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authorTejun Heo <tj@kernel.org>2015-11-16 11:13:34 -0500
committerAlex Shi <alex.shi@linaro.org>2016-11-29 15:25:30 +0800
commitfe10f4e69509a4c384e550e3404b2606f4fd8b87 (patch)
tree3db5212033a2664302ee490012198164cf6af779
parent07d48acbfee2ac01687514ab6bf3144bbd2c0e13 (diff)
downloadlinux-linaro-stable-v4.4/topic/wb-cg2.tar.gz
cgroup: replace unified-hierarchy.txt with a proper cgroup v2 documentationv4.4/topic/wb-cg2
Now that cgroup v2 is almost out of the door, replace the development documentation unified-hierarchy.txt with Documentation/cgroup.txt which is a superset of unified-hierarchy.txt and authoritatively describes all userland-visible aspects of cgroup. v2: Updated to include all information from blkio-controller.txt and list filesystems which support cgroup writeback as suggested by Vivek. Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Li Zefan <lizefan@huawei.com> Cc: Vivek Goyal <vgoyal@redhat.com> (cherry picked from commit 6c2920926b10e8303378408e3c2b8952071d4344) Signed-off-by: Alex Shi <alex.shi@linaro.org>
-rw-r--r--Documentation/cgroup-legacy/blkio-controller.txt79
-rw-r--r--Documentation/cgroup-legacy/unified-hierarchy.txt645
-rw-r--r--Documentation/cgroup.txt1293
3 files changed, 1293 insertions, 724 deletions
diff --git a/Documentation/cgroup-legacy/blkio-controller.txt b/Documentation/cgroup-legacy/blkio-controller.txt
index 52fa9f3..4ecc954 100644
--- a/Documentation/cgroup-legacy/blkio-controller.txt
+++ b/Documentation/cgroup-legacy/blkio-controller.txt
@@ -374,82 +374,3 @@ One can experience an overall throughput drop if you have created multiple
groups and put applications in that group which are not driving enough
IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle
on individual groups and throughput should improve.
-
-Writeback
-=========
-
-Page cache is dirtied through buffered writes and shared mmaps and
-written asynchronously to the backing filesystem by the writeback
-mechanism. Writeback sits between the memory and IO domains and
-regulates the proportion of dirty memory by balancing dirtying and
-write IOs.
-
-On traditional cgroup hierarchies, relationships between different
-controllers cannot be established making it impossible for writeback
-to operate accounting for cgroup resource restrictions and all
-writeback IOs are attributed to the root cgroup.
-
-If both the blkio and memory controllers are used on the v2 hierarchy
-and the filesystem supports cgroup writeback, writeback operations
-correctly follow the resource restrictions imposed by both memory and
-blkio controllers.
-
-Writeback examines both system-wide and per-cgroup dirty memory status
-and enforces the more restrictive of the two. Also, writeback control
-parameters which are absolute values - vm.dirty_bytes and
-vm.dirty_background_bytes - are distributed across cgroups according
-to their current writeback bandwidth.
-
-There's a peculiarity stemming from the discrepancy in ownership
-granularity between memory controller and writeback. While memory
-controller tracks ownership per page, writeback operates on inode
-basis. cgroup writeback bridges the gap by tracking ownership by
-inode but migrating ownership if too many foreign pages, pages which
-don't match the current inode ownership, have been encountered while
-writing back the inode.
-
-This is a conscious design choice as writeback operations are
-inherently tied to inodes making strictly following page ownership
-complicated and inefficient. The only use case which suffers from
-this compromise is multiple cgroups concurrently dirtying disjoint
-regions of the same inode, which is an unlikely use case and decided
-to be unsupported. Note that as memory controller assigns page
-ownership on the first use and doesn't update it until the page is
-released, even if cgroup writeback strictly follows page ownership,
-multiple cgroups dirtying overlapping areas wouldn't work as expected.
-In general, write-sharing an inode across multiple cgroups is not well
-supported.
-
-Filesystem support for cgroup writeback
----------------------------------------
-
-A filesystem can make writeback IOs cgroup-aware by updating
-address_space_operations->writepage[s]() to annotate bio's using the
-following two functions.
-
-* wbc_init_bio(@wbc, @bio)
-
- Should be called for each bio carrying writeback data and associates
- the bio with the inode's owner cgroup. Can be called anytime
- between bio allocation and submission.
-
-* wbc_account_io(@wbc, @page, @bytes)
-
- Should be called for each data segment being written out. While
- this function doesn't care exactly when it's called during the
- writeback session, it's the easiest and most natural to call it as
- data segments are added to a bio.
-
-With writeback bio's annotated, cgroup support can be enabled per
-super_block by setting MS_CGROUPWB in ->s_flags. This allows for
-selective disabling of cgroup writeback support which is helpful when
-certain filesystem features, e.g. journaled data mode, are
-incompatible.
-
-wbc_init_bio() binds the specified bio to its cgroup. Depending on
-the configuration, the bio may be executed at a lower priority and if
-the writeback session is holding shared resources, e.g. a journal
-entry, may lead to priority inversion. There is no one easy solution
-for the problem. Filesystems can try to work around specific problem
-cases by skipping wbc_init_bio() or using bio_associate_blkcg()
-directly.
diff --git a/Documentation/cgroup-legacy/unified-hierarchy.txt b/Documentation/cgroup-legacy/unified-hierarchy.txt
deleted file mode 100644
index c1f0e87..0000000
--- a/Documentation/cgroup-legacy/unified-hierarchy.txt
+++ /dev/null
@@ -1,645 +0,0 @@
-
-Cgroup unified hierarchy
-
-April, 2014 Tejun Heo <tj@kernel.org>
-
-This document describes the changes made by unified hierarchy and
-their rationales. It will eventually be merged into the main cgroup
-documentation.
