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