits > available memory) 1735and letting global memory pressure to distribute memory according to 1736usage is a viable strategy. 1737 1738Because breach of the high limit doesn't trigger the OOM killer but 1739throttles the offending cgroup, a management agent has ample 1740opportunities to monitor and take appropriate actions such as granting 1741more memory or terminating the workload. 1742 1743Determining whether a cgroup has enough memory is not trivial as 1744memory usage doesn't indicate whether the workload can benefit from 1745more memory. For example, a workload which writes data received from 1746network to a file can use all available memory but can also operate as 1747performant with a small amount of memory. A measure of memory 1748pressure - how much the workload is being impacted due to lack of 1749memory - is necessary to determine whether a workload needs more 1750memory; unfortunately, memory pressure monitoring mechanism isn't 1751implemented yet. 1752 1753 1754Memory Ownership 1755~~~~~~~~~~~~~~~~ 1756 1757A memory area is charged to the cgroup which instantiated it and stays 1758charged to the cgroup until the area is released. Migrating a process 1759to a different cgroup doesn't move the memory usages that it 1760instantiated while in the previous cgroup to the new cgroup. 1761 1762A memory area may be used by processes belonging to different cgroups. 1763To which cgroup the area will be charged is in-deterministic; however, 1764over time, the memory area is likely to end up in a cgroup which has 1765enough memory allowance to avoid high reclaim pressure. 1766 1767If a cgroup sweeps a considerable amount of memory which is expected 1768to be accessed repeatedly by other cgroups, it may make sense to use 1769POSIX_FADV_DONTNEED to relinquish the ownership of memory areas 1770belonging to the affected files to ensure correct memory ownership. 1771 1772 1773IO 1774-- 1775 1776The "io" controller regulates the distribution of IO resources. This 1777controller implements both weight based and absolute bandwidth or IOPS 1778limit distribution; however, weight based distribution is available 1779only if cfq-iosched is in use and neither scheme is available for 1780blk-mq devices. 1781 1782 1783IO Interface Files 1784~~~~~~~~~~~~~~~~~~ 1785 1786 io.stat 1787 A read-only nested-keyed file. 1788 1789 Lines are keyed by $MAJ:$MIN device numbers and not ordered. 1790 The following nested keys are defined. 1791 1792 ====== ===================== 1793 rbytes Bytes read 1794 wbytes Bytes written 1795 rios Number of read IOs 1796 wios Number of write IOs 1797 dbytes Bytes discarded 1798 dios Number of discard IOs 1799 ====== ===================== 1800 1801 An example read output follows:: 1802 1803 8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0 1804 8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021 1805 1806 io.cost.qos 1807 A read-write nested-keyed file which exists only on the root 1808 cgroup. 1809 1810 This file configures the Quality of Service of the IO cost 1811 model based controller (CONFIG_BLK_CGROUP_IOCOST) which 1812 currently implements "io.weight" proportional control. Lines 1813 are keyed by $MAJ:$MIN device numbers and not ordered. The 1814 line for a given device is populated on the first write for 1815 the device on "io.cost.qos" or "io.cost.model". The following 1816 nested keys are defined. 1817 1818 ====== ===================================== 1819 enable Weight-based control enable 1820 ctrl "auto" or "user" 1821 rpct Read latency percentile [0, 100] 1822 rlat Read latency threshold 1823 wpct Write latency percentile [0, 100] 1824 wlat Write latency threshold 1825 min Minimum scaling percentage [1, 10000] 1826 max Maximum scaling percentage [1, 10000] 1827 ====== ===================================== 1828 1829 The controller is disabled by default and can be enabled by 1830 setting "enable" to 1. "rpct" and "wpct" parameters default 1831 to zero and the controller uses internal device saturation 1832 state to adjust the overall IO rate between "min" and "max". 1833 1834 When a better control quality is needed, latency QoS 1835 parameters can be configured. For example:: 1836 1837 8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0 1838 1839 shows that on sdb, the controller is enabled, will consider 1840 the device saturated if the 95th percentile of read completion 1841 latencies is above 75ms or write 150ms, and adjust the overall 1842 IO issue rate between 50% and 150% accordingly. 1843 1844 The lower the saturation point, the better the latency QoS at 1845 the cost of aggregate bandwidth. The narrower the allowed 1846 adjustment range between "min" and "max", the more conformant 1847 to the cost model the IO behavior. Note that the IO issue 1848 base rate may be far off from 100% and setting "min" and "max" 1849 blindly can lead to a significant loss of device capacity or 1850 control quality. "min" and "max" are useful for regulating 1851 devices which show wide temporary behavior changes - e.g. a 1852 ssd which accepts writes at the line speed for a while and 1853 then completely stalls for multiple seconds. 1854 1855 When "ctrl" is "auto", the parameters are controlled by the 1856 kernel and may change automatically. Setting "ctrl" to "user" 1857 or setting any of the percentile and latency parameters puts 1858 it into "user" mode and disables the automatic changes. The 1859 automatic mode can be restored by setting "ctrl" to "auto". 1860 1861 io.cost.model 1862 A read-write nested-keyed file which exists only on the root 1863 cgroup. 1864 1865 This file configures the cost model of the IO cost model based 1866 controller (CONFIG_BLK_CGROUP_IOCOST) which currently 1867 implements "io.weight" proportional control. Lines are keyed 1868 by $MAJ:$MIN device numbers and not ordered. The line for a 1869 given device is populated on the first write for the device on 1870 "io.cost.qos" or "io.cost.model". The following nested keys 1871 are defined. 1872 1873 ===== ================================ 1874 ctrl "auto" or "user" 1875 model The cost model in use - "linear" 1876 ===== ================================ 1877 1878 When "ctrl" is "auto", the kernel may change all parameters 1879 dynamically. When "ctrl" is set to "user" or any other 1880 parameters are written to, "ctrl" become "user" and the 1881 automatic changes are disabled. 1882 1883 When "model" is "linear", the following model parameters are 1884 defined. 1885 1886 ============= ======================================== 1887 [r|w]bps The maximum sequential IO throughput 1888 [r|w]seqiops The maximum 4k sequential IOs per second 1889 [r|w]randiops The maximum 4k random IOs per second 1890 ============= ======================================== 1891 1892 From the above, the builtin linear model determines the base 1893 costs of a sequential and random IO and the cost coefficient 1894 for the IO size. While simple, this model can cover most 1895 common device classes acceptably. 1896 1897 The IO cost model isn't expected to be accurate in absolute 1898 sense and is scaled to the device behavior dynamically. 1899 1900 If needed, tools/cgroup/iocost_coef_gen.py can be used to 1901 generate device-specific coefficients. 1902 1903 io.weight 1904 A read-write flat-keyed file which exists on non-root cgroups. 1905 The default is "default 100". 1906 1907 The first line is the default weight applied to devices 1908 without specific override. The rest are overrides keyed by 1909 $MAJ:$MIN device numbers and not ordered. The weights are in 1910 the range [1, 10000] and specifies the relative amount IO time 1911 the cgroup can use in relation to its siblings. 