-
-CONTENTS
-
-1. Background
-2. Basic Operation
- 2-1. Mounting
- 2-2. cgroup.subtree_control
- 2-3. cgroup.controllers
-3. Structural Constraints
- 3-1. Top-down
- 3-2. No internal tasks
-4. Delegation
- 4-1. Model of delegation
- 4-2. Common ancestor rule
-5. Other Changes
- 5-1. [Un]populated Notification
- 5-2. Other Core Changes
- 5-3. Controller File Conventions
- 5-3-1. Format
- 5-3-2. Control Knobs
- 5-4. Per-Controller Changes
- 5-4-1. io
- 5-4-2. cpuset
- 5-4-3. memory
-6. Planned Changes
- 6-1. CAP for resource control
-
-
-1. Background
-
-cgroup allows an arbitrary number of hierarchies and each hierarchy
-can host any number of controllers. While this seems to provide a
-high level of flexibility, it isn't quite useful in practice.
-
-For example, as there is only one instance of each controller, utility
-type controllers such as freezer which can be useful in all
-hierarchies can only be used in one. The issue is exacerbated by the
-fact that controllers can't be moved around once hierarchies are
-populated. Another issue is that all controllers bound to a hierarchy
-are forced to have exactly the same view of the hierarchy. It isn't
-possible to vary the granularity depending on the specific controller.
-
-In practice, these issues heavily limit which controllers can be put
-on the same hierarchy and most configurations resort to putting each
-controller on its own hierarchy. Only closely related ones, such as
-the cpu and cpuacct controllers, make sense to put on the same
-hierarchy. This often means that userland ends up managing multiple
-similar hierarchies repeating the same steps on each hierarchy
-whenever a hierarchy management operation is necessary.
-
-Unfortunately, support for multiple hierarchies comes at a steep cost.
-Internal implementation in cgroup core proper is dazzlingly
-complicated but more importantly the support for multiple hierarchies
-restricts how cgroup is used in general and what controllers can do.
-
-There's no limit on how many hierarchies there may be, which means
-that a task's cgroup membership can't be described in finite length.
-The key may contain any varying number of entries and is unlimited in
-length, which makes it highly awkward to handle and leads to addition
-of controllers which exist only to identify membership, which in turn
-exacerbates the original problem.
-
-Also, as a controller can't have any expectation regarding what shape
-of hierarchies other controllers would be on, each controller has to
-assume that all other controllers are operating on completely
-orthogonal hierarchies. This makes it impossible, or at least very
-cumbersome, for controllers to cooperate with each other.
-
-In most use cases, putting controllers on hierarchies which are
-completely orthogonal to each other isn't necessary. What usually is
-called for is the ability to have differing levels of granularity
-depending on the specific controller. In other words, hierarchy may
-be collapsed from leaf towards root when viewed from specific
-controllers. For example, a given configuration might not care about
-how memory is distributed beyond a certain level while still wanting
-to control how CPU cycles are distributed.
-
-Unified hierarchy is the next version of cgroup interface. It aims to
-address the aforementioned issues by having more structure while
-retaining enough flexibility for most use cases. Various other
-general and controller-specific interface issues are also addressed in
-the process.
-
-
-2. Basic Operation
-
-2-1. Mounting
-
-Unified hierarchy can be mounted with the following mount command.
-
- mount -t cgroup2 none $MOUNT_POINT
-
-All controllers which support the unified hierarchy and are not bound
-to other hierarchies are automatically bound to unified hierarchy and
-show up at the root of it. Controllers which are enabled only in the
-root of unified hierarchy can be bound to other hierarchies. This
-allows mixing unified hierarchy with the traditional multiple
-hierarchies in a fully backward compatible way.
-
-A controller can be moved across hierarchies only after the controller
-is no longer referenced in its current hierarchy. Because per-cgroup
-controller states are destroyed asynchronously and controllers may
-have lingering references, a controller may not show up immediately on
-the unified hierarchy after the final umount of the previous
-hierarchy. Similarly, a controller should be fully disabled to be
-moved out of the unified hierarchy and it may take some time for the
-disabled controller to become available for other hierarchies;
-furthermore, due to dependencies among controllers, other controllers
-may need to be disabled too.
-
-While useful for development and manual configurations, dynamically
-moving controllers between the unified and other hierarchies is
-strongly discouraged for production use. It is recommended to decide
-the hierarchies and controller associations before starting using the
-controllers.
-
-
-2-2. cgroup.subtree_control
-
-All cgroups on unified hierarchy have a "cgroup.subtree_control" file
-which governs which controllers are enabled on the children of the
-cgroup. Let's assume a hierarchy like the following.
-
- root - A - B - C
- \ D
-
-root's "cgroup.subtree_control" file determines which controllers are
-enabled on A. A's on B. B's on C and D. This coincides with the
-fact that controllers on the immediate sub-level are used to
-distribute the resources of the parent. In fact, it's natural to
-assume that resource control knobs of a child belong to its parent.
-Enabling a controller in a "cgroup.subtree_control" file declares that
-distribution of the respective resources of the cgroup will be
-controlled. Note that this means that controller enable states are
-shared among siblings.
-
-When read, the file contains a space-separated list of currently
-enabled controllers. A write to the file should contain a
-space-separated list of controllers with '+' or '-' prefixed (without
-the quotes). Controllers prefixed with '+' are enabled and '-'
-disabled. If a controller is listed multiple times, the last entry
-wins. The specific operations are executed atomically - either all
-succeed or fail.
-
-
-2-3. cgroup.controllers
-
-Read-only "cgroup.controllers" file contains a space-separated list of
-controllers which can be enabled in the cgroup's
-"cgroup.subtree_control" file.
-
-In the root cgroup, this lists controllers which are not bound to
-other hierarchies and the content changes as controllers are bound to
-and unbound from other hierarchies.
-
-In non-root cgroups, the content of this file equals that of the
-parent's "cgroup.subtree_control" file as only controllers enabled
-from the parent can be used in its children.
-
-
-3. Structural Constraints
-
-3-1. Top-down
-
-As it doesn't make sense to nest control of an uncontrolled resource,
-all non-root "cgroup.subtree_control" files can only contain
-controllers which are enabled in the parent's "cgroup.subtree_control"
-file. A controller can be enabled only if the parent has the
-controller enabled and a controller can't be disabled if one or more
-children have it enabled.
