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