1912 1913 The default weight can be updated by writing either "default 1914 $WEIGHT" or simply "$WEIGHT". Overrides can be set by writing 1915 "$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default". 1916 1917 An example read output follows:: 1918 1919 default 100 1920 8:16 200 1921 8:0 50 1922 1923 io.max 1924 A read-write nested-keyed file which exists on non-root 1925 cgroups. 1926 1927 BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN 1928 device numbers and not ordered. The following nested keys are 1929 defined. 1930 1931 ===== ================================== 1932 rbps Max read bytes per second 1933 wbps Max write bytes per second 1934 riops Max read IO operations per second 1935 wiops Max write IO operations per second 1936 ===== ================================== 1937 1938 When writing, any number of nested key-value pairs can be 1939 specified in any order. "max" can be specified as the value 1940 to remove a specific limit. If the same key is specified 1941 multiple times, the outcome is undefined. 1942 1943 BPS and IOPS are measured in each IO direction and IOs are 1944 delayed if limit is reached. Temporary bursts are allowed. 1945 1946 Setting read limit at 2M BPS and write at 120 IOPS for 8:16:: 1947 1948 echo "8:16 rbps=2097152 wiops=120" > io.max 1949 1950 Reading returns the following:: 1951 1952 8:16 rbps=2097152 wbps=max riops=max wiops=120 1953 1954 Write IOPS limit can be removed by writing the following:: 1955 1956 echo "8:16 wiops=max" > io.max 1957 1958 Reading now returns the following:: 1959 1960 8:16 rbps=2097152 wbps=max riops=max wiops=max 1961 1962 io.pressure 1963 A read-only nested-keyed file. 1964 1965 Shows pressure stall information for IO. See 1966 :ref:`Documentation/accounting/psi.rst <psi>` for details. 1967 1968 1969Writeback 1970~~~~~~~~~ 1971 1972Page cache is dirtied through buffered writes and shared mmaps and 1973written asynchronously to the backing filesystem by the writeback 1974mechanism. Writeback sits between the memory and IO domains and 1975regulates the proportion of dirty memory by balancing dirtying and 1976write IOs. 1977 1978The io controller, in conjunction with the memory controller, 1979implements control of page cache writeback IOs. The memory controller 1980defines the memory domain that dirty memory ratio is calculated and 1981maintained for and the io controller defines the io domain which 1982writes out dirty pages for the memory domain. Both system-wide and 1983per-cgroup dirty memory states are examined and the more restrictive 1984of the two is enforced. 1985 1986cgroup writeback requires explicit support from the underlying 1987filesystem. Currently, cgroup writeback is implemented on ext2, ext4, 1988btrfs, f2fs, and xfs. On other filesystems, all writeback IOs are 1989attributed to the root cgroup. 1990 1991There are inherent differences in memory and writeback management 1992which affects how cgroup ownership is tracked. Memory is tracked per 1993page while writeback per inode. For the purpose of writeback, an 1994inode is assigned to a cgroup and all IO requests to write dirty pages 1995from the inode are attributed to that cgroup. 1996 1997As cgroup ownership for memory is tracked per page, there can be pages 1998which are associated with different cgroups than the one the inode is 1999associated with. These are called foreign pages. The writeback 2000constantly keeps track of foreign pages and, if a particular foreign 2001cgroup becomes the majority over a certain period of time, switches 2002the ownership of the inode to that cgroup. 2003 2004While this model is enough for most use cases where a given inode is 2005mostly dirtied by a single cgroup even when the main writing cgroup 2006changes over time, use cases where multiple cgroups write to a single 2007inode simultaneously are not supported well. In such circumstances, a 2008significant portion of IOs are likely to be attributed incorrectly. 2009As memory controller assigns page ownership on the first use and 2010doesn't update it until the page is released, even if writeback 2011strictly follows page ownership, multiple cgroups dirtying overlapping 2012areas wouldn't work as expected. It's recommended to avoid such usage 2013patterns. 2014 2015The sysctl knobs which affect writeback behavior are applied to cgroup 2016writeback as follows. 2017 2018 vm.dirty_background_ratio, vm.dirty_ratio 2019 These ratios apply the same to cgroup writeback with the 2020 amount of available memory capped by limits imposed by the 2021 memory controller and system-wide clean memory. 2022 2023 vm.dirty_background_bytes, vm.dirty_bytes 2024 For cgroup writeback, this is calculated into ratio against 2025 total available memory and applied the same way as 2026 vm.dirty[_background]_ratio. 2027 2028 2029IO Latency 2030~~~~~~~~~~ 2031 2032This is a cgroup v2 controller for IO workload protection. You provide a group 2033with a latency target, and if the average latency exceeds that target the 2034controller will throttle any peers that have a lower latency target than the 2035protected workload. 2036 2037The limits are only applied at the peer level in the hierarchy. This means that 2038in the diagram below, only groups A, B, and C will influence each other, and 2039groups D and F will influence each other. Group G will influence nobody:: 2040 2041 [root] 2042 / | \ 2043 A B C 2044 / \ | 2045 D F G 2046 2047 2048So the ideal way to configure this is to set io.latency in groups A, B, and C. 2049Generally you do not want to set a value lower than the latency your device 2050supports. Experiment to find the value that works best for your workload. 2051Start at higher than the expected latency for your device and watch the 2052avg_lat value in io.stat for your workload group to get an idea of the 2053latency you see during normal operation. Use the avg_lat value as a basis for 2054your real setting, setting at 10-15% higher than the value in io.stat. 2055 2056How IO Latency Throttling Works 2057~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2058 2059io.latency is work conserving; so as long as everybody is meeting their latency 2060target the controller doesn't do anything. Once a group starts missing its 2061target it begins throttling any peer group that has a higher target than itself. 2062This throttling takes 2 forms: 2063 2064- Queue depth throttling. This is the number of outstanding IO's a group is 2065 allowed to have. We will clamp down relatively quickly, starting at no limit 2066 and going all the way down to 1 IO at a time. 2067 2068- Artificial delay induction. There are certain types of IO that cannot be 2069 throttled without possibly adversely affecting higher priority groups. This 2070 includes swapping and metadata IO. These types of IO are allowed to occur 2071 normally, however they are "charged" to the originating group. If the 2072 originating group is being throttled you will see the use_delay and delay 2073 fields in io.stat increase. The delay value is how many microseconds that are 2074 being added to any process that runs in this group. Because this number can 2075 grow quite large if there is a lot of swapping or metadata IO occurring we 2076 limit the individual delay events to 1 second at a time. 2077 2078Once the victimized group starts meeting its latency target again it will start 2079unthrottling any peer groups that were throttled previously. If the victimized 2080group simply stops doing IO the global counter will unthrottle appropriately. 2081 2082IO Latency Interface Files 2083~~~~~~~~~~~~~~~~~~~~~~~~~~ 2084 2085 io.latency 2086 This takes a similar format as the other controllers. 