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 pids_localevents 243 The option restores v1-like behavior of pids.events:max, that is only 244 local (inside cgroup proper) fork failures are counted. Without this 245 option pids.events.max represents any pids.max enforcemnt across 246 cgroup's subtree. 247 248 249 250Organizing Processes and Threads 251-------------------------------- 252 253Processes 254~~~~~~~~~ 255 256Initially, only the root cgroup exists to which all processes belong. 257A child cgroup can be created by creating a sub-directory:: 258 259 # mkdir $CGROUP_NAME 260 261A given cgroup may have multiple child cgroups forming a tree 262structure. Each cgroup has a read-writable interface file 263"cgroup.procs". When read, it lists the PIDs of all processes which 264belong to the cgroup one-per-line. The PIDs are not ordered and the 265same PID may show up more than once if the process got moved to 266another cgroup and then back or the PID got recycled while reading. 267 268A process can be migrated into a cgroup by writing its PID to the 269target cgroup's "cgroup.procs" file. Only one process can be migrated 270on a single write(2) call. If a process is composed of multiple 271threads, writing the PID of any thread migrates all threads of the 272process. 273 274When a process forks a child process, the new process is born into the 275cgroup that the forking process belongs to at the time of the 276operation. After exit, a process stays associated with the cgroup 277that it belonged to at the time of exit until it's reaped; however, a 278zombie process does not appear in "cgroup.procs" and thus can't be 279moved to another cgroup. 280 281A cgroup which doesn't have any children or live processes can be 282destroyed by removing the directory. Note that a cgroup which doesn't 283have any children and is associated only with zombie processes is 284considered empty and can be removed:: 285 286 # rmdir $CGROUP_NAME 287 288"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy 289cgroup is in use in the system, this file may contain multiple lines, 290one for each hierarchy. The entry for cgroup v2 is always in the 291format "0::$PATH":: 292 293 # cat /proc/842/cgroup 294 ... 295 0::/test-cgroup/test-cgroup-nested 296 297If the process becomes a zombie and the cgroup it was associated with 298is removed subsequently, " (deleted)" is appended to the path:: 299 300 # cat /proc/842/cgroup 301 ... 302 0::/test-cgroup/test-cgroup-nested (deleted) 303 304 305Threads 306~~~~~~~ 307 308cgroup v2 supports thread granularity for a subset of controllers to 309support use cases requiring hierarchical resource distribution across 310the threads of a group of processes. By default, all threads of a 311process belong to the same cgroup, which also serves as the resource 312domain to host resource consumptions which are not specific to a 313process or thread. The thread mode allows threads to be spread across 314a subtree while still maintaining the common resource domain for them. 315 316Controllers which support thread mode are called threaded controllers. 317The ones which don't are called domain controllers. 318 319Marking a cgroup threaded makes it join the resource domain of its 320parent as a threaded cgroup. The parent may be another threaded 321cgroup whose resource domain is further up in the hierarchy. The root 322of a threaded subtree, that is, the nearest ancestor which is not 323threaded, is called threaded domain or thread root interchangeably and 324serves as the resource domain for the entire subtree. 325 326Inside a threaded subtree, threads of a process can be put in 327different cgroups and are not subject to the no internal process 328constraint - threaded controllers can be enabled on non-leaf cgroups 329whether they have threads in them or not. 330 331As the threaded domain cgroup hosts all the domain resource 332consumptions of the subtree, it is considered to have internal 333resource consumptions whether there are processes in it or not and 334can't have populated child cgroups which aren't threaded. Because the 335root cgroup is not subject to no internal process constraint, it can 336serve both as a threaded domain and a parent to domain cgroups. 337 338The current operation mode or type of the cgroup is shown in the 339"cgroup.type" file which indicates whether the cgroup is a normal 340domain, a domain which is serving as the domain of a threaded subtree, 341or a threaded cgroup. 342 343On creation, a cgroup is always a domain cgroup and can be made 344threaded by writing "threaded" to the "cgroup.type" file. The 345operation is single direction:: 346 347 # echo threaded > cgroup.type 348 349Once threaded, the cgroup can't be made a domain again. To enable the 350thread mode, the following conditions must be met. 351 352- As the cgroup will join the parent's resource domain. The parent 353 must either be a valid (threaded) domain or a threaded cgroup. 354 355- When the parent is an unthreaded domain, it must not have any domain 356 controllers enabled or populated domain children. The root is 357 exempt from this requirement. 358 359Topology-wise, a cgroup can be in an invalid state. Please consider 360the following topology:: 361 362 A (threaded domain) - B (threaded) - C (domain, just created) 363 364C is created as a domain but isn't connected to a parent which can 365host child domains. C can't be used until it is turned into a 366threaded cgroup. "cgroup.type" file will report "domain (invalid)" in 367these cases. Operations which fail due to invalid topology use 368EOPNOTSUPP as the errno. 369 370A domain cgroup is turned into a threaded domain when one of its child 371cgroup becomes threaded or threaded controllers are enabled in the 372"cgroup.subtree_control" file while there are processes in the cgroup. 373A threaded domain reverts to a normal domain when the conditions 374clear. 375 376When read, "cgroup.threads" contains the list of the thread IDs of all 377threads in the cgroup. Except that the operations are per-thread 378instead of per-process, "cgroup.threads" has the same format and 379behaves the same way as "cgroup.procs". While "cgroup.threads" can be 380written to in any cgroup, as it can only move threads inside the same 381threaded domain, its operations are confined inside each threaded 382subtree. 383 384The threaded domain cgroup serves as the resource domain for the whole 385subtree, and, while the threads can be scattered across the subtree, 386all the processes are considered to be in the threaded domain cgroup. 387"cgroup.procs" in a threaded domain cgroup contains the PIDs of all 388processes in the subtree and is not readable in the subtree proper. 389However, "cgroup.procs" can be written to from anywhere in the subtree 390to migrate all threads of the matching process to the cgroup. 391 392Only threaded controllers can be enabled in a threaded subtree. When 393a threaded controller is enabled inside a threaded subtree, it only 394accounts for and controls resource consumptions associated with the 395threads in the cgroup and its descendants. All consumptions which 396aren't tied to a specific thread belong to the threaded domain cgroup. 397 398Because a threaded subtree is exempt from no internal process 399constraint, a threaded controller must be able to handle competition 400between threads in a non-leaf cgroup and its child cgroups. Each 401threaded controller defines how such competitions are handled. 402 403Currently, the following controllers are threaded and can be enabled 404in a threaded cgroup:: 405 406- cpu 407- cpuset 408- perf_event 409- pids 410 411[Un]populated Notification 412-------------------------- 413 414Each non-root cgroup has a "cgroup.events" file which contains 415"populated" field indicating whether the cgroup's sub-hierarchy has 416live processes in it. Its value is 0 if there is no live process in 417the cgroup and its descendants; otherwise, 1. poll and [id]notify 418events are triggered when the value changes. This can be used, for 419example, to start a clean-up operation after all processes of a given 420sub-hierarchy have exited. The populated state updates and 421notifications are recursive. Consider the following sub-hierarchy 422where the numbers in the parentheses represent the numbers of processes 423in each cgroup:: 424 425 A(4) - B(0) - C(1) 426 \ D(0) 427 428A, B and C's "populated" fields would be 1 while D's 0. After the one 429process in C exits, B and C's "populated" fields would flip to "0" and 430file modified events will be generated on the "cgroup.events" files of 431both cgroups. 432 433 434Controlling Controllers 435----------------------- 436 437Enabling and Disabling 438~~~~~~~~~~~~~~~~~~~~~~ 439 440Each cgroup has a "cgroup.controllers" file which lists all 441controllers available for the cgroup to enable:: 442 443 # cat cgroup.controllers 444 cpu io memory 445 446No controller is enabled by default. Controllers can be enabled and 447disabled by writing to the "cgroup.subtree_control" file:: 448 449 # echo "+cpu +memory -io" > cgroup.subtree_control 450 451Only controllers which are listed in "cgroup.controllers" can be 452enabled. When multiple operations are specified as above, either they 453all succeed or fail. If multiple operations on the same controller 454are specified, the last one is effective. 455 456Enabling a controller in a cgroup indicates that the distribution of 457the target resource across its immediate children will be controlled. 458Consider the following sub-hierarchy. The enabled controllers are 459listed in parentheses:: 460 461 A(cpu,memory) - B(memory) - C() 462 \ D() 463 464As A has "cpu" and "memory" enabled, A will control the distribution 465of CPU cycles and memory to its children, in this case, B. As B has 466"memory" enabled but not "CPU", C and D will compete freely on CPU 467cycles but their division of memory available to B will be controlled. 468 469As a controller regulates the distribution of the target resource to 470the cgroup's children, enabling it creates the controller's interface 471files in the child cgroups. In the above example, enabling "cpu" on B 472would create the "cpu." prefixed controller interface files in C and 473D. Likewise, disabling "memory" from B would remove the "memory." 474prefixed controller interface files from C and D. This means that the 475controller interface files - anything which doesn't start with 476"cgroup." are owned by the parent rather than the cgroup itself. 477 478 479Top-down Constraint 480~~~~~~~~~~~~~~~~~~~ 481 482Resources are distributed top-down and a cgroup can further distribute 483a resource only if the resource has been distributed to it from the 484parent. This means that all non-root "cgroup.subtree_control" files 485can only contain controllers which are enabled in the parent's 486"cgroup.subtree_control" file. A controller can be enabled only if 487the parent has the controller enabled and a controller can't be 488disabled if one or more children have it enabled. 489 490 491No Internal Process Constraint 492~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 493 494Non-root cgroups can distribute domain resources to their children 495only when they don't have any processes of their own. In other words, 496only domain cgroups which don't contain any processes can have domain 497controllers enabled in their "cgroup.subtree_control" files. 498 499This guarantees that, when a domain controller is looking at the part 500of the hierarchy which has it enabled, processes are always only on 501the leaves. This rules out situations where child cgroups compete 502against internal processes of the parent. 503 504The root cgroup is exempt from this restriction. Root contains 505processes and anonymous resource consumption which can't be associated 506with any other cgroups and requires special treatment from most 507controllers. How resource consumption in the root cgroup is governed 508is up to each controller (for more information on this topic please 509refer to the Non-normative information section in the Controllers 510chapter). 511 512Note that the restriction doesn't get in the way if there is no 513enabled controller in the cgroup's "cgroup.subtree_control". This is 514important as otherwise it wouldn't be possible to create children of a 515populated cgroup. To control resource distribution of a cgroup, the 516cgroup must create children and transfer all its processes to the 517children before enabling controllers in its "cgroup.subtree_control" 518file. 519 520 521Delegation 522---------- 523 524Model of Delegation 525~~~~~~~~~~~~~~~~~~~ 526 527A cgroup can be delegated in two ways. First, to a less privileged 528user by granting write access of the directory and its "cgroup.procs", 529"cgroup.threads" and "cgroup.subtree_control" files to the user. 530Second, if the "nsdelegate" mount option is set, automatically to a 531cgroup namespace on namespace creation. 532 533Because the resource control interface files in a given directory 534control the distribution of the parent's resources, the delegatee 535shouldn't be allowed to write to them. For the first method, this is 536achieved by not granting access to these files. For the second, the 537kernel rejects writes to all files other than "cgroup.procs" and 538"cgroup.subtree_control" on a namespace root from inside the 539namespace. 540 541The end results are equivalent for both delegation types. Once 542delegated, the user can build sub-hierarchy under the directory, 543organize processes inside it as it sees fit and further distribute the 544resources it received from the parent. The limits and other settings 545of all resource controllers are hierarchical and regardless of what 546happens in the delegated sub-hierarchy, nothing can escape the 547resource restrictions imposed by the parent. 548 549Currently, cgroup doesn't impose any restrictions on the number of 550cgroups in or nesting depth of a delegated sub-hierarchy; however, 551this may be limited explicitly in the future. 552 553 554Delegation Containment 555~~~~~~~~~~~~~~~~~~~~~~ 556 557A delegated sub-hierarchy is contained in the sense that processes 558can't be moved into or out of the sub-hierarchy by the delegatee. 559 560For delegations to a less privileged user, this is achieved by 561requiring the following conditions for a process with a non-root euid 562to migrate a target process into a cgroup by writing its PID to the 563"cgroup.procs" file. 564 565- The writer must have write access to the "cgroup.procs" file. 566 567- The writer must have write access to the "cgroup.procs" file of the 568 common ancestor of the source and destination cgroups. 569 570The above two constraints ensure that while a delegatee may migrate 571processes around freely in the delegated sub-hierarchy it can't pull 572in from or push out to outside the sub-hierarchy. 