-
-
-3-2. No internal tasks
-
-One long-standing issue that cgroup faces is the competition between
-tasks belonging to the parent cgroup and its children cgroups. This
-is inherently nasty as two different types of entities compete and
-there is no agreed-upon obvious way to handle it. Different
-controllers are doing different things.
-
-The cpu controller considers tasks and cgroups as equivalents and maps
-nice levels to cgroup weights. This works for some cases but falls
-flat when children should be allocated specific ratios of CPU cycles
-and the number of internal tasks fluctuates - the ratios constantly
-change as the number of competing entities fluctuates. There also are
-other issues. The mapping from nice level to weight isn't obvious or
-universal, and there are various other knobs which simply aren't
-available for tasks.
-
-The io controller implicitly creates a hidden leaf node for each
-cgroup to host the tasks. The hidden leaf has its own copies of all
-the knobs with "leaf_" prefixed. While this allows equivalent control
-over internal tasks, it's with serious drawbacks. It always adds an
-extra layer of nesting which may not be necessary, makes the interface
-messy and significantly complicates the implementation.
-
-The memory controller currently doesn't have a way to control what
-happens between internal tasks and child cgroups and the behavior is
-not clearly defined. There have been attempts to add ad-hoc behaviors
-and knobs to tailor the behavior to specific workloads. Continuing
-this direction will lead to problems which will be extremely difficult
-to resolve in the long term.
-
-Multiple controllers struggle with internal tasks and came up with
-different ways to deal with it; unfortunately, all the approaches in
-use now are severely flawed and, furthermore, the widely different
-behaviors make cgroup as whole highly inconsistent.
-
-It is clear that this is something which needs to be addressed from
-cgroup core proper in a uniform way so that controllers don't need to
-worry about it and cgroup as a whole shows a consistent and logical
-behavior. To achieve that, unified hierarchy enforces the following
-structural constraint:
-
- Except for the root, only cgroups which don't contain any task may
- have controllers enabled in their "cgroup.subtree_control" files.
-
-Combined with other properties, this guarantees that, when a
-controller is looking at the part of the hierarchy which has it
-enabled, tasks are always only on the leaves. This rules out
-situations where child cgroups compete against internal tasks of the
-parent.
-
-There are two things to note. Firstly, the root cgroup is exempt from
-the restriction. Root contains tasks and anonymous resource
-consumption which can't be associated with any other cgroup and
-requires special treatment from most controllers. How resource
-consumption in the root cgroup is governed is up to each controller.
-
-Secondly, the restriction doesn't take effect if there is no enabled
-controller in the cgroup's "cgroup.subtree_control" file. This is
-important as otherwise it wouldn't be possible to create children of a
-populated cgroup. To control resource distribution of a cgroup, the
-cgroup must create children and transfer all its tasks to the children
-before enabling controllers in its "cgroup.subtree_control" file.
-
-
-4. Delegation
-
-4-1. Model of delegation
-
-A cgroup can be delegated to a less privileged user by granting write
-access of the directory and its "cgroup.procs" file to the user. Note
-that the resource control knobs in a given directory concern the
-resources of the parent and thus must not be delegated along with the
-directory.
-
-Once delegated, the user can build sub-hierarchy under the directory,
-organize processes as it sees fit and further distribute the resources
-it got from the parent. The limits and other settings of all resource
-controllers are hierarchical and regardless of what happens in the
-delegated sub-hierarchy, nothing can escape the resource restrictions
-imposed by the parent.
-
-Currently, cgroup doesn't impose any restrictions on the number of
-cgroups in or nesting depth of a delegated sub-hierarchy; however,
-this may in the future be limited explicitly.
-
-
-4-2. Common ancestor rule
-
-On the unified hierarchy, to write to a "cgroup.procs" file, in
-addition to the usual write permission to the file and uid match, the
-writer must also have write access to the "cgroup.procs" file of the
-common ancestor of the source and destination cgroups. This prevents
-delegatees from smuggling processes across disjoint sub-hierarchies.
-
-Let's say cgroups C0 and C1 have been delegated to user U0 who created
-C00, C01 under C0 and C10 under C1 as follows.
-
- ~~~~~~~~~~~~~ - C0 - C00
- ~ cgroup ~ \ C01
- ~ hierarchy ~
- ~~~~~~~~~~~~~ - C1 - C10
-
-C0 and C1 are separate entities in terms of resource distribution
-regardless of their relative positions in the hierarchy. The
-resources the processes under C0 are entitled to are controlled by
-C0's ancestors and may be completely different from C1. It's clear
-that the intention of delegating C0 to U0 is allowing U0 to organize
-the processes under C0 and further control the distribution of C0's
-resources.
-
-On traditional hierarchies, if a task has write access to "tasks" or
-"cgroup.procs" file of a cgroup and its uid agrees with the target, it
-can move the target to the cgroup. In the above example, U0 will not
-only be able to move processes in each sub-hierarchy but also across
-the two sub-hierarchies, effectively allowing it to violate the
-organizational and resource restrictions implied by the hierarchical
-structure above C0 and C1.
-
-On the unified hierarchy, let's say U0 wants to write the pid of a
-process which has a matching uid and is currently in C10 into
-"C00/cgroup.procs". U0 obviously has write access to the file and
-migration permission on the process; however, the common ancestor of
-the source cgroup C10 and the destination cgroup C00 is above the
-points of delegation and U0 would not have write access to its
-"cgroup.procs" and thus be denied with -EACCES.
-
-
-5. Other Changes
-
-5-1. [Un]populated Notification
-
-cgroup users often need a way to determine when a cgroup's
-subhierarchy becomes empty so that it can be cleaned up. cgroup
-currently provides release_agent for it; unfortunately, this mechanism
-is riddled with issues.
-
-- It delivers events by forking and execing a userland binary
- specified as the release_agent. This is a long deprecated method of
- notification delivery. It's extremely heavy, slow and cumbersome to
- integrate with larger infrastructure.
-
-- There is single monitoring point at the root. There's no way to
- delegate management of a subtree.
-
-- The event isn't recursive. It triggers when a cgroup doesn't have
- any tasks or child cgroups. Events for internal nodes trigger only
- after all children are removed. This again makes it impossible to
- delegate management of a subtree.