2087 2088 "MAJOR:MINOR target=<target time in microseconds>" 2089 2090 io.stat 2091 If the controller is enabled you will see extra stats in io.stat in 2092 addition to the normal ones. 2093 2094 depth 2095 This is the current queue depth for the group. 2096 2097 avg_lat 2098 This is an exponential moving average with a decay rate of 1/exp 2099 bound by the sampling interval. The decay rate interval can be 2100 calculated by multiplying the win value in io.stat by the 2101 corresponding number of samples based on the win value. 2102 2103 win 2104 The sampling window size in milliseconds. This is the minimum 2105 duration of time between evaluation events. Windows only elapse 2106 with IO activity. Idle periods extend the most recent window. 2107 2108IO Priority 2109~~~~~~~~~~~ 2110 2111A single attribute controls the behavior of the I/O priority cgroup policy, 2112namely the io.prio.class attribute. The following values are accepted for 2113that attribute: 2114 2115 no-change 2116 Do not modify the I/O priority class. 2117 2118 promote-to-rt 2119 For requests that have a non-RT I/O priority class, change it into RT. 2120 Also change the priority level of these requests to 4. Do not modify 2121 the I/O priority of requests that have priority class RT. 2122 2123 restrict-to-be 2124 For requests that do not have an I/O priority class or that have I/O 2125 priority class RT, change it into BE. Also change the priority level 2126 of these requests to 0. Do not modify the I/O priority class of 2127 requests that have priority class IDLE. 2128 2129 idle 2130 Change the I/O priority class of all requests into IDLE, the lowest 2131 I/O priority class. 2132 2133 none-to-rt 2134 Deprecated. Just an alias for promote-to-rt. 2135 2136The following numerical values are associated with the I/O priority policies: 2137 2138+----------------+---+ 2139| no-change | 0 | 2140+----------------+---+ 2141| promote-to-rt | 1 | 2142+----------------+---+ 2143| restrict-to-be | 2 | 2144+----------------+---+ 2145| idle | 3 | 2146+----------------+---+ 2147 2148The numerical value that corresponds to each I/O priority class is as follows: 2149 2150+-------------------------------+---+ 2151| IOPRIO_CLASS_NONE | 0 | 2152+-------------------------------+---+ 2153| IOPRIO_CLASS_RT (real-time) | 1 | 2154+-------------------------------+---+ 2155| IOPRIO_CLASS_BE (best effort) | 2 | 2156+-------------------------------+---+ 2157| IOPRIO_CLASS_IDLE | 3 | 2158+-------------------------------+---+ 2159 2160The algorithm to set the I/O priority class for a request is as follows: 2161 2162- If I/O priority class policy is promote-to-rt, change the request I/O 2163 priority class to IOPRIO_CLASS_RT and change the request I/O priority 2164 level to 4. 2165- If I/O priority class policy is not promote-to-rt, translate the I/O priority 2166 class policy into a number, then change the request I/O priority class 2167 into the maximum of the I/O priority class policy number and the numerical 2168 I/O priority class. 2169 2170PID 2171--- 2172 2173The process number controller is used to allow a cgroup to stop any 2174new tasks from being fork()'d or clone()'d after a specified limit is 2175reached. 2176 2177The number of tasks in a cgroup can be exhausted in ways which other 2178controllers cannot prevent, thus warranting its own controller. For 2179example, a fork bomb is likely to exhaust the number of tasks before 2180hitting memory restrictions. 2181 2182Note that PIDs used in this controller refer to TIDs, process IDs as 2183used by the kernel. 2184 2185 2186PID Interface Files 2187~~~~~~~~~~~~~~~~~~~ 2188 2189 pids.max 2190 A read-write single value file which exists on non-root 2191 cgroups. The default is "max". 2192 2193 Hard limit of number of processes. 2194 2195 pids.current 2196 A read-only single value file which exists on non-root cgroups. 2197 2198 The number of processes currently in the cgroup and its 2199 descendants. 2200 2201 pids.peak 2202 A read-only single value file which exists on non-root cgroups. 2203 2204 The maximum value that the number of processes in the cgroup and its 2205 descendants has ever reached. 2206 2207 pids.events 2208 A read-only flat-keyed file which exists on non-root cgroups. The 2209 following entries are defined. Unless specified otherwise, a value 2210 change in this file generates a file modified event. 2211 2212 max 2213 Number of times fork failed because limit was hit. 2214 2215Organisational operations are not blocked by cgroup policies, so it is 2216possible to have pids.current > pids.max. This can be done by either 2217setting the limit to be smaller than pids.current, or attaching enough 2218processes to the cgroup such that pids.current is larger than 2219pids.max. However, it is not possible to violate a cgroup PID policy 2220through fork() or clone(). These will return -EAGAIN if the creation 2221of a new process would cause a cgroup policy to be violated. 2222 2223 2224Cpuset 2225------ 2226 2227The "cpuset" controller provides a mechanism for constraining 2228the CPU and memory node placement of tasks to only the resources 2229specified in the cpuset interface files in a task's current cgroup. 2230This is especially valuable on large NUMA systems where placing jobs 2231on properly sized subsets of the systems with careful processor and 2232memory placement to reduce cross-node memory access and contention 2233can improve overall system performance. 2234 2235The "cpuset" controller is hierarchical. That means the controller 2236cannot use CPUs or memory nodes not allowed in its parent. 2237 2238 2239Cpuset Interface Files 2240~~~~~~~~~~~~~~~~~~~~~~ 2241 2242 cpuset.cpus 2243 A read-write multiple values file which exists on non-root 2244 cpuset-enabled cgroups. 2245 2246 It lists the requested CPUs to be used by tasks within this 2247 cgroup. The actual list of CPUs to be granted, however, is 2248 subjected to constraints imposed by its parent and can differ 2249 from the requested CPUs. 2250 2251 The CPU numbers are comma-separated numbers or ranges. 2252 For example:: 2253 2254 # cat cpuset.cpus 2255 0-4,6,8-10 2256 2257 An empty value indicates that the cgroup is using the same 2258 setting as the nearest cgroup ancestor with a non-empty 2259 "cpuset.cpus" or all the available CPUs if none is found. 2260 2261 The value of "cpuset.cpus" stays constant until the next update 2262 and won't be affected by any CPU hotplug events. 2263 2264 cpuset.cpus.effective 2265 A read-only multiple values file which exists on all 2266 cpuset-enabled cgroups. 2267 2268 It lists the onlined CPUs that are actually granted to this 2269 cgroup by its parent. These CPUs are allowed to be used by 2270 tasks within the current cgroup. 2271 2272 If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows 2273 all the CPUs from the parent cgroup that can be available to 2274 be used by this cgroup. Otherwise, it should be a subset of 2275 "cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus" 2276 can be granted. In this case, it will be treated just like an 2277 empty "cpuset.cpus". 2278 2279 Its value will be affected by CPU hotplug events. 2280 2281 cpuset.mems 2282 A read-write multiple values file which exists on non-root 2283 cpuset-enabled cgroups. 2284 2285 It lists the requested memory nodes to be used by tasks within 2286 this cgroup. The actual list of memory nodes granted, however, 2287 is subjected to constraints imposed by its parent and can differ 2288 from the requested memory nodes. 2289 2290 The memory node numbers are comma-separated numbers or ranges. 