573 574For an example, let's assume cgroups C0 and C1 have been delegated to 575user U0 who created C00, C01 under C0 and C10 under C1 as follows and 576all processes under C0 and C1 belong to U0:: 577 578 ~~~~~~~~~~~~~ - C0 - C00 579 ~ cgroup ~ \ C01 580 ~ hierarchy ~ 581 ~~~~~~~~~~~~~ - C1 - C10 582 583Let's also say U0 wants to write the PID of a process which is 584currently in C10 into "C00/cgroup.procs". U0 has write access to the 585file; however, the common ancestor of the source cgroup C10 and the 586destination cgroup C00 is above the points of delegation and U0 would 587not have write access to its "cgroup.procs" files and thus the write 588will be denied with -EACCES. 589 590For delegations to namespaces, containment is achieved by requiring 591that both the source and destination cgroups are reachable from the 592namespace of the process which is attempting the migration. If either 593is not reachable, the migration is rejected with -ENOENT. 594 595 596Guidelines 597---------- 598 599Organize Once and Control 600~~~~~~~~~~~~~~~~~~~~~~~~~ 601 602Migrating a process across cgroups is a relatively expensive operation 603and stateful resources such as memory are not moved together with the 604process. This is an explicit design decision as there often exist 605inherent trade-offs between migration and various hot paths in terms 606of synchronization cost. 607 608As such, migrating processes across cgroups frequently as a means to 609apply different resource restrictions is discouraged. A workload 610should be assigned to a cgroup according to the system's logical and 611resource structure once on start-up. Dynamic adjustments to resource 612distribution can be made by changing controller configuration through 613the interface files. 614 615 616Avoid Name Collisions 617~~~~~~~~~~~~~~~~~~~~~ 618 619Interface files for a cgroup and its children cgroups occupy the same 620directory and it is possible to create children cgroups which collide 621with interface files. 622 623All cgroup core interface files are prefixed with "cgroup." and each 624controller's interface files are prefixed with the controller name and 625a dot. A controller's name is composed of lower case alphabets and 626'_'s but never begins with an '_' so it can be used as the prefix 627character for collision avoidance. Also, interface file names won't 628start or end with terms which are often used in categorizing workloads 629such as job, service, slice, unit or workload. 630 631cgroup doesn't do anything to prevent name collisions and it's the 632user's responsibility to avoid them. 633 634 635Resource Distribution Models 636============================ 637 638cgroup controllers implement several resource distribution schemes 639depending on the resource type and expected use cases. This section 640describes major schemes in use along with their expected behaviors. 641 642 643Weights 644------- 645 646A parent's resource is distributed by adding up the weights of all 647active children and giving each the fraction matching the ratio of its 648weight against the sum. As only children which can make use of the 649resource at the moment participate in the distribution, this is 650work-conserving. Due to the dynamic nature, this model is usually 651used for stateless resources. 652 653All weights are in the range [1, 10000] with the default at 100. This 654allows symmetric multiplicative biases in both directions at fine 655enough granularity while staying in the intuitive range. 656 657As long as the weight is in range, all configuration combinations are 658valid and there is no reason to reject configuration changes or 659process migrations. 660 661"cpu.weight" proportionally distributes CPU cycles to active children 662and is an example of this type. 663 664 665.. _cgroupv2-limits-distributor: 666 667Limits 668------ 669 670A child can only consume up to the configured amount of the resource. 671Limits can be over-committed - the sum of the limits of children can 672exceed the amount of resource available to the parent. 673 674Limits are in the range [0, max] and defaults to "max", which is noop. 675 676As limits can be over-committed, all configuration combinations are 677valid and there is no reason to reject configuration changes or 678process migrations. 679 680"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume 681on an IO device and is an example of this type. 682 683.. _cgroupv2-protections-distributor: 684 685Protections 686----------- 687 688A cgroup is protected up to the configured amount of the resource 689as long as the usages of all its ancestors are under their 690protected levels. Protections can be hard guarantees or best effort 691soft boundaries. Protections can also be over-committed in which case 692only up to the amount available to the parent is protected among 693children. 694 695Protections are in the range [0, max] and defaults to 0, which is 696noop. 697 698As protections can be over-committed, all configuration combinations 699are valid and there is no reason to reject configuration changes or 700process migrations. 701 702"memory.low" implements best-effort memory protection and is an 703example of this type. 704 705 706Allocations 707----------- 708 709A cgroup is exclusively allocated a certain amount of a finite 710resource. Allocations can't be over-committed - the sum of the 711allocations of children can not exceed the amount of resource 712available to the parent. 713 714Allocations are in the range [0, max] and defaults to 0, which is no 715resource. 716 717As allocations can't be over-committed, some configuration 718combinations are invalid and should be rejected. Also, if the 719resource is mandatory for execution of processes, process migrations 720may be rejected. 721 722"cpu.rt.max" hard-allocates realtime slices and is an example of this 723type. 724 725 726Interface Files 727=============== 728 729Format 730------ 731 732All interface files should be in one of the following formats whenever 733possible:: 734 735 New-line separated values 736 (when only one value can be written at once) 737 738 VAL0\n 739 VAL1\n 740 ... 741 742 Space separated values 743 (when read-only or multiple values can be written at once) 744 745 VAL0 VAL1 ...\n 746 747 Flat keyed 748 749 KEY0 VAL0\n 750 KEY1 VAL1\n 751 ... 752 753 Nested keyed 754 755 KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01... 756 KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11... 757 ... 758 759For a writable file, the format for writing should generally match 760reading; however, controllers may allow omitting later fields or 761implement restricted shortcuts for most common use cases. 762 763For both flat and nested keyed files, only the values for a single key 764can be written at a time. For nested keyed files, the sub key pairs 765may be specified in any order and not all pairs have to be specified. 766 767 768Conventions 769----------- 770 771- Settings for a single feature should be contained in a single file. 772 773- The root cgroup should be exempt from resource control and thus 774 shouldn't have resource control interface files. 775 776- The default time unit is microseconds. If a different unit is ever 777 used, an explicit unit suffix must be present. 778 779- A parts-per quantity should use a percentage decimal with at least 780 two digit fractional part - e.g. 13.40. 781 782- If a controller implements weight based resource distribution, its 783 interface file should be named "weight" and have the range [1, 784 10000] with 100 as the default. The values are chosen to allow 785 enough and symmetric bias in both directions while keeping it 786 intuitive (the default is 100%). 787 788- If a controller implements an absolute resource guarantee and/or 789 limit, the interface files should be named "min" and "max" 790 respectively. If a controller implements best effort resource 791 guarantee and/or limit, the interface files should be named "low" 792 and "high" respectively. 793 794 In the above four control files, the special token "max" should be 795 used to represent upward infinity for both reading and writing. 796 797- If a setting has a configurable default value and keyed specific 798 overrides, the default entry should be keyed with "default" and 799 appear as the first entry in the file. 800 801 The default value can be updated by writing either "default $VAL" or 802 "$VAL". 803 804 When writing to update a specific override, "default" can be used as 805 the value to indicate removal of the override. Override entries 806 with "default" as the value must not appear when read. 807 808 For example, a setting which is keyed by major:minor device numbers 809 with integer values may look like the following:: 810 811 # cat cgroup-example-interface-file 812 default 150 813 8:0 300 814 815 The default value can be updated by:: 816 817 # echo 125 > cgroup-example-interface-file 818 819 or:: 820 821 # echo "default 125" > cgroup-example-interface-file 822 823 An override can be set by:: 824 825 # echo "8:16 170" > cgroup-example-interface-file 826 827 and cleared by:: 828 829 # echo "8:0 default" > cgroup-example-interface-file 830 # cat cgroup-example-interface-file 831 default 125 832 8:16 170 833 834- For events which are not very high frequency, an interface file 835 "events" should be created which lists event key value pairs. 836 Whenever a notifiable event happens, file modified event should be 837 generated on the file. 838 839 840Core Interface Files 841-------------------- 842 843All cgroup core files are prefixed with "cgroup." 844 845 cgroup.type 846 A read-write single value file which exists on non-root 847 cgroups. 848 849 When read, it indicates the current type of the cgroup, which 850 can be one of the following values. 851 852 - "domain" : A normal valid domain cgroup. 853 854 - "domain threaded" : A threaded domain cgroup which is 855 serving as the root of a threaded subtree. 856 857 - "domain invalid" : A cgroup which is in an invalid state. 858 It can't be populated or have controllers enabled. It may 859 be allowed to become a threaded cgroup. 860 861 - "threaded" : A threaded cgroup which is a member of a 862 threaded subtree. 863 864 A cgroup can be turned into a threaded cgroup by writing 865 "threaded" to this file. 866 867 cgroup.procs 868 A read-write new-line separated values file which exists on 869 all cgroups. 870 871 When read, it lists the PIDs of all processes which belong to 872 the cgroup one-per-line. The PIDs are not ordered and the 873 same PID may show up more than once if the process got moved 874 to another cgroup and then back or the PID got recycled while 875 reading. 876 877 A PID can be written to migrate the process associated with 878 the PID to the cgroup. The writer should match all of the 879 following conditions. 880 881 - It must have write access to the "cgroup.procs" file. 882 883 - It must have write access to the "cgroup.procs" file of the 884 common ancestor of the source and destination cgroups. 885 886 When delegating a sub-hierarchy, write access to this file 887 should be granted along with the containing directory. 888 889 In a threaded cgroup, reading this file fails with EOPNOTSUPP 890 as all the processes belong to the thread root. Writing is 891 supported and moves every thread of the process to the cgroup. 892 893 cgroup.threads 894 A read-write new-line separated values file which exists on 895 all cgroups. 896 897 When read, it lists the TIDs of all threads which belong to 898 the cgroup one-per-line. The TIDs are not ordered and the 899 same TID may show up more than once if the thread got moved to 900 another cgroup and then back or the TID got recycled while 901 reading. 902 903 A TID can be written to migrate the thread associated with the 904 TID to the cgroup. The writer should match all of the 905 following conditions. 906 907 - It must have write access to the "cgroup.threads" file. 908 909 - The cgroup that the thread is currently in must be in the 910 same resource domain as the destination cgroup. 911 912 - It must have write access to the "cgroup.procs" file of the 913 common ancestor of the source and destination cgroups. 914 915 When delegating a sub-hierarchy, write access to this file 916 should be granted along with the containing directory. 917 918 cgroup.controllers 919 A read-only space separated values file which exists on all 920 cgroups. 921 922 It shows space separated list of all controllers available to 923 the cgroup. The controllers are not ordered. 924 925 cgroup.subtree_control 926 A read-write space separated values file which exists on all 927 cgroups. Starts out empty. 928 929 When read, it shows space separated list of the controllers 930 which are enabled to control resource distribution from the 931 cgroup to its children. 932 933 Space separated list of controllers prefixed with '+' or '-' 934 can be written to enable or disable controllers. A controller 935 name prefixed with '+' enables the controller and '-' 936 disables. If a controller appears more than once on the list, 937 the last one is effective. When multiple enable and disable 938 operations are specified, either all succeed or all fail. 939 940 cgroup.events 941 A read-only flat-keyed file which exists on non-root cgroups. 942 The following entries are defined. Unless specified 943 otherwise, a value change in this file generates a file 944 modified event. 945 946 populated 947 1 if the cgroup or its descendants contains any live 948 processes; otherwise, 0. 949 frozen 950 1 if the cgroup is frozen; otherwise, 0. 951 952 cgroup.max.descendants 953 A read-write single value files. The default is "max". 954 955 Maximum allowed number of descent cgroups. 956 If the actual number of descendants is equal or larger, 957 an attempt to create a new cgroup in the hierarchy will fail. 958 959 cgroup.max.depth 960 A read-write single value files. The default is "max". 961 962 Maximum allowed descent depth below the current cgroup. 963 If the actual descent depth is equal or larger, 964 an attempt to create a new child cgroup will fail. 965 966 cgroup.stat 967 A read-only flat-keyed file with the following entries: 968 969 nr_descendants 970 Total number of visible descendant cgroups. 971 972 nr_dying_descendants 973 Total number of dying descendant cgroups. A cgroup becomes 974 dying after being deleted by a user. The cgroup will remain 975 in dying state for some time undefined time (which can depend 976 on system load) before being completely destroyed. 977 978 A process can't enter a dying cgroup under any circumstances, 979 a dying cgroup can't revive. 980 981 A dying cgroup can consume system resources not exceeding 982 limits, which were active at the moment of cgroup deletion. 983 984 cgroup.freeze 985 A read-write single value file which exists on non-root cgroups. 986 Allowed values are "0" and "1". The default is "0". 987 988 Writing "1" to the file causes freezing of the cgroup and all 989 descendant cgroups. This means that all belonging processes will 990 be stopped and will not run until the cgroup will be explicitly 991 unfrozen. Freezing of the cgroup may take some time; when this action 992 is completed, the "frozen" value in the cgroup.events control file 993 will be updated to "1" and the corresponding notification will be 994 issued. 