-
-- Events are filtered from the kernel side. A "notify_on_release"
- file is used to subscribe to or suppress release events. This is
- unnecessarily complicated and probably done this way because event
- delivery itself was expensive.
-
-Unified hierarchy implements "populated" field in "cgroup.events"
-interface file which can be used to monitor whether the cgroup's
-subhierarchy has tasks in it or not. Its value is 0 if there is no
-task in the cgroup and its descendants; otherwise, 1. poll and
-[id]notify events are triggered when the value changes.
-
-This is significantly lighter and simpler and trivially allows
-delegating management of subhierarchy - subhierarchy monitoring can
-block further propagation simply by putting itself or another process
-in the subhierarchy and monitor events that it's interested in from
-there without interfering with monitoring higher in the tree.
-
-In unified hierarchy, the release_agent mechanism is no longer
-supported and the interface files "release_agent" and
-"notify_on_release" do not exist.
-
-
-5-2. Other Core Changes
-
-- None of the mount options is allowed.
-
-- remount is disallowed.
-
-- rename(2) is disallowed.
-
-- The "tasks" file is removed. Everything should at process
- granularity. Use the "cgroup.procs" file instead.
-
-- The "cgroup.procs" file is not sorted. pids will be unique unless
- they got recycled in-between reads.
-
-- The "cgroup.clone_children" file is removed.
-
-- /proc/PID/cgroup keeps reporting the cgroup that a zombie belonged
- to before exiting. If the cgroup is removed before the zombie is
- reaped, " (deleted)" is appeneded to the path.
-
-
-5-3. Controller File Conventions
-
-5-3-1. Format
-
-In general, all controller files should be in one of the following
-formats whenever possible.
-
-- Values only files
-
- VAL0 VAL1...\n
-
-- Flat keyed files
-
- KEY0 VAL0\n
- KEY1 VAL1\n
- ...
-
-- Nested keyed files
-
- KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
- KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
- ...
-
-For a writeable file, the format for writing should generally match
-reading; however, controllers may allow omitting later fields or
-implement restricted shortcuts for most common use cases.
-
-For both flat and nested keyed files, only the values for a single key
-can be written at a time. For nested keyed files, the sub key pairs
-may be specified in any order and not all pairs have to be specified.
-
-
-5-3-2. Control Knobs
-
-- Settings for a single feature should generally be implemented in a
- single file.
-
-- In general, the root cgroup should be exempt from resource control
- and thus shouldn't have resource control knobs.
-
-- If a controller implements ratio based resource distribution, the
- control knob should be named "weight" and have the range [1, 10000]
- and 100 should be the default value. The values are chosen to allow
- enough and symmetric bias in both directions while keeping it
- intuitive (the default is 100%).
-
-- If a controller implements an absolute resource guarantee and/or
- limit, the control knobs should be named "min" and "max"
- respectively. If a controller implements best effort resource
- gurantee and/or limit, the control knobs should be named "low" and
- "high" respectively.
-
- In the above four control files, the special token "max" should be
- used to represent upward infinity for both reading and writing.
-
-- If a setting has configurable default value and specific overrides,
- the default settings should be keyed with "default" and appear as
- the first entry in the file. Specific entries can use "default" as
- its value to indicate inheritance of the default value.
-
-- For events which are not very high frequency, an interface file
- "events" should be created which lists event key value pairs.
- Whenever a notifiable event happens, file modified event should be
- generated on the file.
-
-
-5-4. Per-Controller Changes
-
-5-4-1. io
-
-- blkio is renamed to io. The interface is overhauled anyway. The
- new name is more in line with the other two major controllers, cpu
- and memory, and better suited given that it may be used for cgroup
- writeback without involving block layer.
-
-- Everything including stat is always hierarchical making separate
- recursive stat files pointless and, as no internal node can have
- tasks, leaf weights are meaningless. The operation model is
- simplified and the interface is overhauled accordingly.
-
- io.stat
-
- The stat file. The reported stats are from the point where
- bio's are issued to request_queue. The stats are counted
- independent of which policies are enabled. Each line in the
- file follows the following format. More fields may later be
- added at the end.
-
- $MAJ:$MIN rbytes=$RBYTES wbytes=$WBYTES rios=$RIOS wrios=$WIOS
-
- io.weight
-
- The weight setting, currently only available and effective if
- cfq-iosched is in use for the target device. The weight is
- between 1 and 10000 and defaults to 100. The first line
- always contains the default weight in the following format to
- use when per-device setting is missing.
-
- default $WEIGHT
-
- Subsequent lines list per-device weights of the following
- format.
-
- $MAJ:$MIN $WEIGHT
-
- Writing "$WEIGHT" or "default $WEIGHT" changes the default
- setting. Writing "$MAJ:$MIN $WEIGHT" sets per-device weight
- while "$MAJ:$MIN default" clears it.
-
- This file is available only on non-root cgroups.
-
- io.max
-
- The maximum bandwidth and/or iops setting, only available if
- blk-throttle is enabled. The file is of the following format.
-
- $MAJ:$MIN rbps=$RBPS wbps=$WBPS riops=$RIOPS wiops=$WIOPS
-
- ${R|W}BPS are read/write bytes per second and ${R|W}IOPS are
- read/write IOs per second. "max" indicates no limit. Writing
- to the file follows the same format but the individual
- settings may be omitted or specified in any order.
-
- This file is available only on non-root cgroups.
-
-
-5-4-2. cpuset
-
-- Tasks are kept in empty cpusets after hotplug and take on the masks
- of the nearest non-empty ancestor, instead of being moved to it.
-
-- A task can be moved into an empty cpuset, and again it takes on the
- masks of the nearest non-empty ancestor.
-
-
-5-4-3. memory
-
-- use_hierarchy is on by default and the cgroup file for the flag is
- not created.
-
-- The original lower boundary, the soft limit, is defined as a limit
- that is per default unset. As a result, the set of cgroups that
- global reclaim prefers is opt-in, rather than opt-out. The costs
- for optimizing these mostly negative lookups are so high that the
- implementation, despite its enormous size, does not even provide the
- basic desirable behavior. First off, the soft limit has no
- hierarchical meaning. All configured groups are organized in a
- global rbtree and treated like equal peers, regardless where they
- are located in the hierarchy. This makes subtree delegation
- impossible. Second, the soft limit reclaim pass is so aggressive
- that it not just introduces high allocation latencies into the
- system, but also impacts system performance due to overreclaim, to
- the point where the feature becomes self-defeating.