2291 For example:: 2292 2293 # cat cpuset.mems 2294 0-1,3 2295 2296 An empty value indicates that the cgroup is using the same 2297 setting as the nearest cgroup ancestor with a non-empty 2298 "cpuset.mems" or all the available memory nodes if none 2299 is found. 2300 2301 The value of "cpuset.mems" stays constant until the next update 2302 and won't be affected by any memory nodes hotplug events. 2303 2304 Setting a non-empty value to "cpuset.mems" causes memory of 2305 tasks within the cgroup to be migrated to the designated nodes if 2306 they are currently using memory outside of the designated nodes. 2307 2308 There is a cost for this memory migration. The migration 2309 may not be complete and some memory pages may be left behind. 2310 So it is recommended that "cpuset.mems" should be set properly 2311 before spawning new tasks into the cpuset. Even if there is 2312 a need to change "cpuset.mems" with active tasks, it shouldn't 2313 be done frequently. 2314 2315 cpuset.mems.effective 2316 A read-only multiple values file which exists on all 2317 cpuset-enabled cgroups. 2318 2319 It lists the onlined memory nodes that are actually granted to 2320 this cgroup by its parent. These memory nodes are allowed to 2321 be used by tasks within the current cgroup. 2322 2323 If "cpuset.mems" is empty, it shows all the memory nodes from the 2324 parent cgroup that will be available to be used by this cgroup. 2325 Otherwise, it should be a subset of "cpuset.mems" unless none of 2326 the memory nodes listed in "cpuset.mems" can be granted. In this 2327 case, it will be treated just like an empty "cpuset.mems". 2328 2329 Its value will be affected by memory nodes hotplug events. 2330 2331 cpuset.cpus.exclusive 2332 A read-write multiple values file which exists on non-root 2333 cpuset-enabled cgroups. 2334 2335 It lists all the exclusive CPUs that are allowed to be used 2336 to create a new cpuset partition. Its value is not used 2337 unless the cgroup becomes a valid partition root. See the 2338 "cpuset.cpus.partition" section below for a description of what 2339 a cpuset partition is. 2340 2341 When the cgroup becomes a partition root, the actual exclusive 2342 CPUs that are allocated to that partition are listed in 2343 "cpuset.cpus.exclusive.effective" which may be different 2344 from "cpuset.cpus.exclusive". If "cpuset.cpus.exclusive" 2345 has previously been set, "cpuset.cpus.exclusive.effective" 2346 is always a subset of it. 2347 2348 Users can manually set it to a value that is different from 2349 "cpuset.cpus". The only constraint in setting it is that the 2350 list of CPUs must be exclusive with respect to its sibling. 2351 2352 For a parent cgroup, any one of its exclusive CPUs can only 2353 be distributed to at most one of its child cgroups. Having an 2354 exclusive CPU appearing in two or more of its child cgroups is 2355 not allowed (the exclusivity rule). A value that violates the 2356 exclusivity rule will be rejected with a write error. 2357 2358 The root cgroup is a partition root and all its available CPUs 2359 are in its exclusive CPU set. 2360 2361 cpuset.cpus.exclusive.effective 2362 A read-only multiple values file which exists on all non-root 2363 cpuset-enabled cgroups. 2364 2365 This file shows the effective set of exclusive CPUs that 2366 can be used to create a partition root. The content of this 2367 file will always be a subset of "cpuset.cpus" and its parent's 2368 "cpuset.cpus.exclusive.effective" if its parent is not the root 2369 cgroup. It will also be a subset of "cpuset.cpus.exclusive" 2370 if it is set. If "cpuset.cpus.exclusive" is not set, it is 2371 treated to have an implicit value of "cpuset.cpus" in the 2372 formation of local partition. 2373 2374 cpuset.cpus.isolated 2375 A read-only and root cgroup only multiple values file. 2376 2377 This file shows the set of all isolated CPUs used in existing 2378 isolated partitions. It will be empty if no isolated partition 2379 is created. 2380 2381 cpuset.cpus.partition 2382 A read-write single value file which exists on non-root 2383 cpuset-enabled cgroups. This flag is owned by the parent cgroup 2384 and is not delegatable. 2385 2386 It accepts only the following input values when written to. 2387 2388 ========== ===================================== 2389 "member" Non-root member of a partition 2390 "root" Partition root 2391 "isolated" Partition root without load balancing 2392 ========== ===================================== 2393 2394 A cpuset partition is a collection of cpuset-enabled cgroups with 2395 a partition root at the top of the hierarchy and its descendants 2396 except those that are separate partition roots themselves and 2397 their descendants. A partition has exclusive access to the 2398 set of exclusive CPUs allocated to it. Other cgroups outside 2399 of that partition cannot use any CPUs in that set. 2400 2401 There are two types of partitions - local and remote. A local 2402 partition is one whose parent cgroup is also a valid partition 2403 root. A remote partition is one whose parent cgroup is not a 2404 valid partition root itself. Writing to "cpuset.cpus.exclusive" 2405 is optional for the creation of a local partition as its 2406 "cpuset.cpus.exclusive" file will assume an implicit value that 2407 is the same as "cpuset.cpus" if it is not set. Writing the 2408 proper "cpuset.cpus.exclusive" values down the cgroup hierarchy 2409 before the target partition root is mandatory for the creation 2410 of a remote partition. 2411 2412 Currently, a remote partition cannot be created under a local 2413 partition. All the ancestors of a remote partition root except 2414 the root cgroup cannot be a partition root. 2415 2416 The root cgroup is always a partition root and its state cannot 2417 be changed. All other non-root cgroups start out as "member". 2418 2419 When set to "root", the current cgroup is the root of a new 2420 partition or scheduling domain. The set of exclusive CPUs is 2421 determined by the value of its "cpuset.cpus.exclusive.effective". 2422 2423 When set to "isolated", the CPUs in that partition will be in 2424 an isolated state without any load balancing from the scheduler 2425 and excluded from the unbound workqueues. Tasks placed in such 2426 a partition with multiple CPUs should be carefully distributed 2427 and bound to each of the individual CPUs for optimal performance. 2428 2429 A partition root ("root" or "isolated") can be in one of the 2430 two possible states - valid or invalid. An invalid partition 2431 root is in a degraded state where some state information may 2432 be retained, but behaves more like a "member". 2433 2434 All possible state transitions among "member", "root" and 2435 "isolated" are allowed. 2436 2437 On read, the "cpuset.cpus.partition" file can show the following 2438 values. 2439 2440 ============================= ===================================== 2441 "member" Non-root member of a partition 2442 "root" Partition root 2443 "isolated" Partition root without load balancing 2444 "root invalid (<reason>)" Invalid partition root 2445 "isolated invalid (<reason>)" Invalid isolated partition root 2446 ============================= ===================================== 2447 2448 In the case of an invalid partition root, a descriptive string on 2449 why the partition is invalid is included within parentheses. 2450 2451 For a local partition root to be valid, the following conditions 2452 must be met. 2453 2454 1) The parent cgroup is a valid partition root. 2455 2) The "cpuset.cpus.exclusive.effective" file cannot be empty, 2456 though it may contain offline CPUs. 2457 3) The "cpuset.cpus.effective" cannot be empty unless there is 2458 no task associated with this partition. 