995 996 A cgroup can be frozen either by its own settings, or by settings 997 of any ancestor cgroups. If any of ancestor cgroups is frozen, the 998 cgroup will remain frozen. 999 1000 Processes in the frozen cgroup can be killed by a fatal signal. 1001 They also can enter and leave a frozen cgroup: either by an explicit 1002 move by a user, or if freezing of the cgroup races with fork(). 1003 If a process is moved to a frozen cgroup, it stops. If a process is 1004 moved out of a frozen cgroup, it becomes running. 1005 1006 Frozen status of a cgroup doesn't affect any cgroup tree operations: 1007 it's possible to delete a frozen (and empty) cgroup, as well as 1008 create new sub-cgroups. 1009 1010 cgroup.kill 1011 A write-only single value file which exists in non-root cgroups. 1012 The only allowed value is "1". 1013 1014 Writing "1" to the file causes the cgroup and all descendant cgroups to 1015 be killed. This means that all processes located in the affected cgroup 1016 tree will be killed via SIGKILL. 1017 1018 Killing a cgroup tree will deal with concurrent forks appropriately and 1019 is protected against migrations. 1020 1021 In a threaded cgroup, writing this file fails with EOPNOTSUPP as 1022 killing cgroups is a process directed operation, i.e. it affects 1023 the whole thread-group. 1024 1025 cgroup.pressure 1026 A read-write single value file that allowed values are "0" and "1". 1027 The default is "1". 1028 1029 Writing "0" to the file will disable the cgroup PSI accounting. 1030 Writing "1" to the file will re-enable the cgroup PSI accounting. 1031 1032 This control attribute is not hierarchical, so disable or enable PSI 1033 accounting in a cgroup does not affect PSI accounting in descendants 1034 and doesn't need pass enablement via ancestors from root. 1035 1036 The reason this control attribute exists is that PSI accounts stalls for 1037 each cgroup separately and aggregates it at each level of the hierarchy. 1038 This may cause non-negligible overhead for some workloads when under 1039 deep level of the hierarchy, in which case this control attribute can 1040 be used to disable PSI accounting in the non-leaf cgroups. 1041 1042 irq.pressure 1043 A read-write nested-keyed file. 1044 1045 Shows pressure stall information for IRQ/SOFTIRQ. See 1046 :ref:`Documentation/accounting/psi.rst <psi>` for details. 1047 1048Controllers 1049=========== 1050 1051.. _cgroup-v2-cpu: 1052 1053CPU 1054--- 1055 1056The "cpu" controllers regulates distribution of CPU cycles. This 1057controller implements weight and absolute bandwidth limit models for 1058normal scheduling policy and absolute bandwidth allocation model for 1059realtime scheduling policy. 1060 1061In all the above models, cycles distribution is defined only on a temporal 1062base and it does not account for the frequency at which tasks are executed. 1063The (optional) utilization clamping support allows to hint the schedutil 1064cpufreq governor about the minimum desired frequency which should always be 1065provided by a CPU, as well as the maximum desired frequency, which should not 1066be exceeded by a CPU. 1067 1068WARNING: cgroup2 doesn't yet support control of realtime processes. For 1069a kernel built with the CONFIG_RT_GROUP_SCHED option enabled for group 1070scheduling of realtime processes, the cpu controller can only be enabled 1071when all RT processes are in the root cgroup. This limitation does 1072not apply if CONFIG_RT_GROUP_SCHED is disabled. Be aware that system 1073management software may already have placed RT processes into nonroot 1074cgroups during the system boot process, and these processes may need 1075to be moved to the root cgroup before the cpu controller can be enabled 1076with a CONFIG_RT_GROUP_SCHED enabled kernel. 1077 1078 1079CPU Interface Files 1080~~~~~~~~~~~~~~~~~~~ 1081 1082All time durations are in microseconds. 1083 1084 cpu.stat 1085 A read-only flat-keyed file. 1086 This file exists whether the controller is enabled or not. 1087 1088 It always reports the following three stats: 1089 1090 - usage_usec 1091 - user_usec 1092 - system_usec 1093 1094 and the following five when the controller is enabled: 1095 1096 - nr_periods 1097 - nr_throttled 1098 - throttled_usec 1099 - nr_bursts 1100 - burst_usec 1101 1102 cpu.weight 1103 A read-write single value file which exists on non-root 1104 cgroups. The default is "100". 1105 1106 For non idle groups (cpu.idle = 0), the weight is in the 1107 range [1, 10000]. 1108 1109 If the cgroup has been configured to be SCHED_IDLE (cpu.idle = 1), 1110 then the weight will show as a 0. 1111 1112 cpu.weight.nice 1113 A read-write single value file which exists on non-root 1114 cgroups. The default is "0". 1115 1116 The nice value is in the range [-20, 19]. 1117 1118 This interface file is an alternative interface for 1119 "cpu.weight" and allows reading and setting weight using the 1120 same values used by nice(2). Because the range is smaller and 1121 granularity is coarser for the nice values, the read value is 1122 the closest approximation of the current weight. 1123 1124 cpu.max 1125 A read-write two value file which exists on non-root cgroups. 1126 The default is "max 100000". 1127 1128 The maximum bandwidth limit. It's in the following format:: 1129 1130 $MAX $PERIOD 1131 1132 which indicates that the group may consume up to $MAX in each 1133 $PERIOD duration. "max" for $MAX indicates no limit. If only 1134 one number is written, $MAX is updated. 1135 1136 cpu.max.burst 1137 A read-write single value file which exists on non-root 1138 cgroups. The default is "0". 1139 1140 The burst in the range [0, $MAX]. 1141 1142 cpu.pressure 1143 A read-write nested-keyed file. 1144 1145 Shows pressure stall information for CPU. See 1146 :ref:`Documentation/accounting/psi.rst <psi>` for details. 1147 1148 cpu.uclamp.min 1149 A read-write single value file which exists on non-root cgroups. 1150 The default is "0", i.e. no utilization boosting. 1151 1152 The requested minimum utilization (protection) as a percentage 1153 rational number, e.g. 12.34 for 12.34%. 1154 1155 This interface allows reading and setting minimum utilization clamp 1156 values similar to the sched_setattr(2). This minimum utilization 1157 value is used to clamp the task specific minimum utilization clamp. 1158 1159 The requested minimum utilization (protection) is always capped by 1160 the current value for the maximum utilization (limit), i.e. 1161 `cpu.uclamp.max`. 1162 1163 cpu.uclamp.max 1164 A read-write single value file which exists on non-root cgroups. 1165 The default is "max". i.e. no utilization capping 1166 1167 The requested maximum utilization (limit) as a percentage rational 1168 number, e.g. 98.76 for 98.76%. 1169 1170 This interface allows reading and setting maximum utilization clamp 1171 values similar to the sched_setattr(2). This maximum utilization 1172 value is used to clamp the task specific maximum utilization clamp. 1173 1174 cpu.idle 1175 A read-write single value file which exists on non-root cgroups. 1176 The default is 0. 1177 1178 This is the cgroup analog of the per-task SCHED_IDLE sched policy. 1179 Setting this value to a 1 will make the scheduling policy of the 1180 cgroup SCHED_IDLE. The threads inside the cgroup will retain their 1181 own relative priorities, but the cgroup itself will be treated as 1182 very low priority relative to its peers. 1183 1184 1185 1186Memory 1187------ 1188 1189The "memory" controller regulates distribution of memory. Memory is 1190stateful and implements both limit and protection models. Due to the 1191intertwining between memory usage and reclaim pressure and the 1192stateful nature of memory, the distribution model is relatively 1193complex. 1194 1195While not completely water-tight, all major memory usages by a given 1196cgroup are tracked so that the total memory consumption can be 1197accounted and controlled to a reasonable extent. Currently, the 1198following types of memory usages are tracked. 1199 1200- Userland memory - page cache and anonymous memory. 1201 1202- Kernel data structures such as dentries and inodes. 1203 1204- TCP socket buffers. 1205 1206The above list may expand in the future for better coverage. 1207 1208 1209Memory Interface Files 1210~~~~~~~~~~~~~~~~~~~~~~ 1211 1212All memory amounts are in bytes. If a value which is not aligned to 1213PAGE_SIZE is written, the value may be rounded up to the closest 1214PAGE_SIZE multiple when read back. 1215 1216 memory.current 1217 A read-only single value file which exists on non-root 1218 cgroups. 1219 1220 The total amount of memory currently being used by the cgroup 1221 and its descendants. 1222 1223 memory.min 1224 A read-write single value file which exists on non-root 1225 cgroups. The default is "0". 1226 1227 Hard memory protection. If the memory usage of a cgroup 1228 is within its effective min boundary, the cgroup's memory 1229 won't be reclaimed under any conditions. If there is no 1230 unprotected reclaimable memory available, OOM killer 1231 is invoked. Above the effective min boundary (or 1232 effective low boundary if it is higher), pages are reclaimed 1233 proportionally to the overage, reducing reclaim pressure for 1234 smaller overages. 1235 1236 Effective min boundary is limited by memory.min values of 1237 all ancestor cgroups. If there is memory.min overcommitment 1238 (child cgroup or cgroups are requiring more protected memory 1239 than parent will allow), then each child cgroup will get 1240 the part of parent's protection proportional to its 1241 actual memory usage below memory.min. 1242 1243 Putting more memory than generally available under this 1244 protection is discouraged and may lead to constant OOMs. 1245 1246 If a memory cgroup is not populated with processes, 1247 its memory.min is ignored. 1248 1249 memory.low 1250 A read-write single value file which exists on non-root 1251 cgroups. The default is "0". 1252 1253 Best-effort memory protection. If the memory usage of a 1254 cgroup is within its effective low boundary, the cgroup's 1255 memory won't be reclaimed unless there is no reclaimable 1256 memory available in unprotected cgroups. 1257 Above the effective low boundary (or 1258 effective min boundary if it is higher), pages are reclaimed 1259 proportionally to the overage, reducing reclaim pressure for 1260 smaller overages. 1261 1262 Effective low boundary is limited by memory.low values of 1263 all ancestor cgroups. If there is memory.low overcommitment 1264 (child cgroup or cgroups are requiring more protected memory 1265 than parent will allow), then each child cgroup will get 1266 the part of parent's protection proportional to its 1267 actual memory usage below memory.low. 1268 1269 Putting more memory than generally available under this 1270 protection is discouraged. 1271 1272 memory.high 1273 A read-write single value file which exists on non-root 1274 cgroups. The default is "max". 1275 1276 Memory usage throttle limit. If a cgroup's usage goes 1277 over the high boundary, the processes of the cgroup are 1278 throttled and put under heavy reclaim pressure. 1279 1280 Going over the high limit never invokes the OOM killer and 1281 under extreme conditions the limit may be breached. The high 1282 limit should be used in scenarios where an external process 1283 monitors the limited cgroup to alleviate heavy reclaim 1284 pressure. 1285 1286 memory.max 1287 A read-write single value file which exists on non-root 1288 cgroups. The default is "max". 1289 1290 Memory usage hard limit. This is the main mechanism to limit 1291 memory usage of a cgroup. If a cgroup's memory usage reaches 1292 this limit and can't be reduced, the OOM killer is invoked in 1293 the cgroup. Under certain circumstances, the usage may go 1294 over the limit temporarily. 1295 1296 In default configuration regular 0-order allocations always 1297 succeed unless OOM killer chooses current task as a victim. 1298 1299 Some kinds of allocations don't invoke the OOM killer. 1300 Caller could retry them differently, return into userspace 1301 as -ENOMEM or silently ignore in cases like disk readahead. 1302 1303 memory.reclaim 1304 A write-only nested-keyed file which exists for all cgroups. 1305 1306 This is a simple interface to trigger memory reclaim in the 1307 target cgroup. 1308 1309 Example:: 1310 1311 echo "1G" > memory.reclaim 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 1324The following nested keys are defined. 1325 1326 ========== ================================ 1327 swappiness Swappiness value to reclaim with 1328 ========== ================================ 1329 1330 Specifying a swappiness value instructs the kernel to perform 1331 the reclaim with that swappiness value. Note that this has the 1332 same semantics as vm.swappiness applied to memcg reclaim with 1333 all the existing limitations and potential future extensions. 1334 1335 memory.peak 1336 A read-write single value file which exists on non-root cgroups. 1337 1338 The max memory usage recorded for the cgroup and its descendants since 1339 either the creation of the cgroup or the most recent reset for that FD. 1340 1341 A write of any non-empty string to this file resets it to the 1342 current memory usage for subsequent reads through the same 1343 file descriptor. 1344 1345 memory.oom.group 1346 A read-write single value file which exists on non-root 1347 cgroups. The default value is "0". 1348 1349 Determines whether the cgroup should be treated as 1350 an indivisible workload by the OOM killer. If set, 1351 all tasks belonging to the cgroup or to its descendants 1352 (if the memory cgroup is not a leaf cgroup) are killed 1353 together or not at all. This can be used to avoid 1354 partial kills to guarantee workload integrity. 1355 1356 Tasks with the OOM protection (oom_score_adj set to -1000) 1357 are treated as an exception and are never killed. 1358 1359 If the OOM killer is invoked in a cgroup, it's not going 1360 to kill any tasks outside of this cgroup, regardless 1361 memory.oom.group values of ancestor cgroups. 1362 1363 memory.events 1364 A read-only flat-keyed file which exists on non-root cgroups. 1365 The following entries are defined. Unless specified 1366 otherwise, a value change in this file generates a file 1367 modified event. 1368 1369 Note that all fields in this file are hierarchical and the 1370 file modified event can be generated due to an event down the 1371 hierarchy. For the local events at the cgroup level see 1372 memory.events.local. 1373 1374 low 1375 The number of times the cgroup is reclaimed due to 1376 high memory pressure even though its usage is under 1377 the low boundary. This usually indicates that the low 1378 boundary is over-committed. 