-
- The memory.low boundary on the other hand is a top-down allocated
- reserve. A cgroup enjoys reclaim protection when it and all its
- ancestors are below their low boundaries, which makes delegation of
- subtrees possible. Secondly, new cgroups have no reserve per
- default and in the common case most cgroups are eligible for the
- preferred reclaim pass. This allows the new low boundary to be
- efficiently implemented with just a minor addition to the generic
- reclaim code, without the need for out-of-band data structures and
- reclaim passes. Because the generic reclaim code considers all
- cgroups except for the ones running low in the preferred first
- reclaim pass, overreclaim of individual groups is eliminated as
- well, resulting in much better overall workload performance.
-
-- The original high boundary, the hard limit, is defined as a strict
- limit that can not budge, even if the OOM killer has to be called.
- But this generally goes against the goal of making the most out of
- the available memory. The memory consumption of workloads varies
- during runtime, and that requires users to overcommit. But doing
- that with a strict upper limit requires either a fairly accurate
- prediction of the working set size or adding slack to the limit.
- Since working set size estimation is hard and error prone, and
- getting it wrong results in OOM kills, most users tend to err on the
- side of a looser limit and end up wasting precious resources.
-
- The memory.high boundary on the other hand can be set much more
- conservatively. When hit, it throttles allocations by forcing them
- into direct reclaim to work off the excess, but it never invokes the
- OOM killer. As a result, a high boundary that is chosen too
- aggressively will not terminate the processes, but instead it will
- lead to gradual performance degradation. The user can monitor this
- and make corrections until the minimal memory footprint that still
- gives acceptable performance is found.
-
- In extreme cases, with many concurrent allocations and a complete
- breakdown of reclaim progress within the group, the high boundary
- can be exceeded. But even then it's mostly better to satisfy the
- allocation from the slack available in other groups or the rest of
- the system than killing the group. Otherwise, memory.max is there
- to limit this type of spillover and ultimately contain buggy or even
- malicious applications.
-
-- The original control file names are unwieldy and inconsistent in
- many different ways. For example, the upper boundary hit count is
- exported in the memory.failcnt file, but an OOM event count has to
- be manually counted by listening to memory.oom_control events, and
- lower boundary / soft limit events have to be counted by first
- setting a threshold for that value and then counting those events.
- Also, usage and limit files encode their units in the filename.
- That makes the filenames very long, even though this is not
- information that a user needs to be reminded of every time they type
- out those names.
-
- To address these naming issues, as well as to signal clearly that
- the new interface carries a new configuration model, the naming
- conventions in it necessarily differ from the old interface.
-
-- The original limit files indicate the state of an unset limit with a
- Very High Number, and a configured limit can be unset by echoing -1
- into those files. But that very high number is implementation and
- architecture dependent and not very descriptive. And while -1 can
- be understood as an underflow into the highest possible value, -2 or
- -10M etc. do not work, so it's not consistent.
-
- memory.low, memory.high, and memory.max will use the string "max" to
- indicate and set the highest possible value.
-
-6. Planned Changes
-
-6-1. CAP for resource control
-
-Unified hierarchy will require one of the capabilities(7), which is
-yet to be decided, for all resource control related knobs. Process
-organization operations - creation of sub-cgroups and migration of
-processes in sub-hierarchies may be delegated by changing the
-ownership and/or permissions on the cgroup directory and
-"cgroup.procs" interface file; however, all operations which affect
-resource control - writes to a "cgroup.subtree_control" file or any
-controller-specific knobs - will require an explicit CAP privilege.
-
-This, in part, is to prevent the cgroup interface from being
-inadvertently promoted to programmable API used by non-privileged
-binaries. cgroup exposes various aspects of the system in ways which
-aren't properly abstracted for direct consumption by regular programs.
-This is an administration interface much closer to sysctl knobs than
-system calls. Even the basic access model, being filesystem path
-based, isn't suitable for direct consumption. There's no way to
-access "my cgroup" in a race-free way or make multiple operations
-atomic against migration to another cgroup.
-
-Another aspect is that, for better or for worse, the cgroup interface
-goes through far less scrutiny than regular interfaces for
-unprivileged userland. The upside is that cgroup is able to expose
-useful features which may not be suitable for general consumption in a
-reasonable time frame. It provides a relatively short path between
-internal details and userland-visible interface. Of course, this
-shortcut comes with high risk. We go through what we go through for
-general kernel APIs for good reasons. It may end up leaking internal
-details in a way which can exert significant pain by locking the
-kernel into a contract that can't be maintained in a reasonable
-manner.
-
-Also, due to the specific nature, cgroup and its controllers don't
-tend to attract attention from a wide scope of developers. cgroup's
-short history is already fraught with severely mis-designed
-interfaces, unnecessary commitments to and exposing of internal
-details, broken and dangerous implementations of various features.
-
-Keeping cgroup as an administration interface is both advantageous for
-its role and imperative given its nature. Some of the cgroup features
-may make sense for unprivileged access. If deemed justified, those
-must be further abstracted and implemented as a different interface,
-be it a system call or process-private filesystem, and survive through
-the scrutiny that any interface for general consumption is required to
-go through.
-
-Requiring CAP is not a complete solution but should serve as a
-significant deterrent against spraying cgroup usages in non-privileged
-programs.
diff --git a/Documentation/cgroup.txt b/Documentation/cgroup.txt
new file mode 100644
index 0000000..31d1f7b
--- /dev/null
+++ b/Documentation/cgroup.txt
@@ -0,0 +1,1293 @@
+
+Control Group v2
+
+October, 2015 Tejun Heo <tj@kernel.org>
+
+This is the authoritative documentation on the design, interface and
+conventions of cgroup v2. It describes all userland-visible aspects
+of cgroup including core and specific controller behaviors. All
+future changes must be reflected in this document. Documentation for
+v1 is available under Documentation/cgroup-legacy/.