2459 2460 For a remote partition root to be valid, all the above conditions 2461 except the first one must be met. 2462 2463 External events like hotplug or changes to "cpuset.cpus" or 2464 "cpuset.cpus.exclusive" can cause a valid partition root to 2465 become invalid and vice versa. Note that a task cannot be 2466 moved to a cgroup with empty "cpuset.cpus.effective". 2467 2468 A valid non-root parent partition may distribute out all its CPUs 2469 to its child local partitions when there is no task associated 2470 with it. 2471 2472 Care must be taken to change a valid partition root to "member" 2473 as all its child local partitions, if present, will become 2474 invalid causing disruption to tasks running in those child 2475 partitions. These inactivated partitions could be recovered if 2476 their parent is switched back to a partition root with a proper 2477 value in "cpuset.cpus" or "cpuset.cpus.exclusive". 2478 2479 Poll and inotify events are triggered whenever the state of 2480 "cpuset.cpus.partition" changes. That includes changes caused 2481 by write to "cpuset.cpus.partition", cpu hotplug or other 2482 changes that modify the validity status of the partition. 2483 This will allow user space agents to monitor unexpected changes 2484 to "cpuset.cpus.partition" without the need to do continuous 2485 polling. 2486 2487 A user can pre-configure certain CPUs to an isolated state 2488 with load balancing disabled at boot time with the "isolcpus" 2489 kernel boot command line option. If those CPUs are to be put 2490 into a partition, they have to be used in an isolated partition. 2491 2492 2493Device controller 2494----------------- 2495 2496Device controller manages access to device files. It includes both 2497creation of new device files (using mknod), and access to the 2498existing device files. 2499 2500Cgroup v2 device controller has no interface files and is implemented 2501on top of cgroup BPF. To control access to device files, a user may 2502create bpf programs of type BPF_PROG_TYPE_CGROUP_DEVICE and attach 2503them to cgroups with BPF_CGROUP_DEVICE flag. On an attempt to access a 2504device file, corresponding BPF programs will be executed, and depending 2505on the return value the attempt will succeed or fail with -EPERM. 2506 2507A BPF_PROG_TYPE_CGROUP_DEVICE program takes a pointer to the 2508bpf_cgroup_dev_ctx structure, which describes the device access attempt: 2509access type (mknod/read/write) and device (type, major and minor numbers). 2510If the program returns 0, the attempt fails with -EPERM, otherwise it 2511succeeds. 2512 2513An example of BPF_PROG_TYPE_CGROUP_DEVICE program may be found in 2514tools/testing/selftests/bpf/progs/dev_cgroup.c in the kernel source tree. 2515 2516 2517RDMA 2518---- 2519 2520The "rdma" controller regulates the distribution and accounting of 2521RDMA resources. 2522 2523RDMA Interface Files 2524~~~~~~~~~~~~~~~~~~~~ 2525 2526 rdma.max 2527 A readwrite nested-keyed file that exists for all the cgroups 2528 except root that describes current configured resource limit 2529 for a RDMA/IB device. 2530 2531 Lines are keyed by device name and are not ordered. 2532 Each line contains space separated resource name and its configured 2533 limit that can be distributed. 2534 2535 The following nested keys are defined. 2536 2537 ========== ============================= 2538 hca_handle Maximum number of HCA Handles 2539 hca_object Maximum number of HCA Objects 2540 ========== ============================= 2541 2542 An example for mlx4 and ocrdma device follows:: 2543 2544 mlx4_0 hca_handle=2 hca_object=2000 2545 ocrdma1 hca_handle=3 hca_object=max 2546 2547 rdma.current 2548 A read-only file that describes current resource usage. 2549 It exists for all the cgroup except root. 2550 2551 An example for mlx4 and ocrdma device follows:: 2552 2553 mlx4_0 hca_handle=1 hca_object=20 2554 ocrdma1 hca_handle=1 hca_object=23 2555 2556HugeTLB 2557------- 2558 2559The HugeTLB controller allows to limit the HugeTLB usage per control group and 2560enforces the controller limit during page fault. 2561 2562HugeTLB Interface Files 2563~~~~~~~~~~~~~~~~~~~~~~~ 2564 2565 hugetlb.<hugepagesize>.current 2566 Show current usage for "hugepagesize" hugetlb. It exists for all 2567 the cgroup except root. 2568 2569 hugetlb.<hugepagesize>.max 2570 Set/show the hard limit of "hugepagesize" hugetlb usage. 2571 The default value is "max". It exists for all the cgroup except root. 2572 2573 hugetlb.<hugepagesize>.events 2574 A read-only flat-keyed file which exists on non-root cgroups. 2575 2576 max 2577 The number of allocation failure due to HugeTLB limit 2578 2579 hugetlb.<hugepagesize>.events.local 2580 Similar to hugetlb.<hugepagesize>.events but the fields in the file 2581 are local to the cgroup i.e. not hierarchical. The file modified event 2582 generated on this file reflects only the local events. 2583 2584 hugetlb.<hugepagesize>.numa_stat 2585 Similar to memory.numa_stat, it shows the numa information of the 2586 hugetlb pages of <hugepagesize> in this cgroup. Only active in 2587 use hugetlb pages are included. The per-node values are in bytes. 2588 2589Misc 2590---- 2591 2592The Miscellaneous cgroup provides the resource limiting and tracking 2593mechanism for the scalar resources which cannot be abstracted like the other 2594cgroup resources. Controller is enabled by the CONFIG_CGROUP_MISC config 2595option. 2596 2597A resource can be added to the controller via enum misc_res_type{} in the 2598include/linux/misc_cgroup.h file and the corresponding name via misc_res_name[] 2599in the kernel/cgroup/misc.c file. Provider of the resource must set its 2600capacity prior to using the resource by calling misc_cg_set_capacity(). 2601 2602Once a capacity is set then the resource usage can be updated using charge and 2603uncharge APIs. All of the APIs to interact with misc controller are in 2604include/linux/misc_cgroup.h. 2605 2606Misc Interface Files 2607~~~~~~~~~~~~~~~~~~~~ 2608 2609Miscellaneous controller provides 3 interface files. If two misc resources (res_a and res_b) are registered then: 2610 2611 misc.capacity 2612 A read-only flat-keyed file shown only in the root cgroup. It shows 2613 miscellaneous scalar resources available on the platform along with 2614 their quantities:: 2615 2616 $ cat misc.capacity 2617 res_a 50 2618 res_b 10 2619 2620 misc.current 2621 A read-only flat-keyed file shown in the all cgroups. It shows 2622 the current usage of the resources in the cgroup and its children.:: 2623 2624 $ cat misc.current 2625 res_a 3 2626 res_b 0 2627 2628 misc.max 2629 A read-write flat-keyed file shown in the non root cgroups. Allowed 2630 maximum usage of the resources in the cgroup and its children.:: 2631 2632 $ cat misc.max 2633 res_a max 2634 res_b 4 2635 2636 Limit can be set by:: 2637 2638 # echo res_a 1 > misc.max 2639 2640 Limit can be set to max by:: 2641 2642 # echo res_a max > misc.max 2643 2644 Limits can be set higher than the capacity value in the misc.capacity 2645 file. 2646 2647 misc.events 2648 A read-only flat-keyed file which exists on non-root cgroups. The 2649 following entries are defined. Unless specified otherwise, a value 2650 change in this file generates a file modified event. All fields in 2651 this file are hierarchical. 2652 2653 max 2654 The number of times the cgroup's resource usage was 2655 about to go over the max boundary. 