1379 1380 high 1381 The number of times processes of the cgroup are 1382 throttled and routed to perform direct memory reclaim 1383 because the high memory boundary was exceeded. For a 1384 cgroup whose memory usage is capped by the high limit 1385 rather than global memory pressure, this event's 1386 occurrences are expected. 1387 1388 max 1389 The number of times the cgroup's memory usage was 1390 about to go over the max boundary. If direct reclaim 1391 fails to bring it down, the cgroup goes to OOM state. 1392 1393 oom 1394 The number of time the cgroup's memory usage was 1395 reached the limit and allocation was about to fail. 1396 1397 This event is not raised if the OOM killer is not 1398 considered as an option, e.g. for failed high-order 1399 allocations or if caller asked to not retry attempts. 1400 1401 oom_kill 1402 The number of processes belonging to this cgroup 1403 killed by any kind of OOM killer. 1404 1405 oom_group_kill 1406 The number of times a group OOM has occurred. 1407 1408 memory.events.local 1409 Similar to memory.events but the fields in the file are local 1410 to the cgroup i.e. not hierarchical. The file modified event 1411 generated on this file reflects only the local events. 1412 1413 memory.stat 1414 A read-only flat-keyed file which exists on non-root cgroups. 1415 1416 This breaks down the cgroup's memory footprint into different 1417 types of memory, type-specific details, and other information 1418 on the state and past events of the memory management system. 1419 1420 All memory amounts are in bytes. 1421 1422 The entries are ordered to be human readable, and new entries 1423 can show up in the middle. Don't rely on items remaining in a 1424 fixed position; use the keys to look up specific values! 1425 1426 If the entry has no per-node counter (or not show in the 1427 memory.numa_stat). We use 'npn' (non-per-node) as the tag 1428 to indicate that it will not show in the memory.numa_stat. 1429 1430 anon 1431 Amount of memory used in anonymous mappings such as 1432 brk(), sbrk(), and mmap(MAP_ANONYMOUS) 1433 1434 file 1435 Amount of memory used to cache filesystem data, 1436 including tmpfs and shared memory. 1437 1438 kernel (npn) 1439 Amount of total kernel memory, including 1440 (kernel_stack, pagetables, percpu, vmalloc, slab) in 1441 addition to other kernel memory use cases. 1442 1443 kernel_stack 1444 Amount of memory allocated to kernel stacks. 1445 1446 pagetables 1447 Amount of memory allocated for page tables. 1448 1449 sec_pagetables 1450 Amount of memory allocated for secondary page tables, 1451 this currently includes KVM mmu allocations on x86 1452 and arm64 and IOMMU page tables. 1453 1454 percpu (npn) 1455 Amount of memory used for storing per-cpu kernel 1456 data structures. 1457 1458 sock (npn) 1459 Amount of memory used in network transmission buffers 1460 1461 vmalloc (npn) 1462 Amount of memory used for vmap backed memory. 1463 1464 shmem 1465 Amount of cached filesystem data that is swap-backed, 1466 such as tmpfs, shm segments, shared anonymous mmap()s 1467 1468 zswap 1469 Amount of memory consumed by the zswap compression backend. 1470 1471 zswapped 1472 Amount of application memory swapped out to zswap. 1473 1474 file_mapped 1475 Amount of cached filesystem data mapped with mmap() 1476 1477 file_dirty 1478 Amount of cached filesystem data that was modified but 1479 not yet written back to disk 1480 1481 file_writeback 1482 Amount of cached filesystem data that was modified and 1483 is currently being written back to disk 1484 1485 swapcached 1486 Amount of swap cached in memory. The swapcache is accounted 1487 against both memory and swap usage. 1488 1489 anon_thp 1490 Amount of memory used in anonymous mappings backed by 1491 transparent hugepages 1492 1493 file_thp 1494 Amount of cached filesystem data backed by transparent 1495 hugepages 1496 1497 shmem_thp 1498 Amount of shm, tmpfs, shared anonymous mmap()s backed by 1499 transparent hugepages 1500 1501 inactive_anon, active_anon, inactive_file, active_file, unevictable 1502 Amount of memory, swap-backed and filesystem-backed, 1503 on the internal memory management lists used by the 1504 page reclaim algorithm. 1505 1506 As these represent internal list state (eg. shmem pages are on anon 1507 memory management lists), inactive_foo + active_foo may not be equal to 1508 the value for the foo counter, since the foo counter is type-based, not 1509 list-based. 1510 1511 slab_reclaimable 1512 Part of "slab" that might be reclaimed, such as 1513 dentries and inodes. 1514 1515 slab_unreclaimable 1516 Part of "slab" that cannot be reclaimed on memory 1517 pressure. 1518 1519 slab (npn) 1520 Amount of memory used for storing in-kernel data 1521 structures. 1522 1523 workingset_refault_anon 1524 Number of refaults of previously evicted anonymous pages. 1525 1526 workingset_refault_file 1527 Number of refaults of previously evicted file pages. 1528 1529 workingset_activate_anon 1530 Number of refaulted anonymous pages that were immediately 1531 activated. 1532 1533 workingset_activate_file 1534 Number of refaulted file pages that were immediately activated. 1535 1536 workingset_restore_anon 1537 Number of restored anonymous pages which have been detected as 1538 an active workingset before they got reclaimed. 1539 1540 workingset_restore_file 1541 Number of restored file pages which have been detected as an 1542 active workingset before they got reclaimed. 1543 1544 workingset_nodereclaim 1545 Number of times a shadow node has been reclaimed 1546 1547 pgscan (npn) 1548 Amount of scanned pages (in an inactive LRU list) 1549 1550 pgsteal (npn) 1551 Amount of reclaimed pages 1552 1553 pgscan_kswapd (npn) 1554 Amount of scanned pages by kswapd (in an inactive LRU list) 1555 1556 pgscan_direct (npn) 1557 Amount of scanned pages directly (in an inactive LRU list) 1558 1559 pgscan_khugepaged (npn) 1560 Amount of scanned pages by khugepaged (in an inactive LRU list) 1561 1562 pgsteal_kswapd (npn) 1563 Amount of reclaimed pages by kswapd 1564 1565 pgsteal_direct (npn) 1566 Amount of reclaimed pages directly 1567 1568 pgsteal_khugepaged (npn) 1569 Amount of reclaimed pages by khugepaged 1570 1571 pgfault (npn) 1572 Total number of page faults incurred 1573 1574 pgmajfault (npn) 1575 Number of major page faults incurred 1576 1577 pgrefill (npn) 1578 Amount of scanned pages (in an active LRU list) 1579 1580 pgactivate (npn) 1581 Amount of pages moved to the active LRU list 1582 1583 pgdeactivate (npn) 1584 Amount of pages moved to the inactive LRU list 1585 1586 pglazyfree (npn) 1587 Amount of pages postponed to be freed under memory pressure 1588 1589 pglazyfreed (npn) 1590 Amount of reclaimed lazyfree pages 1591 1592 zswpin 1593 Number of pages moved in to memory from zswap. 1594 1595 zswpout 1596 Number of pages moved out of memory to zswap. 1597 1598 zswpwb 1599 Number of pages written from zswap to swap. 1600 1601 thp_fault_alloc (npn) 1602 Number of transparent hugepages which were allocated to satisfy 1603 a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE 1604 is not set. 1605 1606 thp_collapse_alloc (npn) 1607 Number of transparent hugepages which were allocated to allow 1608 collapsing an existing range of pages. This counter is not 1609 present when CONFIG_TRANSPARENT_HUGEPAGE is not set. 1610 1611 thp_swpout (npn) 1612 Number of transparent hugepages which are swapout in one piece 1613 without splitting. 1614 1615 thp_swpout_fallback (npn) 1616 Number of transparent hugepages which were split before swapout. 1617 Usually because failed to allocate some continuous swap space 1618 for the huge page. 1619 1620 numa_pages_migrated (npn) 1621 Number of pages migrated by NUMA balancing. 1622 1623 numa_pte_updates (npn) 1624 Number of pages whose page table entries are modified by 1625 NUMA balancing to produce NUMA hinting faults on access. 1626 1627 numa_hint_faults (npn) 1628 Number of NUMA hinting faults. 1629 1630 pgdemote_kswapd 1631 Number of pages demoted by kswapd. 1632 1633 pgdemote_direct 1634 Number of pages demoted directly. 1635 1636 pgdemote_khugepaged 1637 Number of pages demoted by khugepaged. 1638 1639 memory.numa_stat 1640 A read-only nested-keyed file which exists on non-root cgroups. 1641 1642 This breaks down the cgroup's memory footprint into different 1643 types of memory, type-specific details, and other information 1644 per node on the state of the memory management system. 1645 1646 This is useful for providing visibility into the NUMA locality 1647 information within an memcg since the pages are allowed to be 1648 allocated from any physical node. One of the use case is evaluating 1649 application performance by combining this information with the 1650 application's CPU allocation. 1651 1652 All memory amounts are in bytes. 1653 1654 The output format of memory.numa_stat is:: 1655 1656 type N0=<bytes in node 0> N1=<bytes in node 1> ... 1657 1658 The entries are ordered to be human readable, and new entries 1659 can show up in the middle. Don't rely on items remaining in a 1660 fixed position; use the keys to look up specific values! 1661 1662 The entries can refer to the memory.stat. 1663 1664 memory.swap.current 1665 A read-only single value file which exists on non-root 1666 cgroups. 1667 1668 The total amount of swap currently being used by the cgroup 1669 and its descendants. 1670 1671 memory.swap.high 1672 A read-write single value file which exists on non-root 1673 cgroups. The default is "max". 1674 1675 Swap usage throttle limit. If a cgroup's swap usage exceeds 1676 this limit, all its further allocations will be throttled to 1677 allow userspace to implement custom out-of-memory procedures. 1678 1679 This limit marks a point of no return for the cgroup. It is NOT 1680 designed to manage the amount of swapping a workload does 1681 during regular operation. Compare to memory.swap.max, which 1682 prohibits swapping past a set amount, but lets the cgroup 1683 continue unimpeded as long as other memory can be reclaimed. 1684 1685 Healthy workloads are not expected to reach this limit. 1686 1687 memory.swap.peak 1688 A read-write single value file which exists on non-root cgroups. 1689 1690 The max swap usage recorded for the cgroup and its descendants since 1691 the creation of the cgroup or the most recent reset for that FD. 1692 1693 A write of any non-empty string to this file resets it to the 1694 current memory usage for subsequent reads through the same 1695 file descriptor. 1696 1697 memory.swap.max 1698 A read-write single value file which exists on non-root 1699 cgroups. The default is "max". 1700 1701 Swap usage hard limit. If a cgroup's swap usage reaches this 1702 limit, anonymous memory of the cgroup will not be swapped out. 1703 1704 memory.swap.events 1705 A read-only flat-keyed file which exists on non-root cgroups. 1706 The following entries are defined. Unless specified 1707 otherwise, a value change in this file generates a file 1708 modified event. 1709 1710 high 1711 The number of times the cgroup's swap usage was over 1712 the high threshold. 1713 1714 max 1715 The number of times the cgroup's swap usage was about 1716 to go over the max boundary and swap allocation 1717 failed. 1718 1719 fail 1720 The number of times swap allocation failed either 1721 because of running out of swap system-wide or max 1722 limit. 1723 1724 When reduced under the current usage, the existing swap 1725 entries are reclaimed gradually and the swap usage may stay 1726 higher than the limit for an extended period of time. This 1727 reduces the impact on the workload and memory management. 1728 1729 memory.zswap.current 1730 A read-only single value file which exists on non-root 1731 cgroups. 1732 1733 The total amount of memory consumed by the zswap compression 1734 backend. 1735 1736 memory.zswap.max 1737 A read-write single value file which exists on non-root 1738 cgroups. The default is "max". 1739 1740 Zswap usage hard limit. If a cgroup's zswap pool reaches this 1741 limit, it will refuse to take any more stores before existing 1742 entries fault back in or are written out to disk. 1743 1744 memory.zswap.writeback 1745 A read-write single value file. The default value is "1". The 1746 initial value of the root cgroup is 1, and when a new cgroup is 1747 created, it inherits the current value of its parent. 1748 1749 When this is set to 0, all swapping attempts to swapping devices 1750 are disabled. This included both zswap writebacks, and swapping due 1751 to zswap store failures. If the zswap store failures are recurring 1752 (for e.g if the pages are incompressible), users can observe 1753 reclaim inefficiency after disabling writeback (because the same 1754 pages might be rejected again and again). 1755 1756 Note that this is subtly different from setting memory.swap.max to 1757 0, as it still allows for pages to be written to the zswap pool. 1758 This setting has no effect if zswap is disabled, and swapping 1759 is allowed unless memory.swap.max is set to 0. 1760 1761 memory.pressure 1762 A read-only nested-keyed file. 1763 1764 Shows pressure stall information for memory. See 1765 :ref:`Documentation/accounting/psi.rst <psi>` for details. 1766 1767 1768Usage Guidelines 1769~~~~~~~~~~~~~~~~ 1770 1771"memory.high" is the main mechanism to control memory usage. 1772Over-committing on high limit (sum of high limits > available memory) 1773and letting global memory pressure to distribute memory according to 1774usage is a viable strategy. 1775 1776Because breach of the high limit doesn't trigger the OOM killer but 1777throttles the offending cgroup, a management agent has ample 1778opportunities to monitor and take appropriate actions such as granting 1779more memory or terminating the workload. 1780 1781Determining whether a cgroup has enough memory is not trivial as 1782memory usage doesn't indicate whether the workload can benefit from 1783more memory. For example, a workload which writes data received from 1784network to a file can use all available memory but can also operate as 1785performant with a small amount of memory. A measure of memory 1786pressure - how much the workload is being impacted due to lack of 1787memory - is necessary to determine whether a workload needs more 1788memory; unfortunately, memory pressure monitoring mechanism isn't 1789implemented yet. 1790 1791 1792Memory Ownership 1793~~~~~~~~~~~~~~~~ 1794 1795A memory area is charged to the cgroup which instantiated it and stays 1796charged to the cgroup until the area is released. Migrating a process 1797to a different cgroup doesn't move the memory usages that it 1798instantiated while in the previous cgroup to the new cgroup. 1799 1800A memory area may be used by processes belonging to different cgroups. 1801To which cgroup the area will be charged is in-deterministic; however, 1802over time, the memory area is likely to end up in a cgroup which has 1803enough memory allowance to avoid high reclaim pressure. 1804 1805If a cgroup sweeps a considerable amount of memory which is expected 1806to be accessed repeatedly by other cgroups, it may make sense to use 1807POSIX_FADV_DONTNEED to relinquish the ownership of memory areas 1808belonging to the affected files to ensure correct memory ownership. 