+
+CONTENTS
+
+1. Introduction
+ 1-1. Terminology
+ 1-2. What is cgroup?
+2. Basic Operations
+ 2-1. Mounting
+ 2-2. Organizing Processes
+ 2-3. [Un]populated Notification
+ 2-4. Controlling Controllers
+ 2-4-1. Enabling and Disabling
+ 2-4-2. Top-down Constraint
+ 2-4-3. No Internal Process Constraint
+ 2-5. Delegation
+ 2-5-1. Model of Delegation
+ 2-5-2. Delegation Containment
+ 2-6. Guidelines
+ 2-6-1. Organize Once and Control
+ 2-6-2. Avoid Name Collisions
+3. Resource Distribution Models
+ 3-1. Weights
+ 3-2. Limits
+ 3-3. Protections
+ 3-4. Allocations
+4. Interface Files
+ 4-1. Format
+ 4-2. Conventions
+ 4-3. Core Interface Files
+5. Controllers
+ 5-1. CPU
+ 5-1-1. CPU Interface Files
+ 5-2. Memory
+ 5-2-1. Memory Interface Files
+ 5-2-2. Usage Guidelines
+ 5-2-3. Memory Ownership
+ 5-3. IO
+ 5-3-1. IO Interface Files
+ 5-3-2. Writeback
+P. Information on Kernel Programming
+ P-1. Filesystem Support for Writeback
+D. Deprecated v1 Core Features
+R. Issues with v1 and Rationales for v2
+ R-1. Multiple Hierarchies
+ R-2. Thread Granularity
+ R-3. Competition Between Inner Nodes and Threads
+ R-4. Other Interface Issues
+ R-5. Controller Issues and Remedies
+ R-5-1. Memory
+
+
+1. Introduction
+
+1-1. Terminology
+
+"cgroup" stands for "control group" and is never capitalized. The
+singular form is used to designate the whole feature and also as a
+qualifier as in "cgroup controllers". When explicitly referring to
+multiple individual control groups, the plural form "cgroups" is used.
+
+
+1-2. What is cgroup?
+
+cgroup is a mechanism to organize processes hierarchically and
+distribute system resources along the hierarchy in a controlled and
+configurable manner.
+
+cgroup is largely composed of two parts - the core and controllers.
+cgroup core is primarily responsible for hierarchically organizing
+processes. A cgroup controller is usually responsible for
+distributing a specific type of system resource along the hierarchy
+although there are utility controllers which serve purposes other than
+resource distribution.
+
+cgroups form a tree structure and every process in the system belongs
+to one and only one cgroup. All threads of a process belong to the
+same cgroup. On creation, all processes are put in the cgroup that
+the parent process belongs to at the time. A process can be migrated
+to another cgroup. Migration of a process doesn't affect already
+existing descendant processes.
+
+Following certain structural constraints, controllers may be enabled or
+disabled selectively on a cgroup. All controller behaviors are
+hierarchical - if a controller is enabled on a cgroup, it affects all
+processes which belong to the cgroups consisting the inclusive
+sub-hierarchy of the cgroup. When a controller is enabled on a nested
+cgroup, it always restricts the resource distribution further. The
+restrictions set closer to the root in the hierarchy can not be
+overridden from further away.
+
+
+2. Basic Operations
+
+2-1. Mounting
+
+Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2
+hierarchy can be mounted with the following mount command.
+
+ # mount -t cgroup2 none $MOUNT_POINT
+
+cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All
+controllers which support v2 and are not bound to a v1 hierarchy are
+automatically bound to the v2 hierarchy and show up at the root.
+Controllers which are not in active use in the v2 hierarchy can be
+bound to other hierarchies. This allows mixing v2 hierarchy with the
+legacy v1 multiple hierarchies in a fully backward compatible way.
+
+A controller can be moved across hierarchies only after the controller
+is no longer referenced in its current hierarchy. Because per-cgroup
+controller states are destroyed asynchronously and controllers may
+have lingering references, a controller may not show up immediately on
+the v2 hierarchy after the final umount of the previous hierarchy.
+Similarly, a controller should be fully disabled to be moved out of
+the unified hierarchy and it may take some time for the disabled
+controller to become available for other hierarchies; furthermore, due
+to inter-controller dependencies, other controllers may need to be
+disabled too.
+
+While useful for development and manual configurations, moving
+controllers dynamically between the v2 and other hierarchies is
+strongly discouraged for production use. It is recommended to decide
+the hierarchies and controller associations before starting using the
+controllers after system boot.
+
+
+2-2. Organizing Processes
+
+Initially, only the root cgroup exists to which all processes belong.
+A child cgroup can be created by creating a sub-directory.
+
+ # mkdir $CGROUP_NAME
+
+A given cgroup may have multiple child cgroups forming a tree
+structure. Each cgroup has a read-writable interface file
+"cgroup.procs". When read, it lists the PIDs of all processes which
+belong to the cgroup one-per-line. The PIDs are not ordered and the
+same PID may show up more than once if the process got moved to
+another cgroup and then back or the PID got recycled while reading.
+
+A process can be migrated into a cgroup by writing its PID to the
+target cgroup's "cgroup.procs" file. Only one process can be migrated
+on a single write(2) call. If a process is composed of multiple
+threads, writing the PID of any thread migrates all threads of the
+process.
+
+When a process forks a child process, the new process is born into the
+cgroup that the forking process belongs to at the time of the
+operation. After exit, a process stays associated with the cgroup
+that it belonged to at the time of exit until it's reaped; however, a
+zombie process does not appear in "cgroup.procs" and thus can't be
+moved to another cgroup.
+
+A cgroup which doesn't have any children or live processes can be
+destroyed by removing the directory. Note that a cgroup which doesn't
+have any children and is associated only with zombie processes is
+considered empty and can be removed.
+
+ # rmdir $CGROUP_NAME
+
+"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy
+cgroup is in use in the system, this file may contain multiple lines,
+one for each hierarchy. The entry for cgroup v2 is always in the
+format "0::$PATH".
+
+ # cat /proc/842/cgroup
+ ...