2656 2657Migration and Ownership 2658~~~~~~~~~~~~~~~~~~~~~~~ 2659 2660A miscellaneous scalar resource is charged to the cgroup in which it is used 2661first, and stays charged to that cgroup until that resource is freed. Migrating 2662a process to a different cgroup does not move the charge to the destination 2663cgroup where the process has moved. 2664 2665Others 2666------ 2667 2668perf_event 2669~~~~~~~~~~ 2670 2671perf_event controller, if not mounted on a legacy hierarchy, is 2672automatically enabled on the v2 hierarchy so that perf events can 2673always be filtered by cgroup v2 path. The controller can still be 2674moved to a legacy hierarchy after v2 hierarchy is populated. 2675 2676 2677Non-normative information 2678------------------------- 2679 2680This section contains information that isn't considered to be a part of 2681the stable kernel API and so is subject to change. 2682 2683 2684CPU controller root cgroup process behaviour 2685~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2686 2687When distributing CPU cycles in the root cgroup each thread in this 2688cgroup is treated as if it was hosted in a separate child cgroup of the 2689root cgroup. This child cgroup weight is dependent on its thread nice 2690level. 2691 2692For details of this mapping see sched_prio_to_weight array in 2693kernel/sched/core.c file (values from this array should be scaled 2694appropriately so the neutral - nice 0 - value is 100 instead of 1024). 2695 2696 2697IO controller root cgroup process behaviour 2698~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2699 2700Root cgroup processes are hosted in an implicit leaf child node. 2701When distributing IO resources this implicit child node is taken into 2702account as if it was a normal child cgroup of the root cgroup with a 2703weight value of 200. 2704 2705 2706Namespace 2707========= 2708 2709Basics 2710------ 2711 2712cgroup namespace provides a mechanism to virtualize the view of the 2713"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone 2714flag can be used with clone(2) and unshare(2) to create a new cgroup 2715namespace. The process running inside the cgroup namespace will have 2716its "/proc/$PID/cgroup" output restricted to cgroupns root. The 2717cgroupns root is the cgroup of the process at the time of creation of 2718the cgroup namespace. 2719 2720Without cgroup namespace, the "/proc/$PID/cgroup" file shows the 2721complete path of the cgroup of a process. In a container setup where 2722a set of cgroups and namespaces are intended to isolate processes the 2723"/proc/$PID/cgroup" file may leak potential system level information 2724to the isolated processes. For example:: 2725 2726 # cat /proc/self/cgroup 2727 0::/batchjobs/container_id1 2728 2729The path '/batchjobs/container_id1' can be considered as system-data 2730and undesirable to expose to the isolated processes. cgroup namespace 2731can be used to restrict visibility of this path. For example, before 2732creating a cgroup namespace, one would see:: 2733 2734 # ls -l /proc/self/ns/cgroup 2735 lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835] 2736 # cat /proc/self/cgroup 2737 0::/batchjobs/container_id1 2738 2739After unsharing a new namespace, the view changes:: 2740 2741 # ls -l /proc/self/ns/cgroup 2742 lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183] 2743 # cat /proc/self/cgroup 2744 0::/ 2745 2746When some thread from a multi-threaded process unshares its cgroup 2747namespace, the new cgroupns gets applied to the entire process (all 2748the threads). This is natural for the v2 hierarchy; however, for the 2749legacy hierarchies, this may be unexpected. 2750 2751A cgroup namespace is alive as long as there are processes inside or 2752mounts pinning it. When the last usage goes away, the cgroup 2753namespace is destroyed. The cgroupns root and the actual cgroups 2754remain. 2755 2756 2757The Root and Views 2758------------------ 2759 2760The 'cgroupns root' for a cgroup namespace is the cgroup in which the 2761process calling unshare(2) is running. For example, if a process in 2762/batchjobs/container_id1 cgroup calls unshare, cgroup 2763/batchjobs/container_id1 becomes the cgroupns root. For the 2764init_cgroup_ns, this is the real root ('/') cgroup. 2765 2766The cgroupns root cgroup does not change even if the namespace creator 2767process later moves to a different cgroup:: 2768 2769 # ~/unshare -c # unshare cgroupns in some cgroup 2770 # cat /proc/self/cgroup 2771 0::/ 2772 # mkdir sub_cgrp_1 2773 # echo 0 > sub_cgrp_1/cgroup.procs 2774 # cat /proc/self/cgroup 2775 0::/sub_cgrp_1 2776 2777Each process gets its namespace-specific view of "/proc/$PID/cgroup" 2778 2779Processes running inside the cgroup namespace will be able to see 2780cgroup paths (in /proc/self/cgroup) only inside their root cgroup. 2781From within an unshared cgroupns:: 2782 2783 # sleep 100000 & 2784 [1] 7353 2785 # echo 7353 > sub_cgrp_1/cgroup.procs 2786 # cat /proc/7353/cgroup 2787 0::/sub_cgrp_1 2788 2789From the initial cgroup namespace, the real cgroup path will be 2790visible:: 2791 2792 $ cat /proc/7353/cgroup 2793 0::/batchjobs/container_id1/sub_cgrp_1 2794 2795From a sibling cgroup namespace (that is, a namespace rooted at a 2796different cgroup), the cgroup path relative to its own cgroup 2797namespace root will be shown. For instance, if PID 7353's cgroup 2798namespace root is at '/batchjobs/container_id2', then it will see:: 2799 2800 # cat /proc/7353/cgroup 2801 0::/../container_id2/sub_cgrp_1 2802 2803Note that the relative path always starts with '/' to indicate that 2804its relative to the cgroup namespace root of the caller. 2805 2806 2807Migration and setns(2) 2808---------------------- 2809 2810Processes inside a cgroup namespace can move into and out of the 2811namespace root if they have proper access to external cgroups. For 2812example, from inside a namespace with cgroupns root at 2813/batchjobs/container_id1, and assuming that the global hierarchy is 2814still accessible inside cgroupns:: 2815 2816 # cat /proc/7353/cgroup 2817 0::/sub_cgrp_1 2818 # echo 7353 > batchjobs/container_id2/cgroup.procs 2819 # cat /proc/7353/cgroup 2820 0::/../container_id2 2821 2822Note that this kind of setup is not encouraged. A task inside cgroup 2823namespace should only be exposed to its own cgroupns hierarchy. 2824 2825setns(2) to another cgroup namespace is allowed when: 2826 2827(a) the process has CAP_SYS_ADMIN against its current user namespace 2828(b) the process has CAP_SYS_ADMIN against the target cgroup 2829 namespace's userns 2830 2831No implicit cgroup changes happen with attaching to another cgroup 2832namespace. It is expected that the someone moves the attaching 2833process under the target cgroup namespace root. 2834 2835 2836Interaction with Other Namespaces 2837--------------------------------- 2838 2839Namespace specific cgroup hierarchy can be mounted by a process 2840running inside a non-init cgroup namespace:: 2841 2842 # mount -t cgroup2 none $MOUNT_POINT 2843 2844This will mount the unified cgroup hierarchy with cgroupns root as the 2845filesystem root. The process needs CAP_SYS_ADMIN against its user and 2846mount namespaces. 2847 2848The virtualization of /proc/self/cgroup file combined with restricting 2849the view of cgroup hierarchy by namespace-private cgroupfs mount 2850provides a properly isolated cgroup view inside the container. 2851 2852 2853Information on Kernel Programming 2854================================= 2855 2856This section contains kernel programming information in the areas 2857where interacting with cgroup is necessary. cgroup core and 2858controllers are not covered. 