1809 1810 1811IO 1812-- 1813 1814The "io" controller regulates the distribution of IO resources. This 1815controller implements both weight based and absolute bandwidth or IOPS 1816limit distribution; however, weight based distribution is available 1817only if cfq-iosched is in use and neither scheme is available for 1818blk-mq devices. 1819 1820 1821IO Interface Files 1822~~~~~~~~~~~~~~~~~~ 1823 1824 io.stat 1825 A read-only nested-keyed file. 1826 1827 Lines are keyed by $MAJ:$MIN device numbers and not ordered. 1828 The following nested keys are defined. 1829 1830 ====== ===================== 1831 rbytes Bytes read 1832 wbytes Bytes written 1833 rios Number of read IOs 1834 wios Number of write IOs 1835 dbytes Bytes discarded 1836 dios Number of discard IOs 1837 ====== ===================== 1838 1839 An example read output follows:: 1840 1841 8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0 1842 8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021 1843 1844 io.cost.qos 1845 A read-write nested-keyed file which exists only on the root 1846 cgroup. 1847 1848 This file configures the Quality of Service of the IO cost 1849 model based controller (CONFIG_BLK_CGROUP_IOCOST) which 1850 currently implements "io.weight" proportional control. Lines 1851 are keyed by $MAJ:$MIN device numbers and not ordered. The 1852 line for a given device is populated on the first write for 1853 the device on "io.cost.qos" or "io.cost.model". The following 1854 nested keys are defined. 1855 1856 ====== ===================================== 1857 enable Weight-based control enable 1858 ctrl "auto" or "user" 1859 rpct Read latency percentile [0, 100] 1860 rlat Read latency threshold 1861 wpct Write latency percentile [0, 100] 1862 wlat Write latency threshold 1863 min Minimum scaling percentage [1, 10000] 1864 max Maximum scaling percentage [1, 10000] 1865 ====== ===================================== 1866 1867 The controller is disabled by default and can be enabled by 1868 setting "enable" to 1. "rpct" and "wpct" parameters default 1869 to zero and the controller uses internal device saturation 1870 state to adjust the overall IO rate between "min" and "max". 1871 1872 When a better control quality is needed, latency QoS 1873 parameters can be configured. For example:: 1874 1875 8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0 1876 1877 shows that on sdb, the controller is enabled, will consider 1878 the device saturated if the 95th percentile of read completion 1879 latencies is above 75ms or write 150ms, and adjust the overall 1880 IO issue rate between 50% and 150% accordingly. 1881 1882 The lower the saturation point, the better the latency QoS at 1883 the cost of aggregate bandwidth. The narrower the allowed 1884 adjustment range between "min" and "max", the more conformant 1885 to the cost model the IO behavior. Note that the IO issue 1886 base rate may be far off from 100% and setting "min" and "max" 1887 blindly can lead to a significant loss of device capacity or 1888 control quality. "min" and "max" are useful for regulating 1889 devices which show wide temporary behavior changes - e.g. a 1890 ssd which accepts writes at the line speed for a while and 1891 then completely stalls for multiple seconds. 1892 1893 When "ctrl" is "auto", the parameters are controlled by the 1894 kernel and may change automatically. Setting "ctrl" to "user" 1895 or setting any of the percentile and latency parameters puts 1896 it into "user" mode and disables the automatic changes. The 1897 automatic mode can be restored by setting "ctrl" to "auto". 1898 1899 io.cost.model 1900 A read-write nested-keyed file which exists only on the root 1901 cgroup. 1902 1903 This file configures the cost model of the IO cost model based 1904 controller (CONFIG_BLK_CGROUP_IOCOST) which currently 1905 implements "io.weight" proportional control. Lines are keyed 1906 by $MAJ:$MIN device numbers and not ordered. The line for a 1907 given device is populated on the first write for the device on 1908 "io.cost.qos" or "io.cost.model". The following nested keys 1909 are defined. 1910 1911 ===== ================================ 1912 ctrl "auto" or "user" 1913 model The cost model in use - "linear" 1914 ===== ================================ 1915 1916 When "ctrl" is "auto", the kernel may change all parameters 1917 dynamically. When "ctrl" is set to "user" or any other 1918 parameters are written to, "ctrl" become "user" and the 1919 automatic changes are disabled. 1920 1921 When "model" is "linear", the following model parameters are 1922 defined. 1923 1924 ============= ======================================== 1925 [r|w]bps The maximum sequential IO throughput 1926 [r|w]seqiops The maximum 4k sequential IOs per second 1927 [r|w]randiops The maximum 4k random IOs per second 1928 ============= ======================================== 1929 1930 From the above, the builtin linear model determines the base 1931 costs of a sequential and random IO and the cost coefficient 1932 for the IO size. While simple, this model can cover most 1933 common device classes acceptably. 1934 1935 The IO cost model isn't expected to be accurate in absolute 1936 sense and is scaled to the device behavior dynamically. 1937 1938 If needed, tools/cgroup/iocost_coef_gen.py can be used to 1939 generate device-specific coefficients. 1940 1941 io.weight 1942 A read-write flat-keyed file which exists on non-root cgroups. 1943 The default is "default 100". 1944 1945 The first line is the default weight applied to devices 1946 without specific override. The rest are overrides keyed by 1947 $MAJ:$MIN device numbers and not ordered. The weights are in 1948 the range [1, 10000] and specifies the relative amount IO time 1949 the cgroup can use in relation to its siblings. 1950 1951 The default weight can be updated by writing either "default 1952 $WEIGHT" or simply "$WEIGHT". Overrides can be set by writing 1953 "$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default". 1954 1955 An example read output follows:: 1956 1957 default 100 1958 8:16 200 1959 8:0 50 1960 1961 io.max 1962 A read-write nested-keyed file which exists on non-root 1963 cgroups. 1964 1965 BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN 1966 device numbers and not ordered. The following nested keys are 1967 defined. 1968 1969 ===== ================================== 1970 rbps Max read bytes per second 1971 wbps Max write bytes per second 1972 riops Max read IO operations per second 1973 wiops Max write IO operations per second 1974 ===== ================================== 1975 1976 When writing, any number of nested key-value pairs can be 1977 specified in any order. "max" can be specified as the value 1978 to remove a specific limit. If the same key is specified 1979 multiple times, the outcome is undefined. 1980 1981 BPS and IOPS are measured in each IO direction and IOs are 1982 delayed if limit is reached. Temporary bursts are allowed. 1983 1984 Setting read limit at 2M BPS and write at 120 IOPS for 8:16:: 1985 1986 echo "8:16 rbps=2097152 wiops=120" > io.max 1987 1988 Reading returns the following:: 1989 1990 8:16 rbps=2097152 wbps=max riops=max wiops=120 1991 1992 Write IOPS limit can be removed by writing the following:: 1993 1994 echo "8:16 wiops=max" > io.max 1995 1996 Reading now returns the following:: 1997 1998 8:16 rbps=2097152 wbps=max riops=max wiops=max 1999 2000 io.pressure 2001 A read-only nested-keyed file. 2002 2003 Shows pressure stall information for IO. See 2004 :ref:`Documentation/accounting/psi.rst <psi>` for details. 2005 2006 2007Writeback 2008~~~~~~~~~ 2009 2010Page cache is dirtied through buffered writes and shared mmaps and 2011written asynchronously to the backing filesystem by the writeback 2012mechanism. Writeback sits between the memory and IO domains and 2013regulates the proportion of dirty memory by balancing dirtying and 2014write IOs. 2015 2016The io controller, in conjunction with the memory controller, 2017implements control of page cache writeback IOs. The memory controller 2018defines the memory domain that dirty memory ratio is calculated and 2019maintained for and the io controller defines the io domain which 2020writes out dirty pages for the memory domain. Both system-wide and 2021per-cgroup dirty memory states are examined and the more restrictive 2022of the two is enforced. 2023 2024cgroup writeback requires explicit support from the underlying 2025filesystem. Currently, cgroup writeback is implemented on ext2, ext4, 2026btrfs, f2fs, and xfs. On other filesystems, all writeback IOs are 2027attributed to the root cgroup. 2028 2029There are inherent differences in memory and writeback management 2030which affects how cgroup ownership is tracked. Memory is tracked per 2031page while writeback per inode. For the purpose of writeback, an 2032inode is assigned to a cgroup and all IO requests to write dirty pages 2033from the inode are attributed to that cgroup. 2034 2035As cgroup ownership for memory is tracked per page, there can be pages 2036which are associated with different cgroups than the one the inode is 2037associated with. These are called foreign pages. The writeback 2038constantly keeps track of foreign pages and, if a particular foreign 2039cgroup becomes the majority over a certain period of time, switches 2040the ownership of the inode to that cgroup. 2041 2042While this model is enough for most use cases where a given inode is 2043mostly dirtied by a single cgroup even when the main writing cgroup 2044changes over time, use cases where multiple cgroups write to a single 2045inode simultaneously are not supported well. In such circumstances, a 2046significant portion of IOs are likely to be attributed incorrectly. 2047As memory controller assigns page ownership on the first use and 2048doesn't update it until the page is released, even if writeback 2049strictly follows page ownership, multiple cgroups dirtying overlapping 2050areas wouldn't work as expected. It's recommended to avoid such usage 2051patterns. 2052 2053The sysctl knobs which affect writeback behavior are applied to cgroup 2054writeback as follows. 2055 2056 vm.dirty_background_ratio, vm.dirty_ratio 2057 These ratios apply the same to cgroup writeback with the 2058 amount of available memory capped by limits imposed by the 2059 memory controller and system-wide clean memory. 2060 2061 vm.dirty_background_bytes, vm.dirty_bytes 2062 For cgroup writeback, this is calculated into ratio against 2063 total available memory and applied the same way as 2064 vm.dirty[_background]_ratio. 2065 2066 2067IO Latency 2068~~~~~~~~~~ 2069 2070This is a cgroup v2 controller for IO workload protection. You provide a group 2071with a latency target, and if the average latency exceeds that target the 2072controller will throttle any peers that have a lower latency target than the 2073protected workload. 2074 2075The limits are only applied at the peer level in the hierarchy. This means that 2076in the diagram below, only groups A, B, and C will influence each other, and 2077groups D and F will influence each other. Group G will influence nobody:: 2078 2079 [root] 2080 / | \ 2081 A B C 2082 / \ | 2083 D F G 2084 2085 2086So the ideal way to configure this is to set io.latency in groups A, B, and C. 2087Generally you do not want to set a value lower than the latency your device 2088supports. Experiment to find the value that works best for your workload. 2089Start at higher than the expected latency for your device and watch the 2090avg_lat value in io.stat for your workload group to get an idea of the 2091latency you see during normal operation. Use the avg_lat value as a basis for 2092your real setting, setting at 10-15% higher than the value in io.stat. 2093 2094How IO Latency Throttling Works 2095~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2096 2097io.latency is work conserving; so as long as everybody is meeting their latency 2098target the controller doesn't do anything. Once a group starts missing its 2099target it begins throttling any peer group that has a higher target than itself. 2100This throttling takes 2 forms: 2101 2102- Queue depth throttling. This is the number of outstanding IO's a group is 2103 allowed to have. We will clamp down relatively quickly, starting at no limit 2104 and going all the way down to 1 IO at a time. 2105 2106- Artificial delay induction. There are certain types of IO that cannot be 2107 throttled without possibly adversely affecting higher priority groups. This 2108 includes swapping and metadata IO. These types of IO are allowed to occur 2109 normally, however they are "charged" to the originating group. If the 2110 originating group is being throttled you will see the use_delay and delay 2111 fields in io.stat increase. The delay value is how many microseconds that are 2112 being added to any process that runs in this group. Because this number can 2113 grow quite large if there is a lot of swapping or metadata IO occurring we 2114 limit the individual delay events to 1 second at a time. 2115 2116Once the victimized group starts meeting its latency target again it will start 2117unthrottling any peer groups that were throttled previously. If the victimized 2118group simply stops doing IO the global counter will unthrottle appropriately. 2119 2120IO Latency Interface Files 2121~~~~~~~~~~~~~~~~~~~~~~~~~~ 2122 2123 io.latency 2124 This takes a similar format as the other controllers. 2125 2126 "MAJOR:MINOR target=<target time in microseconds>" 2127 2128 io.stat 2129 If the controller is enabled you will see extra stats in io.stat in 2130 addition to the normal ones. 2131 2132 depth 2133 This is the current queue depth for the group. 2134 2135 avg_lat 2136 This is an exponential moving average with a decay rate of 1/exp 2137 bound by the sampling interval. The decay rate interval can be 2138 calculated by multiplying the win value in io.stat by the 2139 corresponding number of samples based on the win value. 2140 2141 win 2142 The sampling window size in milliseconds. This is the minimum 2143 duration of time between evaluation events. Windows only elapse 2144 with IO activity. Idle periods extend the most recent window. 2145 2146IO Priority 2147~~~~~~~~~~~ 2148 2149A single attribute controls the behavior of the I/O priority cgroup policy, 2150namely the io.prio.class attribute. The following values are accepted for 2151that attribute: 2152 2153 no-change 2154 Do not modify the I/O priority class. 2155 2156 promote-to-rt 2157 For requests that have a non-RT I/O priority class, change it into RT. 2158 Also change the priority level of these requests to 4. Do not modify 2159 the I/O priority of requests that have priority class RT. 2160 2161 restrict-to-be 2162 For requests that do not have an I/O priority class or that have I/O 2163 priority class RT, change it into BE. Also change the priority level 2164 of these requests to 0. Do not modify the I/O priority class of 2165 requests that have priority class IDLE. 2166 2167 idle 2168 Change the I/O priority class of all requests into IDLE, the lowest 2169 I/O priority class. 