+ 0::/test-cgroup/test-cgroup-nested
+
+If the process becomes a zombie and the cgroup it was associated with
+is removed subsequently, " (deleted)" is appended to the path.
+
+ # cat /proc/842/cgroup
+ ...
+ 0::/test-cgroup/test-cgroup-nested (deleted)
+
+
+2-3. [Un]populated Notification
+
+Each non-root cgroup has a "cgroup.events" file which contains
+"populated" field indicating whether the cgroup's sub-hierarchy has
+live processes in it. Its value is 0 if there is no live process in
+the cgroup and its descendants; otherwise, 1. poll and [id]notify
+events are triggered when the value changes. This can be used, for
+example, to start a clean-up operation after all processes of a given
+sub-hierarchy have exited. The populated state updates and
+notifications are recursive. Consider the following sub-hierarchy
+where the numbers in the parentheses represent the numbers of processes
+in each cgroup.
+
+ A(4) - B(0) - C(1)
+ \ D(0)
+
+A, B and C's "populated" fields would be 1 while D's 0. After the one
+process in C exits, B and C's "populated" fields would flip to "0" and
+file modified events will be generated on the "cgroup.events" files of
+both cgroups.
+
+
+2-4. Controlling Controllers
+
+2-4-1. Enabling and Disabling
+
+Each cgroup has a "cgroup.controllers" file which lists all
+controllers available for the cgroup to enable.
+
+ # cat cgroup.controllers
+ cpu io memory
+
+No controller is enabled by default. Controllers can be enabled and
+disabled by writing to the "cgroup.subtree_control" file.
+
+ # echo "+cpu +memory -io" > cgroup.subtree_control
+
+Only controllers which are listed in "cgroup.controllers" can be
+enabled. When multiple operations are specified as above, either they
+all succeed or fail. If multiple operations on the same controller
+are specified, the last one is effective.
+
+Enabling a controller in a cgroup indicates that the distribution of
+the target resource across its immediate children will be controlled.
+Consider the following sub-hierarchy. The enabled controllers are
+listed in parentheses.
+
+ A(cpu,memory) - B(memory) - C()
+ \ D()
+
+As A has "cpu" and "memory" enabled, A will control the distribution
+of CPU cycles and memory to its children, in this case, B. As B has
+"memory" enabled but not "CPU", C and D will compete freely on CPU
+cycles but their division of memory available to B will be controlled.
+
+As a controller regulates the distribution of the target resource to
+the cgroup's children, enabling it creates the controller's interface
+files in the child cgroups. In the above example, enabling "cpu" on B
+would create the "cpu." prefixed controller interface files in C and
+D. Likewise, disabling "memory" from B would remove the "memory."
+prefixed controller interface files from C and D. This means that the
+controller interface files - anything which doesn't start with
+"cgroup." are owned by the parent rather than the cgroup itself.
+
+
+2-4-2. Top-down Constraint
+
+Resources are distributed top-down and a cgroup can further distribute
+a resource only if the resource has been distributed to it from the
+parent. This means that all non-root "cgroup.subtree_control" files
+can only contain controllers which are enabled in the parent's
+"cgroup.subtree_control" file. A controller can be enabled only if
+the parent has the controller enabled and a controller can't be
+disabled if one or more children have it enabled.
+
+
+2-4-3. No Internal Process Constraint
+
+Non-root cgroups can only distribute resources to their children when
+they don't have any processes of their own. In other words, only
+cgroups which don't contain any processes can have controllers enabled
+in their "cgroup.subtree_control" files.
+
+This guarantees that, when a controller is looking at the part of the
+hierarchy which has it enabled, processes are always only on the
+leaves. This rules out situations where child cgroups compete against
+internal processes of the parent.
+
+The root cgroup is exempt from this restriction. Root contains
+processes and anonymous resource consumption which can't be associated
+with any other cgroups and requires special treatment from most
+controllers. How resource consumption in the root cgroup is governed
+is up to each controller.
+
+Note that the restriction doesn't get in the way if there is no
+enabled controller in the cgroup's "cgroup.subtree_control". This is
+important as otherwise it wouldn't be possible to create children of a
+populated cgroup. To control resource distribution of a cgroup, the
+cgroup must create children and transfer all its processes to the
+children before enabling controllers in its "cgroup.subtree_control"
+file.
+
+
+2-5. Delegation
+
+2-5-1. Model of Delegation
+
+A cgroup can be delegated to a less privileged user by granting write
+access of the directory and its "cgroup.procs" file to the user. Note
+that resource control interface files in a given directory control the
+distribution of the parent's resources and thus must not be delegated
+along with the directory.
+
+Once delegated, the user can build sub-hierarchy under the directory,
+organize processes as it sees fit and further distribute the resources
+it received from the parent. The limits and other settings of all
+resource controllers are hierarchical and regardless of what happens
+in the delegated sub-hierarchy, nothing can escape the resource
+restrictions imposed by the parent.
+
+Currently, cgroup doesn't impose any restrictions on the number of
+cgroups in or nesting depth of a delegated sub-hierarchy; however,
+this may be limited explicitly in the future.
+
+
+2-5-2. Delegation Containment
+
+A delegated sub-hierarchy is contained in the sense that processes
+can't be moved into or out of the sub-hierarchy by the delegatee. For
+a process with a non-root euid to migrate a target process into a
+cgroup by writing its PID to the "cgroup.procs" file, the following
+conditions must be met.
+
+- The writer's euid must match either uid or suid of the target process.
+
+- The writer must have write access to the "cgroup.procs" file.
+
+- The writer must have write access to the "cgroup.procs" file of the
+ common ancestor of the source and destination cgroups.
+
+The above three constraints ensure that while a delegatee may migrate
+processes around freely in the delegated sub-hierarchy it can't pull
+in from or push out to outside the sub-hierarchy.
+
+For an example, let's assume cgroups C0 and C1 have been delegated to
+user U0 who created C00, C01 under C0 and C10 under C1 as follows and
+all processes under C0 and C1 belong to U0.