2859 2860 2861Filesystem Support for Writeback 2862-------------------------------- 2863 2864A filesystem can support cgroup writeback by updating 2865address_space_operations->writepage[s]() to annotate bio's using the 2866following two functions. 2867 2868 wbc_init_bio(@wbc, @bio) 2869 Should be called for each bio carrying writeback data and 2870 associates the bio with the inode's owner cgroup and the 2871 corresponding request queue. This must be called after 2872 a queue (device) has been associated with the bio and 2873 before submission. 2874 2875 wbc_account_cgroup_owner(@wbc, @page, @bytes) 2876 Should be called for each data segment being written out. 2877 While this function doesn't care exactly when it's called 2878 during the writeback session, it's the easiest and most 2879 natural to call it as data segments are added to a bio. 2880 2881With writeback bio's annotated, cgroup support can be enabled per 2882super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for 2883selective disabling of cgroup writeback support which is helpful when 2884certain filesystem features, e.g. journaled data mode, are 2885incompatible. 2886 2887wbc_init_bio() binds the specified bio to its cgroup. Depending on 2888the configuration, the bio may be executed at a lower priority and if 2889the writeback session is holding shared resources, e.g. a journal 2890entry, may lead to priority inversion. There is no one easy solution 2891for the problem. Filesystems can try to work around specific problem 2892cases by skipping wbc_init_bio() and using bio_associate_blkg() 2893directly. 2894 2895 2896Deprecated v1 Core Features 2897=========================== 2898 2899- Multiple hierarchies including named ones are not supported. 2900 2901- All v1 mount options are not supported. 2902 2903- The "tasks" file is removed and "cgroup.procs" is not sorted. 2904 2905- "cgroup.clone_children" is removed. 2906 2907- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file 2908 at the root instead. 2909 2910 2911Issues with v1 and Rationales for v2 2912==================================== 2913 2914Multiple Hierarchies 2915-------------------- 2916 2917cgroup v1 allowed an arbitrary number of hierarchies and each 2918hierarchy could host any number of controllers. While this seemed to 2919provide a high level of flexibility, it wasn't useful in practice. 2920 2921For example, as there is only one instance of each controller, utility 2922type controllers such as freezer which can be useful in all 2923hierarchies could only be used in one. The issue is exacerbated by 2924the fact that controllers couldn't be moved to another hierarchy once 2925hierarchies were populated. Another issue was that all controllers 2926bound to a hierarchy were forced to have exactly the same view of the 2927hierarchy. It wasn't possible to vary the granularity depending on 2928the specific controller. 2929 2930In practice, these issues heavily limited which controllers could be 2931put on the same hierarchy and most configurations resorted to putting 2932each controller on its own hierarchy. Only closely related ones, such 2933as the cpu and cpuacct controllers, made sense to be put on the same 2934hierarchy. This often meant that userland ended up managing multiple 2935similar hierarchies repeating the same steps on each hierarchy 2936whenever a hierarchy management operation was necessary. 2937 2938Furthermore, support for multiple hierarchies came at a steep cost. 2939It greatly complicated cgroup core implementation but more importantly 2940the support for multiple hierarchies restricted how cgroup could be 2941used in general and what controllers was able to do. 2942 2943There was no limit on how many hierarchies there might be, which meant 2944that a thread's cgroup membership couldn't be described in finite 2945length. The key might contain any number of entries and was unlimited 2946in length, which made it highly awkward to manipulate and led to 2947addition of controllers which existed only to identify membership, 2948which in turn exacerbated the original problem of proliferating number 2949of hierarchies. 2950 2951Also, as a controller couldn't have any expectation regarding the 2952topologies of hierarchies other controllers might be on, each 2953controller had to assume that all other controllers were attached to 2954completely orthogonal hierarchies. This made it impossible, or at 2955least very cumbersome, for controllers to cooperate with each other. 2956 2957In most use cases, putting controllers on hierarchies which are 2958completely orthogonal to each other isn't necessary. What usually is 2959called for is the ability to have differing levels of granularity 2960depending on the specific controller. In other words, hierarchy may 2961be collapsed from leaf towards root when viewed from specific 2962controllers. For example, a given configuration might not care about 2963how memory is distributed beyond a certain level while still wanting 2964to control how CPU cycles are distributed. 2965 2966 2967Thread Granularity 2968------------------ 2969 2970cgroup v1 allowed threads of a process to belong to different cgroups. 2971This didn't make sense for some controllers and those controllers 2972ended up implementing different ways to ignore such situations but 2973much more importantly it blurred the line between API exposed to 2974individual applications and system management interface. 2975 2976Generally, in-process knowledge is available only to the process 2977itself; thus, unlike service-level organization of processes, 2978categorizing threads of a process requires active participation from 2979the application which owns the target process. 2980 2981cgroup v1 had an ambiguously defined delegation model which got abused 2982in combination with thread granularity. cgroups were delegated to 2983individual applications so that they can create and manage their own 2984sub-hierarchies and control resource distributions along them. This 2985effectively raised cgroup to the status of a syscall-like API exposed 2986to lay programs. 2987 2988First of all, cgroup has a fundamentally inadequate interface to be 2989exposed this way. For a process to access its own knobs, it has to 2990extract the path on the target hierarchy from /proc/self/cgroup, 2991construct the path by appending the name of the knob to the path, open 2992and then read and/or write to it. This is not only extremely clunky 2993and unusual but also inherently racy. There is no conventional way to 2994define transaction across the required steps and nothing can guarantee 2995that the process would actually be operating on its own sub-hierarchy. 2996 2997cgroup controllers implemented a number of knobs which would never be 2998accepted as public APIs because they were just adding control knobs to 2999system-management pseudo filesystem. cgroup ended up with interface 3000knobs which were not properly abstracted or refined and directly 3001revealed kernel internal details. These knobs got exposed to 3002individual applications through the ill-defined delegation mechanism 3003effectively abusing cgroup as a shortcut to implementing public APIs 3004without going through the required scrutiny. 3005 3006This was painful for both userland and kernel. Userland ended up with 3007misbehaving and poorly abstracted interfaces and kernel exposing and 3008locked into constructs inadvertently. 3009 3010 3011Competition Between Inner Nodes and Threads 3012------------------------------------------- 3013 3014cgroup v1 allowed threads to be in any cgroups which created an 3015interesting problem where threads belonging to a parent cgroup and its 3016children cgroups competed for resources. This was nasty as two 3017different types of entities competed and there was no obvious way to 3018settle it. Different controllers did different things. 