2170 2171 none-to-rt 2172 Deprecated. Just an alias for promote-to-rt. 2173 2174The following numerical values are associated with the I/O priority policies: 2175 2176+----------------+---+ 2177| no-change | 0 | 2178+----------------+---+ 2179| promote-to-rt | 1 | 2180+----------------+---+ 2181| restrict-to-be | 2 | 2182+----------------+---+ 2183| idle | 3 | 2184+----------------+---+ 2185 2186The numerical value that corresponds to each I/O priority class is as follows: 2187 2188+-------------------------------+---+ 2189| IOPRIO_CLASS_NONE | 0 | 2190+-------------------------------+---+ 2191| IOPRIO_CLASS_RT (real-time) | 1 | 2192+-------------------------------+---+ 2193| IOPRIO_CLASS_BE (best effort) | 2 | 2194+-------------------------------+---+ 2195| IOPRIO_CLASS_IDLE | 3 | 2196+-------------------------------+---+ 2197 2198The algorithm to set the I/O priority class for a request is as follows: 2199 2200- If I/O priority class policy is promote-to-rt, change the request I/O 2201 priority class to IOPRIO_CLASS_RT and change the request I/O priority 2202 level to 4. 2203- If I/O priority class policy is not promote-to-rt, translate the I/O priority 2204 class policy into a number, then change the request I/O priority class 2205 into the maximum of the I/O priority class policy number and the numerical 2206 I/O priority class. 2207 2208PID 2209--- 2210 2211The process number controller is used to allow a cgroup to stop any 2212new tasks from being fork()'d or clone()'d after a specified limit is 2213reached. 2214 2215The number of tasks in a cgroup can be exhausted in ways which other 2216controllers cannot prevent, thus warranting its own controller. For 2217example, a fork bomb is likely to exhaust the number of tasks before 2218hitting memory restrictions. 2219 2220Note that PIDs used in this controller refer to TIDs, process IDs as 2221used by the kernel. 2222 2223 2224PID Interface Files 2225~~~~~~~~~~~~~~~~~~~ 2226 2227 pids.max 2228 A read-write single value file which exists on non-root 2229 cgroups. The default is "max". 2230 2231 Hard limit of number of processes. 2232 2233 pids.current 2234 A read-only single value file which exists on non-root cgroups. 2235 2236 The number of processes currently in the cgroup and its 2237 descendants. 2238 2239 pids.peak 2240 A read-only single value file which exists on non-root cgroups. 2241 2242 The maximum value that the number of processes in the cgroup and its 2243 descendants has ever reached. 2244 2245 pids.events 2246 A read-only flat-keyed file which exists on non-root cgroups. Unless 2247 specified otherwise, a value change in this file generates a file 2248 modified event. The following entries are defined. 2249 2250 max 2251 The number of times the cgroup's total number of processes hit the pids.max 2252 limit (see also pids_localevents). 2253 2254 pids.events.local 2255 Similar to pids.events but the fields in the file are local 2256 to the cgroup i.e. not hierarchical. The file modified event 2257 generated on this file reflects only the local events. 2258 2259Organisational operations are not blocked by cgroup policies, so it is 2260possible to have pids.current > pids.max. This can be done by either 2261setting the limit to be smaller than pids.current, or attaching enough 2262processes to the cgroup such that pids.current is larger than 2263pids.max. However, it is not possible to violate a cgroup PID policy 2264through fork() or clone(). These will return -EAGAIN if the creation 2265of a new process would cause a cgroup policy to be violated. 2266 2267 2268Cpuset 2269------ 2270 2271The "cpuset" controller provides a mechanism for constraining 2272the CPU and memory node placement of tasks to only the resources 2273specified in the cpuset interface files in a task's current cgroup. 2274This is especially valuable on large NUMA systems where placing jobs 2275on properly sized subsets of the systems with careful processor and 2276memory placement to reduce cross-node memory access and contention 2277can improve overall system performance. 2278 2279The "cpuset" controller is hierarchical. That means the controller 2280cannot use CPUs or memory nodes not allowed in its parent. 2281 2282 2283Cpuset Interface Files 2284~~~~~~~~~~~~~~~~~~~~~~ 2285 2286 cpuset.cpus 2287 A read-write multiple values file which exists on non-root 2288 cpuset-enabled cgroups. 2289 2290 It lists the requested CPUs to be used by tasks within this 2291 cgroup. The actual list of CPUs to be granted, however, is 2292 subjected to constraints imposed by its parent and can differ 2293 from the requested CPUs. 2294 2295 The CPU numbers are comma-separated numbers or ranges. 2296 For example:: 2297 2298 # cat cpuset.cpus 2299 0-4,6,8-10 2300 2301 An empty value indicates that the cgroup is using the same 2302 setting as the nearest cgroup ancestor with a non-empty 2303 "cpuset.cpus" or all the available CPUs if none is found. 2304 2305 The value of "cpuset.cpus" stays constant until the next update 2306 and won't be affected by any CPU hotplug events. 2307 2308 cpuset.cpus.effective 2309 A read-only multiple values file which exists on all 2310 cpuset-enabled cgroups. 2311 2312 It lists the onlined CPUs that are actually granted to this 2313 cgroup by its parent. These CPUs are allowed to be used by 2314 tasks within the current cgroup. 2315 2316 If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows 2317 all the CPUs from the parent cgroup that can be available to 2318 be used by this cgroup. Otherwise, it should be a subset of 2319 "cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus" 2320 can be granted. In this case, it will be treated just like an 2321 empty "cpuset.cpus". 2322 2323 Its value will be affected by CPU hotplug events. 2324 2325 cpuset.mems 2326 A read-write multiple values file which exists on non-root 2327 cpuset-enabled cgroups. 2328 2329 It lists the requested memory nodes to be used by tasks within 2330 this cgroup. The actual list of memory nodes granted, however, 2331 is subjected to constraints imposed by its parent and can differ 2332 from the requested memory nodes. 2333 2334 The memory node numbers are comma-separated numbers or ranges. 2335 For example:: 2336 2337 # cat cpuset.mems 2338 0-1,3 2339 2340 An empty value indicates that the cgroup is using the same 2341 setting as the nearest cgroup ancestor with a non-empty 2342 "cpuset.mems" or all the available memory nodes if none 2343 is found. 2344 2345 The value of "cpuset.mems" stays constant until the next update 2346 and won't be affected by any memory nodes hotplug events. 2347 2348 Setting a non-empty value to "cpuset.mems" causes memory of 2349 tasks within the cgroup to be migrated to the designated nodes if 2350 they are currently using memory outside of the designated nodes. 2351 2352 There is a cost for this memory migration. The migration 2353 may not be complete and some memory pages may be left behind. 2354 So it is recommended that "cpuset.mems" should be set properly 2355 before spawning new tasks into the cpuset. Even if there is 2356 a need to change "cpuset.mems" with active tasks, it shouldn't 2357 be done frequently. 2358 2359 cpuset.mems.effective 2360 A read-only multiple values file which exists on all 2361 cpuset-enabled cgroups. 2362 2363 It lists the onlined memory nodes that are actually granted to 2364 this cgroup by its parent. These memory nodes are allowed to 2365 be used by tasks within the current cgroup. 2366 2367 If "cpuset.mems" is empty, it shows all the memory nodes from the 2368 parent cgroup that will be available to be used by this cgroup. 2369 Otherwise, it should be a subset of "cpuset.mems" unless none of 2370 the memory nodes listed in "cpuset.mems" can be granted. In this 2371 case, it will be treated just like an empty "cpuset.mems". 2372 2373 Its value will be affected by memory nodes hotplug events. 2374 2375 cpuset.cpus.exclusive 2376 A read-write multiple values file which exists on non-root 2377 cpuset-enabled cgroups. 2378 2379 It lists all the exclusive CPUs that are allowed to be used 2380 to create a new cpuset partition. Its value is not used 2381 unless the cgroup becomes a valid partition root. See the 2382 "cpuset.cpus.partition" section below for a description of what 2383 a cpuset partition is. 2384 2385 When the cgroup becomes a partition root, the actual exclusive 2386 CPUs that are allocated to that partition are listed in 2387 "cpuset.cpus.exclusive.effective" which may be different 2388 from "cpuset.cpus.exclusive". If "cpuset.cpus.exclusive" 2389 has previously been set, "cpuset.cpus.exclusive.effective" 2390 is always a subset of it. 2391 2392 Users can manually set it to a value that is different from 2393 "cpuset.cpus". One constraint in setting it is that the list of 2394 CPUs must be exclusive with respect to "cpuset.cpus.exclusive" 2395 of its sibling. If "cpuset.cpus.exclusive" of a sibling cgroup 2396 isn't set, its "cpuset.cpus" value, if set, cannot be a subset 2397 of it to leave at least one CPU available when the exclusive 2398 CPUs are taken away. 2399 2400 For a parent cgroup, any one of its exclusive CPUs can only 2401 be distributed to at most one of its child cgroups. Having an 2402 exclusive CPU appearing in two or more of its child cgroups is 2403 not allowed (the exclusivity rule). A value that violates the 2404 exclusivity rule will be rejected with a write error. 2405 2406 The root cgroup is a partition root and all its available CPUs 2407 are in its exclusive CPU set. 2408 2409 cpuset.cpus.exclusive.effective 2410 A read-only multiple values file which exists on all non-root 2411 cpuset-enabled cgroups. 2412 2413 This file shows the effective set of exclusive CPUs that 2414 can be used to create a partition root. The content 2415 of this file will always be a subset of its parent's 2416 "cpuset.cpus.exclusive.effective" if its parent is not the root 2417 cgroup. It will also be a subset of "cpuset.cpus.exclusive" 2418 if it is set. If "cpuset.cpus.exclusive" is not set, it is 2419 treated to have an implicit value of "cpuset.cpus" in the 2420 formation of local partition. 2421 2422 cpuset.cpus.isolated 2423 A read-only and root cgroup only multiple values file. 2424 2425 This file shows the set of all isolated CPUs used in existing 2426 isolated partitions. It will be empty if no isolated partition 2427 is created. 2428 2429 cpuset.cpus.partition 2430 A read-write single value file which exists on non-root 2431 cpuset-enabled cgroups. This flag is owned by the parent cgroup 2432 and is not delegatable. 2433 2434 It accepts only the following input values when written to. 2435 2436 ========== ===================================== 2437 "member" Non-root member of a partition 2438 "root" Partition root 2439 "isolated" Partition root without load balancing 2440 ========== ===================================== 2441 2442 A cpuset partition is a collection of cpuset-enabled cgroups with 2443 a partition root at the top of the hierarchy and its descendants 2444 except those that are separate partition roots themselves and 2445 their descendants. A partition has exclusive access to the 2446 set of exclusive CPUs allocated to it. Other cgroups outside 2447 of that partition cannot use any CPUs in that set. 2448 2449 There are two types of partitions - local and remote. A local 2450 partition is one whose parent cgroup is also a valid partition 2451 root. A remote partition is one whose parent cgroup is not a 2452 valid partition root itself. Writing to "cpuset.cpus.exclusive" 2453 is optional for the creation of a local partition as its 2454 "cpuset.cpus.exclusive" file will assume an implicit value that 2455 is the same as "cpuset.cpus" if it is not set. Writing the 2456 proper "cpuset.cpus.exclusive" values down the cgroup hierarchy 2457 before the target partition root is mandatory for the creation 2458 of a remote partition. 2459 2460 Currently, a remote partition cannot be created under a local 2461 partition. All the ancestors of a remote partition root except 2462 the root cgroup cannot be a partition root. 2463 2464 The root cgroup is always a partition root and its state cannot 2465 be changed. All other non-root cgroups start out as "member". 2466 2467 When set to "root", the current cgroup is the root of a new 2468 partition or scheduling domain. The set of exclusive CPUs is 2469 determined by the value of its "cpuset.cpus.exclusive.effective". 2470 2471 When set to "isolated", the CPUs in that partition will be in 2472 an isolated state without any load balancing from the scheduler 2473 and excluded from the unbound workqueues. Tasks placed in such 2474 a partition with multiple CPUs should be carefully distributed 2475 and bound to each of the individual CPUs for optimal performance. 2476 2477 A partition root ("root" or "isolated") can be in one of the 2478 two possible states - valid or invalid. An invalid partition 2479 root is in a degraded state where some state information may 2480 be retained, but behaves more like a "member". 2481 2482 All possible state transitions among "member", "root" and 2483 "isolated" are allowed. 2484 2485 On read, the "cpuset.cpus.partition" file can show the following 2486 values. 2487 2488 ============================= ===================================== 2489 "member" Non-root member of a partition 2490 "root" Partition root 2491 "isolated" Partition root without load balancing 2492 "root invalid (<reason>)" Invalid partition root 2493 "isolated invalid (<reason>)" Invalid isolated partition root 2494 ============================= ===================================== 2495 2496 In the case of an invalid partition root, a descriptive string on 2497 why the partition is invalid is included within parentheses. 2498 2499 For a local partition root to be valid, the following conditions 2500 must be met. 2501 2502 1) The parent cgroup is a valid partition root. 2503 2) The "cpuset.cpus.exclusive.effective" file cannot be empty, 2504 though it may contain offline CPUs. 2505 3) The "cpuset.cpus.effective" cannot be empty unless there is 2506 no task associated with this partition. 2507 2508 For a remote partition root to be valid, all the above conditions 2509 except the first one must be met. 2510 2511 External events like hotplug or changes to "cpuset.cpus" or 2512 "cpuset.cpus.exclusive" can cause a valid partition root to 2513 become invalid and vice versa. Note that a task cannot be 2514 moved to a cgroup with empty "cpuset.cpus.effective". 2515 2516 A valid non-root parent partition may distribute out all its CPUs 2517 to its child local partitions when there is no task associated 2518 with it. 2519 2520 Care must be taken to change a valid partition root to "member" 2521 as all its child local partitions, if present, will become 2522 invalid causing disruption to tasks running in those child 2523 partitions. These inactivated partitions could be recovered if 2524 their parent is switched back to a partition root with a proper 2525 value in "cpuset.cpus" or "cpuset.cpus.exclusive". 2526 2527 Poll and inotify events are triggered whenever the state of 2528 "cpuset.cpus.partition" changes. That includes changes caused 2529 by write to "cpuset.cpus.partition", cpu hotplug or other 2530 changes that modify the validity status of the partition. 2531 This will allow user space agents to monitor unexpected changes 2532 to "cpuset.cpus.partition" without the need to do continuous 2533 polling. 2534 2535 A user can pre-configure certain CPUs to an isolated state 2536 with load balancing disabled at boot time with the "isolcpus" 2537 kernel boot command line option. If those CPUs are to be put 2538 into a partition, they have to be used in an isolated partition. 2539 2540 2541Device controller 2542----------------- 2543 2544Device controller manages access to device files. It includes both 2545creation of new device files (using mknod), and access to the 2546existing device files. 2547 2548Cgroup v2 device controller has no interface files and is implemented 2549on top of cgroup BPF. To control access to device files, a user may 2550create bpf programs of type BPF_PROG_TYPE_CGROUP_DEVICE and attach 2551them to cgroups with BPF_CGROUP_DEVICE flag. On an attempt to access a 2552device file, corresponding BPF programs will be executed, and depending 2553on the return value the attempt will succeed or fail with -EPERM. 2554 2555A BPF_PROG_TYPE_CGROUP_DEVICE program takes a pointer to the 2556bpf_cgroup_dev_ctx structure, which describes the device access attempt: 2557access type (mknod/read/write) and device (type, major and minor numbers). 2558If the program returns 0, the attempt fails with -EPERM, otherwise it 2559succeeds. 2560 2561An example of BPF_PROG_TYPE_CGROUP_DEVICE program may be found in 2562tools/testing/selftests/bpf/progs/dev_cgroup.c in the kernel source tree. 2563 2564 2565RDMA 2566---- 2567 2568The "rdma" controller regulates the distribution and accounting of 2569RDMA resources. 2570 2571RDMA Interface Files 2572~~~~~~~~~~~~~~~~~~~~ 2573 2574 rdma.max 2575 A readwrite nested-keyed file that exists for all the cgroups 2576 except root that describes current configured resource limit 2577 for a RDMA/IB device. 2578 2579 Lines are keyed by device name and are not ordered. 2580 Each line contains space separated resource name and its configured 2581 limit that can be distributed. 2582 2583 The following nested keys are defined. 2584 2585 ========== ============================= 2586 hca_handle Maximum number of HCA Handles 2587 hca_object Maximum number of HCA Objects 2588 ========== ============================= 2589 2590 An example for mlx4 and ocrdma device follows:: 2591 2592 mlx4_0 hca_handle=2 hca_object=2000 2593 ocrdma1 hca_handle=3 hca_object=max 2594 2595 rdma.current 2596 A read-only file that describes current resource usage. 2597 It exists for all the cgroup except root. 2598 2599 An example for mlx4 and ocrdma device follows:: 2600 2601 mlx4_0 hca_handle=1 hca_object=20 2602 ocrdma1 hca_handle=1 hca_object=23 2603 2604HugeTLB 2605------- 2606 2607The HugeTLB controller allows to limit the HugeTLB usage per control group and 2608enforces the controller limit during page fault. 2609 2610HugeTLB Interface Files 2611~~~~~~~~~~~~~~~~~~~~~~~ 2612 2613 hugetlb.<hugepagesize>.current 2614 Show current usage for "hugepagesize" hugetlb. It exists for all 2615 the cgroup except root. 2616 2617 hugetlb.<hugepagesize>.max 2618 Set/show the hard limit of "hugepagesize" hugetlb usage. 2619 The default value is "max". It exists for all the cgroup except root. 2620 2621 hugetlb.<hugepagesize>.events 2622 A read-only flat-keyed file which exists on non-root cgroups. 2623 2624 max 2625 The number of allocation failure due to HugeTLB limit 2626 2627 hugetlb.<hugepagesize>.events.local 2628 Similar to hugetlb.<hugepagesize>.events but the fields in the file 2629 are local to the cgroup i.e. not hierarchical. The file modified event 2630 generated on this file reflects only the local events. 2631 2632 hugetlb.<hugepagesize>.numa_stat 2633 Similar to memory.numa_stat, it shows the numa information of the 2634 hugetlb pages of <hugepagesize> in this cgroup. Only active in 2635 use hugetlb pages are included. The per-node values are in bytes. 2636 2637Misc 2638---- 2639 2640The Miscellaneous cgroup provides the resource limiting and tracking 2641mechanism for the scalar resources which cannot be abstracted like the other 2642cgroup resources. Controller is enabled by the CONFIG_CGROUP_MISC config 2643option. 2644 2645A resource can be added to the controller via enum misc_res_type{} in the 2646include/linux/misc_cgroup.h file and the corresponding name via misc_res_name[] 2647in the kernel/cgroup/misc.c file. Provider of the resource must set its 2648capacity prior to using the resource by calling misc_cg_set_capacity(). 2649 2650Once a capacity is set then the resource usage can be updated using charge and 2651uncharge APIs. All of the APIs to interact with misc controller are in 2652include/linux/misc_cgroup.h. 2653 2654Misc Interface Files 2655~~~~~~~~~~~~~~~~~~~~ 2656 2657Miscellaneous controller provides 3 interface files. If two misc resources (res_a and res_b) are registered then: 2658 2659 misc.capacity 2660 A read-only flat-keyed file shown only in the root cgroup. It shows 2661 miscellaneous scalar resources available on the platform along with 2662 their quantities:: 2663 2664 $ cat misc.capacity 2665 res_a 50 2666 res_b 10 2667 2668 misc.current 2669 A read-only flat-keyed file shown in the all cgroups. It shows 2670 the current usage of the resources in the cgroup and its children.:: 2671 2672 $ cat misc.current 2673 res_a 3 2674 res_b 0 2675 2676 misc.peak 2677 A read-only flat-keyed file shown in all cgroups. It shows the 2678 historical maximum usage of the resources in the cgroup and its 2679 children.:: 2680 2681 $ cat misc.peak 2682 res_a 10 2683 res_b 8 2684 2685 misc.max 2686 A read-write flat-keyed file shown in the non root cgroups. Allowed 2687 maximum usage of the resources in the cgroup and its children.:: 2688 2689 $ cat misc.max 2690 res_a max 2691 res_b 4 2692 2693 Limit can be set by:: 2694 2695 # echo res_a 1 > misc.max 2696 2697 Limit can be set to max by:: 2698 2699 # echo res_a max > misc.max 2700 2701 Limits can be set higher than the capacity value in the misc.capacity 2702 file. 2703 2704 misc.events 2705 A read-only flat-keyed file which exists on non-root cgroups. The 2706 following entries are defined. Unless specified otherwise, a value 2707 change in this file generates a file modified event. All fields in 2708 this file are hierarchical. 2709 2710 max 2711 The number of times the cgroup's resource usage was 2712 about to go over the max boundary. 2713 2714 misc.events.local 2715 Similar to misc.events but the fields in the file are local to the 2716 cgroup i.e. not hierarchical. The file modified event generated on 2717 this file reflects only the local events. 2718 2719Migration and Ownership 2720~~~~~~~~~~~~~~~~~~~~~~~ 2721 2722A miscellaneous scalar resource is charged to the cgroup in which it is used 2723first, and stays charged to that cgroup until that resource is freed. Migrating 2724a process to a different cgroup does not move the charge to the destination 2725cgroup where the process has moved. 2726 2727Others 2728------ 2729 2730perf_event 2731~~~~~~~~~~ 2732 2733perf_event controller, if not mounted on a legacy hierarchy, is 2734automatically enabled on the v2 hierarchy so that perf events can 2735always be filtered by cgroup v2 path. The controller can still be 2736moved to a legacy hierarchy after v2 hierarchy is populated. 2737 2738 2739Non-normative information 2740------------------------- 2741 2742This section contains information that isn't considered to be a part of 2743the stable kernel API and so is subject to change. 2744 2745 2746CPU controller root cgroup process behaviour 2747~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2748 2749When distributing CPU cycles in the root cgroup each thread in this 2750cgroup is treated as if it was hosted in a separate child cgroup of the 2751root cgroup. This child cgroup weight is dependent on its thread nice 2752level. 2753 2754For details of this mapping see sched_prio_to_weight array in 2755kernel/sched/core.c file (values from this array should be scaled 2756appropriately so the neutral - nice 0 - value is 100 instead of 1024). 2757 2758 2759IO controller root cgroup process behaviour 2760~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2761 2762Root cgroup processes are hosted in an implicit leaf child node. 2763When distributing IO resources this implicit child node is taken into 2764account as if it was a normal child cgroup of the root cgroup with a 2765weight value of 200. 2766 2767 2768Namespace 2769========= 2770 2771Basics 2772------ 2773 2774cgroup namespace provides a mechanism to virtualize the view of the 2775"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone 2776flag can be used with clone(2) and unshare(2) to create a new cgroup 2777namespace. The process running inside the cgroup namespace will have 2778its "/proc/$PID/cgroup" output restricted to cgroupns root. The 2779cgroupns root is the cgroup of the process at the time of creation of 2780the cgroup namespace. 2781 2782Without cgroup namespace, the "/proc/$PID/cgroup" file shows the 2783complete path of the cgroup of a process. In a container setup where 2784a set of cgroups and namespaces are intended to isolate processes the 2785"/proc/$PID/cgroup" file may leak potential system level information 2786to the isolated processes. For example:: 2787 2788 # cat /proc/self/cgroup 2789 0::/batchjobs/container_id1 2790 2791The path '/batchjobs/container_id1' can be considered as system-data 2792and undesirable to expose to the isolated processes. cgroup namespace 2793can be used to restrict visibility of this path. For example, before 2794creating a cgroup namespace, one would see:: 2795 2796 # ls -l /proc/self/ns/cgroup 2797 lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835] 2798 # cat /proc/self/cgroup 2799 0::/batchjobs/container_id1 2800 2801After unsharing a new namespace, the view changes:: 2802 2803 # ls -l /proc/self/ns/cgroup 2804 lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183] 2805 # cat /proc/self/cgroup 2806 0::/ 2807 2808When some thread from a multi-threaded process unshares its cgroup 2809namespace, the new cgroupns gets applied to the entire process (all 2810the threads). This is natural for the v2 hierarchy; however, for the 2811legacy hierarchies, this may be unexpected. 2812 2813A cgroup namespace is alive as long as there are processes inside or 2814mounts pinning it. When the last usage goes away, the cgroup 2815namespace is destroyed. The cgroupns root and the actual cgroups 2816remain. 2817 2818 2819The Root and Views 2820------------------ 2821 2822The 'cgroupns root' for a cgroup namespace is the cgroup in which the 2823process calling unshare(2) is running. For example, if a process in 2824/batchjobs/container_id1 cgroup calls unshare, cgroup 2825/batchjobs/container_id1 becomes the cgroupns root. For the 2826init_cgroup_ns, this is the real root ('/') cgroup. 2827 2828The cgroupns root cgroup does not change even if the namespace creator 2829process later moves to a different cgroup:: 2830 2831 # ~/unshare -c # unshare cgroupns in some cgroup 2832 # cat /proc/self/cgroup 2833 0::/ 2834 # mkdir sub_cgrp_1 2835 # echo 0 > sub_cgrp_1/cgroup.procs 2836 # cat /proc/self/cgroup 2837 0::/sub_cgrp_1 2838 2839Each process gets its namespace-specific view of "/proc/$PID/cgroup" 2840 2841Processes running inside the cgroup namespace will be able to see 2842cgroup paths (in /proc/self/cgroup) only inside their root cgroup. 2843From within an unshared cgroupns:: 2844 2845 # sleep 100000 & 2846 [1] 7353 2847 # echo 7353 > sub_cgrp_1/cgroup.procs 2848 # cat /proc/7353/cgroup 2849 0::/sub_cgrp_1 2850 2851From the initial cgroup namespace, the real cgroup path will be 2852visible:: 2853 2854 $ cat /proc/7353/cgroup 2855 0::/batchjobs/container_id1/sub_cgrp_1 2856 2857From a sibling cgroup namespace (that is, a namespace rooted at a 2858different cgroup), the cgroup path relative to its own cgroup 2859namespace root will be shown. For instance, if PID 7353's cgroup 2860namespace root is at '/batchjobs/container_id2', then it will see:: 2861 2862 # cat /proc/7353/cgroup 2863 0::/../container_id2/sub_cgrp_1 2864 2865Note that the relative path always starts with '/' to indicate that 2866its relative to the cgroup namespace root of the caller. 2867 2868 2869Migration and setns(2) 2870---------------------- 2871 2872Processes inside a cgroup namespace can move into and out of the 2873namespace root if they have proper access to external cgroups. For 2874example, from inside a namespace with cgroupns root at 2875/batchjobs/container_id1, and assuming that the global hierarchy is 2876still accessible inside cgroupns:: 2877 2878 # cat /proc/7353/cgroup 2879 0::/sub_cgrp_1 2880 # echo 7353 > batchjobs/container_id2/cgroup.procs 2881 # cat /proc/7353/cgroup 2882 0::/../container_id2 2883 2884Note that this kind of setup is not encouraged. A task inside cgroup 2885namespace should only be exposed to its own cgroupns hierarchy. 2886 2887setns(2) to another cgroup namespace is allowed when: 2888 2889(a) the process has CAP_SYS_ADMIN against its current user namespace 2890(b) the process has CAP_SYS_ADMIN against the target cgroup 2891 namespace's userns 2892 2893No implicit cgroup changes happen with attaching to another cgroup 2894namespace. It is expected that the someone moves the attaching 2895process under the target cgroup namespace root. 2896 2897 2898Interaction with Other Namespaces 2899--------------------------------- 2900 2901Namespace specific cgroup hierarchy can be mounted by a process 2902running inside a non-init cgroup namespace:: 2903 2904 # mount -t cgroup2 none $MOUNT_POINT 2905 2906This will mount the unified cgroup hierarchy with cgroupns root as the 2907filesystem root. The process needs CAP_SYS_ADMIN against its user and 2908mount namespaces. 2909 2910The virtualization of /proc/self/cgroup file combined with restricting 2911the view of cgroup hierarchy by namespace-private cgroupfs mount 2912provides a properly isolated cgroup view inside the container. 2913 2914 2915Information on Kernel Programming 2916================================= 2917 2918This section contains kernel programming information in the areas 2919where interacting with cgroup is necessary. cgroup core and 2920controllers are not covered. 2921 2922 2923Filesystem Support for Writeback 2924-------------------------------- 2925 2926A filesystem can support cgroup writeback by updating 2927address_space_operations->writepage[s]() to annotate bio's using the 2928following two functions. 