3019 3020The cpu controller considered threads and cgroups as equivalents and 3021mapped nice levels to cgroup weights. This worked for some cases but 3022fell flat when children wanted to be allocated specific ratios of CPU 3023cycles and the number of internal threads fluctuated - the ratios 3024constantly changed as the number of competing entities fluctuated. 3025There also were other issues. The mapping from nice level to weight 3026wasn't obvious or universal, and there were various other knobs which 3027simply weren't available for threads. 3028 3029The io controller implicitly created a hidden leaf node for each 3030cgroup to host the threads. The hidden leaf had its own copies of all 3031the knobs with ``leaf_`` prefixed. While this allowed equivalent 3032control over internal threads, it was with serious drawbacks. It 3033always added an extra layer of nesting which wouldn't be necessary 3034otherwise, made the interface messy and significantly complicated the 3035implementation. 3036 3037The memory controller didn't have a way to control what happened 3038between internal tasks and child cgroups and the behavior was not 3039clearly defined. There were attempts to add ad-hoc behaviors and 3040knobs to tailor the behavior to specific workloads which would have 3041led to problems extremely difficult to resolve in the long term. 3042 3043Multiple controllers struggled with internal tasks and came up with 3044different ways to deal with it; unfortunately, all the approaches were 3045severely flawed and, furthermore, the widely different behaviors 3046made cgroup as a whole highly inconsistent. 3047 3048This clearly is a problem which needs to be addressed from cgroup core 3049in a uniform way. 3050 3051 3052Other Interface Issues 3053---------------------- 3054 3055cgroup v1 grew without oversight and developed a large number of 3056idiosyncrasies and inconsistencies. One issue on the cgroup core side 3057was how an empty cgroup was notified - a userland helper binary was 3058forked and executed for each event. The event delivery wasn't 3059recursive or delegatable. The limitations of the mechanism also led 3060to in-kernel event delivery filtering mechanism further complicating 3061the interface. 3062 3063Controller interfaces were problematic too. An extreme example is 3064controllers completely ignoring hierarchical organization and treating 3065all cgroups as if they were all located directly under the root 3066cgroup. Some controllers exposed a large amount of inconsistent 3067implementation details to userland. 3068 3069There also was no consistency across controllers. When a new cgroup 3070was created, some controllers defaulted to not imposing extra 3071restrictions while others disallowed any resource usage until 3072explicitly configured. Configuration knobs for the same type of 3073control used widely differing naming schemes and formats. Statistics 3074and information knobs were named arbitrarily and used different 3075formats and units even in the same controller. 3076 3077cgroup v2 establishes common conventions where appropriate and updates 3078controllers so that they expose minimal and consistent interfaces. 3079 3080 3081Controller Issues and Remedies 3082------------------------------ 3083 3084Memory 3085~~~~~~ 3086 3087The original lower boundary, the soft limit, is defined as a limit 3088that is per default unset. As a result, the set of cgroups that 3089global reclaim prefers is opt-in, rather than opt-out. The costs for 3090optimizing these mostly negative lookups are so high that the 3091implementation, despite its enormous size, does not even provide the 3092basic desirable behavior. First off, the soft limit has no 3093hierarchical meaning. All configured groups are organized in a global 3094rbtree and treated like equal peers, regardless where they are located 3095in the hierarchy. This makes subtree delegation impossible. Second, 3096the soft limit reclaim pass is so aggressive that it not just 3097introduces high allocation latencies into the system, but also impacts 3098system performance due to overreclaim, to the point where the feature 3099becomes self-defeating. 3100 3101The memory.low boundary on the other hand is a top-down allocated 3102reserve. A cgroup enjoys reclaim protection when it's within its 3103effective low, which makes delegation of subtrees possible. It also 3104enjoys having reclaim pressure proportional to its overage when 3105above its effective low. 3106 3107The original high boundary, the hard limit, is defined as a strict 3108limit that can not budge, even if the OOM killer has to be called. 3109But this generally goes against the goal of making the most out of the 3110available memory. The memory consumption of workloads varies during 3111runtime, and that requires users to overcommit. But doing that with a 3112strict upper limit requires either a fairly accurate prediction of the 3113working set size or adding slack to the limit. Since working set size 3114estimation is hard and error prone, and getting it wrong results in 3115OOM kills, most users tend to err on the side of a looser limit and 3116end up wasting precious resources. 3117 3118The memory.high boundary on the other hand can be set much more 3119conservatively. When hit, it throttles allocations by forcing them 3120into direct reclaim to work off the excess, but it never invokes the 3121OOM killer. As a result, a high boundary that is chosen too 3122aggressively will not terminate the processes, but instead it will 3123lead to gradual performance degradation. The user can monitor this 3124and make corrections until the minimal memory footprint that still 3125gives acceptable performance is found. 3126 3127In extreme cases, with many concurrent allocations and a complete 3128breakdown of reclaim progress within the group, the high boundary can 3129be exceeded. But even then it's mostly better to satisfy the 3130allocation from the slack available in other groups or the rest of the 3131system than killing the group. Otherwise, memory.max is there to 3132limit this type of spillover and ultimately contain buggy or even 3133malicious applications. 3134 3135Setting the original memory.limit_in_bytes below the current usage was 3136subject to a race condition, where concurrent charges could cause the 3137limit setting to fail. memory.max on the other hand will first set the 3138limit to prevent new charges, and then reclaim and OOM kill until the 3139new limit is met - or the task writing to memory.max is killed. 3140 3141The combined memory+swap accounting and limiting is replaced by real 3142control over swap space. 3143 3144The main argument for a combined memory+swap facility in the original 3145cgroup design was that global or parental pressure would always be 3146able to swap all anonymous memory of a child group, regardless of the 3147child's own (possibly untrusted) configuration. However, untrusted 3148groups can sabotage swapping by other means - such as referencing its 3149anonymous memory in a tight loop - and an admin can not assume full 3150swappability when overcommitting untrusted jobs. 3151 3152For trusted jobs, on the other hand, a combined counter is not an 3153intuitive userspace interface, and it flies in the face of the idea 3154that cgroup controllers should account and limit specific physical 3155resources. Swap space is a resource like all others in the system, 3156and that's why unified hierarchy allows distributing it separately. 3157