2929 2930 wbc_init_bio(@wbc, @bio) 2931 Should be called for each bio carrying writeback data and 2932 associates the bio with the inode's owner cgroup and the 2933 corresponding request queue. This must be called after 2934 a queue (device) has been associated with the bio and 2935 before submission. 2936 2937 wbc_account_cgroup_owner(@wbc, @page, @bytes) 2938 Should be called for each data segment being written out. 2939 While this function doesn't care exactly when it's called 2940 during the writeback session, it's the easiest and most 2941 natural to call it as data segments are added to a bio. 2942 2943With writeback bio's annotated, cgroup support can be enabled per 2944super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for 2945selective disabling of cgroup writeback support which is helpful when 2946certain filesystem features, e.g. journaled data mode, are 2947incompatible. 2948 2949wbc_init_bio() binds the specified bio to its cgroup. Depending on 2950the configuration, the bio may be executed at a lower priority and if 2951the writeback session is holding shared resources, e.g. a journal 2952entry, may lead to priority inversion. There is no one easy solution 2953for the problem. Filesystems can try to work around specific problem 2954cases by skipping wbc_init_bio() and using bio_associate_blkg() 2955directly. 2956 2957 2958Deprecated v1 Core Features 2959=========================== 2960 2961- Multiple hierarchies including named ones are not supported. 2962 2963- All v1 mount options are not supported. 2964 2965- The "tasks" file is removed and "cgroup.procs" is not sorted. 2966 2967- "cgroup.clone_children" is removed. 2968 2969- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file 2970 at the root instead. 2971 2972 2973Issues with v1 and Rationales for v2 2974==================================== 2975 2976Multiple Hierarchies 2977-------------------- 2978 2979cgroup v1 allowed an arbitrary number of hierarchies and each 2980hierarchy could host any number of controllers. While this seemed to 2981provide a high level of flexibility, it wasn't useful in practice. 2982 2983For example, as there is only one instance of each controller, utility 2984type controllers such as freezer which can be useful in all 2985hierarchies could only be used in one. The issue is exacerbated by 2986the fact that controllers couldn't be moved to another hierarchy once 2987hierarchies were populated. Another issue was that all controllers 2988bound to a hierarchy were forced to have exactly the same view of the 2989hierarchy. It wasn't possible to vary the granularity depending on 2990the specific controller. 2991 2992In practice, these issues heavily limited which controllers could be 2993put on the same hierarchy and most configurations resorted to putting 2994each controller on its own hierarchy. Only closely related ones, such 2995as the cpu and cpuacct controllers, made sense to be put on the same 2996hierarchy. This often meant that userland ended up managing multiple 2997similar hierarchies repeating the same steps on each hierarchy 2998whenever a hierarchy management operation was necessary. 2999 3000Furthermore, support for multiple hierarchies came at a steep cost. 3001It greatly complicated cgroup core implementation but more importantly 3002the support for multiple hierarchies restricted how cgroup could be 3003used in general and what controllers was able to do. 3004 3005There was no limit on how many hierarchies there might be, which meant 3006that a thread's cgroup membership couldn't be described in finite 3007length. The key might contain any number of entries and was unlimited 3008in length, which made it highly awkward to manipulate and led to 3009addition of controllers which existed only to identify membership, 3010which in turn exacerbated the original problem of proliferating number 3011of hierarchies. 3012 3013Also, as a controller couldn't have any expectation regarding the 3014topologies of hierarchies other controllers might be on, each 3015controller had to assume that all other controllers were attached to 3016completely orthogonal hierarchies. This made it impossible, or at 3017least very cumbersome, for controllers to cooperate with each other. 3018 3019In most use cases, putting controllers on hierarchies which are 3020completely orthogonal to each other isn't necessary. What usually is 3021called for is the ability to have differing levels of granularity 3022depending on the specific controller. In other words, hierarchy may 3023be collapsed from leaf towards root when viewed from specific 3024controllers. For example, a given configuration might not care about 3025how memory is distributed beyond a certain level while still wanting 3026to control how CPU cycles are distributed. 3027 3028 3029Thread Granularity 3030------------------ 3031 3032cgroup v1 allowed threads of a process to belong to different cgroups. 3033This didn't make sense for some controllers and those controllers 3034ended up implementing different ways to ignore such situations but 3035much more importantly it blurred the line between API exposed to 3036individual applications and system management interface. 3037 3038Generally, in-process knowledge is available only to the process 3039itself; thus, unlike service-level organization of processes, 3040categorizing threads of a process requires active participation from 3041the application which owns the target process. 3042 3043cgroup v1 had an ambiguously defined delegation model which got abused 3044in combination with thread granularity. cgroups were delegated to 3045individual applications so that they can create and manage their own 3046sub-hierarchies and control resource distributions along them. This 3047effectively raised cgroup to the status of a syscall-like API exposed 3048to lay programs. 3049 3050First of all, cgroup has a fundamentally inadequate interface to be 3051exposed this way. For a process to access its own knobs, it has to 3052extract the path on the target hierarchy from /proc/self/cgroup, 3053construct the path by appending the name of the knob to the path, open 3054and then read and/or write to it. This is not only extremely clunky 3055and unusual but also inherently racy. There is no conventional way to 3056define transaction across the required steps and nothing can guarantee 3057that the process would actually be operating on its own sub-hierarchy. 3058 3059cgroup controllers implemented a number of knobs which would never be 3060accepted as public APIs because they were just adding control knobs to 3061system-management pseudo filesystem. cgroup ended up with interface 3062knobs which were not properly abstracted or refined and directly 3063revealed kernel internal details. These knobs got exposed to 3064individual applications through the ill-defined delegation mechanism 3065effectively abusing cgroup as a shortcut to implementing public APIs 3066without going through the required scrutiny. 3067 3068This was painful for both userland and kernel. Userland ended up with 3069misbehaving and poorly abstracted interfaces and kernel exposing and 3070locked into constructs inadvertently. 3071 3072 3073Competition Between Inner Nodes and Threads 3074------------------------------------------- 3075 3076cgroup v1 allowed threads to be in any cgroups which created an 3077interesting problem where threads belonging to a parent cgroup and its 3078children cgroups competed for resources. This was nasty as two 3079different types of entities competed and there was no obvious way to 3080settle it. Different controllers did different things. 3081 3082The cpu controller considered threads and cgroups as equivalents and 3083mapped nice levels to cgroup weights. This worked for some cases but 3084fell flat when children wanted to be allocated specific ratios of CPU 3085cycles and the number of internal threads fluctuated - the ratios 3086constantly changed as the number of competing entities fluctuated. 3087There also were other issues. The mapping from nice level to weight 3088wasn't obvious or universal, and there were various other knobs which 3089simply weren't available for threads. 3090 3091The io controller implicitly created a hidden leaf node for each 3092cgroup to host the threads. The hidden leaf had its own copies of all 3093the knobs with ``leaf_`` prefixed. While this allowed equivalent 3094control over internal threads, it was with serious drawbacks. It 3095always added an extra layer of nesting which wouldn't be necessary 3096otherwise, made the interface messy and significantly complicated the 3097implementation. 3098 3099The memory controller didn't have a way to control what happened 3100between internal tasks and child cgroups and the behavior was not 3101clearly defined. There were attempts to add ad-hoc behaviors and 3102knobs to tailor the behavior to specific workloads which would have 3103led to problems extremely difficult to resolve in the long term. 3104 3105Multiple controllers struggled with internal tasks and came up with 3106different ways to deal with it; unfortunately, all the approaches were 3107severely flawed and, furthermore, the widely different behaviors 3108made cgroup as a whole highly inconsistent. 3109 3110This clearly is a problem which needs to be addressed from cgroup core 3111in a uniform way. 3112 3113 3114Other Interface Issues 3115---------------------- 3116 3117cgroup v1 grew without oversight and developed a large number of 3118idiosyncrasies and inconsistencies. One issue on the cgroup core side 3119was how an empty cgroup was notified - a userland helper binary was 3120forked and executed for each event. The event delivery wasn't 3121recursive or delegatable. The limitations of the mechanism also led 3122to in-kernel event delivery filtering mechanism further complicating 3123the interface. 3124 3125Controller interfaces were problematic too. An extreme example is 3126controllers completely ignoring hierarchical organization and treating 3127all cgroups as if they were all located directly under the root 3128cgroup. Some controllers exposed a large amount of inconsistent 3129implementation details to userland. 3130 3131There also was no consistency across controllers. When a new cgroup 3132was created, some controllers defaulted to not imposing extra 3133restrictions while others disallowed any resource usage until 3134explicitly configured. Configuration knobs for the same type of 3135control used widely differing naming schemes and formats. Statistics 3136and information knobs were named arbitrarily and used different 3137formats and units even in the same controller. 3138 3139cgroup v2 establishes common conventions where appropriate and updates 3140controllers so that they expose minimal and consistent interfaces. 3141 3142 3143Controller Issues and Remedies 3144------------------------------ 3145 3146Memory 3147~~~~~~ 3148 3149The original lower boundary, the soft limit, is defined as a limit 3150that is per default unset. As a result, the set of cgroups that 3151global reclaim prefers is opt-in, rather than opt-out. The costs for 3152optimizing these mostly negative lookups are so high that the 3153implementation, despite its enormous size, does not even provide the 3154basic desirable behavior. First off, the soft limit has no 3155hierarchical meaning. All configured groups are organized in a global 3156rbtree and treated like equal peers, regardless where they are located 3157in the hierarchy. This makes subtree delegation impossible. Second, 3158the soft limit reclaim pass is so aggressive that it not just 3159introduces high allocation latencies into the system, but also impacts 3160system performance due to overreclaim, to the point where the feature 3161becomes self-defeating. 3162 3163The memory.low boundary on the other hand is a top-down allocated 3164reserve. A cgroup enjoys reclaim protection when it's within its 3165effective low, which makes delegation of subtrees possible. It also 3166enjoys having reclaim pressure proportional to its overage when 3167above its effective low. 3168 3169The original high boundary, the hard limit, is defined as a strict 3170limit that can not budge, even if the OOM killer has to be called. 3171But this generally goes against the goal of making the most out of the 3172available memory. The memory consumption of workloads varies during 3173runtime, and that requires users to overcommit. But doing that with a 3174strict upper limit requires either a fairly accurate prediction of the 3175working set size or adding slack to the limit. Since working set size 3176estimation is hard and error prone, and getting it wrong results in 3177OOM kills, most users tend to err on the side of a looser limit and 3178end up wasting precious resources. 3179 3180The memory.high boundary on the other hand can be set much more 3181conservatively. When hit, it throttles allocations by forcing them 3182into direct reclaim to work off the excess, but it never invokes the 3183OOM killer. As a result, a high boundary that is chosen too 3184aggressively will not terminate the processes, but instead it will 3185lead to gradual performance degradation. The user can monitor this 3186and make corrections until the minimal memory footprint that still 3187gives acceptable performance is found. 3188 3189In extreme cases, with many concurrent allocations and a complete 3190breakdown of reclaim progress within the group, the high boundary can 3191be exceeded. But even then it's mostly better to satisfy the 3192allocation from the slack available in other groups or the rest of the 3193system than killing the group. Otherwise, memory.max is there to 3194limit this type of spillover and ultimately contain buggy or even 3195malicious applications. 3196 3197Setting the original memory.limit_in_bytes below the current usage was 3198subject to a race condition, where concurrent charges could cause the 3199limit setting to fail. memory.max on the other hand will first set the 3200limit to prevent new charges, and then reclaim and OOM kill until the 3201new limit is met - or the task writing to memory.max is killed. 3202 3203The combined memory+swap accounting and limiting is replaced by real 3204control over swap space. 3205 3206The main argument for a combined memory+swap facility in the original 3207cgroup design was that global or parental pressure would always be 3208able to swap all anonymous memory of a child group, regardless of the 3209child's own (possibly untrusted) configuration. However, untrusted 3210groups can sabotage swapping by other means - such as referencing its 3211anonymous memory in a tight loop - and an admin can not assume full 3212swappability when overcommitting untrusted jobs. 3213 3214For trusted jobs, on the other hand, a combined counter is not an 3215intuitive userspace interface, and it flies in the face of the idea 3216that cgroup controllers should account and limit specific physical 3217resources. Swap space is a resource like all others in the system, 3218and that's why unified hierarchy allows distributing it separately. 3219