+
+ ~~~~~~~~~~~~~ - C0 - C00
+ ~ cgroup ~ \ C01
+ ~ hierarchy ~
+ ~~~~~~~~~~~~~ - C1 - C10
+
+Let's also say U0 wants to write the PID of a process which is
+currently in C10 into "C00/cgroup.procs". U0 has write access to the
+file and uid match on the process; however, the common ancestor of the
+source cgroup C10 and the destination cgroup C00 is above the points
+of delegation and U0 would not have write access to its "cgroup.procs"
+files and thus the write will be denied with -EACCES.
+
+
+2-6. Guidelines
+
+2-6-1. Organize Once and Control
+
+Migrating a process across cgroups is a relatively expensive operation
+and stateful resources such as memory are not moved together with the
+process. This is an explicit design decision as there often exist
+inherent trade-offs between migration and various hot paths in terms
+of synchronization cost.
+
+As such, migrating processes across cgroups frequently as a means to
+apply different resource restrictions is discouraged. A workload
+should be assigned to a cgroup according to the system's logical and
+resource structure once on start-up. Dynamic adjustments to resource
+distribution can be made by changing controller configuration through
+the interface files.
+
+
+2-6-2. Avoid Name Collisions
+
+Interface files for a cgroup and its children cgroups occupy the same
+directory and it is possible to create children cgroups which collide
+with interface files.
+
+All cgroup core interface files are prefixed with "cgroup." and each
+controller's interface files are prefixed with the controller name and
+a dot. A controller's name is composed of lower case alphabets and
+'_'s but never begins with an '_' so it can be used as the prefix
+character for collision avoidance. Also, interface file names won't
+start or end with terms which are often used in categorizing workloads
+such as job, service, slice, unit or workload.
+
+cgroup doesn't do anything to prevent name collisions and it's the
+user's responsibility to avoid them.
+
+
+3. Resource Distribution Models
+
+cgroup controllers implement several resource distribution schemes
+depending on the resource type and expected use cases. This section
+describes major schemes in use along with their expected behaviors.
+
+
+3-1. Weights
+
+A parent's resource is distributed by adding up the weights of all
+active children and giving each the fraction matching the ratio of its
+weight against the sum. As only children which can make use of the
+resource at the moment participate in the distribution, this is
+work-conserving. Due to the dynamic nature, this model is usually
+used for stateless resources.
+
+All weights are in the range [1, 10000] with the default at 100. This
+allows symmetric multiplicative biases in both directions at fine
+enough granularity while staying in the intuitive range.
+
+As long as the weight is in range, all configuration combinations are
+valid and there is no reason to reject configuration changes or
+process migrations.
+
+"cpu.weight" proportionally distributes CPU cycles to active children
+and is an example of this type.
+
+
+3-2. Limits
+
+A child can only consume upto the configured amount of the resource.
+Limits can be over-committed - the sum of the limits of children can
+exceed the amount of resource available to the parent.
+
+Limits are in the range [0, max] and defaults to "max", which is noop.
+
+As limits can be over-committed, all configuration combinations are
+valid and there is no reason to reject configuration changes or
+process migrations.
+
+"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
+on an IO device and is an example of this type.
+
+
+3-3. Protections
+
+A cgroup is protected to be allocated upto the configured amount of
+the resource if the usages of all its ancestors are under their
+protected levels. Protections can be hard guarantees or best effort
+soft boundaries. Protections can also be over-committed in which case
+only upto the amount available to the parent is protected among
+children.
+
+Protections are in the range [0, max] and defaults to 0, which is
+noop.
+
+As protections can be over-committed, all configuration combinations
+are valid and there is no reason to reject configuration changes or
+process migrations.
+
+"memory.low" implements best-effort memory protection and is an
+example of this type.
+
+
+3-4. Allocations
+
+A cgroup is exclusively allocated a certain amount of a finite
+resource. Allocations can't be over-committed - the sum of the
+allocations of children can not exceed the amount of resource
+available to the parent.
+
+Allocations are in the range [0, max] and defaults to 0, which is no
+resource.
+
+As allocations can't be over-committed, some configuration
+combinations are invalid and should be rejected. Also, if the
+resource is mandatory for execution of processes, process migrations
+may be rejected.
+
+"cpu.rt.max" hard-allocates realtime slices and is an example of this
+type.
+
+
+4. Interface Files
+
+4-1. Format
+
+All interface files should be in one of the following formats whenever
+possible.
+
+ New-line separated values
+ (when only one value can be written at once)
+
+ VAL0\n
+ VAL1\n
+ ...
+
+ Space separated values
+ (when read-only or multiple values can be written at once)
+
+ VAL0 VAL1 ...\n
+
+ Flat keyed
+
+ KEY0 VAL0\n
+ KEY1 VAL1\n
+ ...
+
+ Nested keyed
+
+ KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
+ KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
+ ...
+
+For a writable file, the format for writing should generally match
+reading; however, controllers may allow omitting later fields or
+implement restricted shortcuts for most common use cases.
+
+For both flat and nested keyed files, only the values for a single key
+can be written at a time. For nested keyed files, the sub key pairs
+may be specified in any order and not all pairs have to be specified.
+
+
+4-2. Conventions
+
+- Settings for a single feature should be contained in a single file.
+
+- The root cgroup should be exempt from resource control and thus
+ shouldn't have resource control interface files. Also,
+ informational files on the root cgroup which end up showing global
+ information available elsewhere shouldn't exist.
+
+- If a controller implements weight based resource distribution, its
+ interface file should be named "weight" and have the range [1,
+ 10000] with 100 as the default. The values are chosen to allow
+ enough and symmetric bias in both directions while keeping it
+ intuitive (the default is 100%).
+
+- If a controller implements an absolute resource guarantee and/or
+ limit, the interface files should be named "min" and "max"
+ respectively. If a controller implements best effort resource
+ guarantee and/or limit, the interface files should be named "low"
+ and "high" respectively.
+
+ In the above four control files, the special token "max" should be
+ used to represent upward infinity for both reading and writing.
+
+- If a setting has a configurable default value and keyed specific
+ overrides, the default entry should be keyed with "default" and
+ appear as the first entry in the file.
+
+ The default value can be updated by writing either "default $VAL" or
+ "$VAL".
+
+ When writing to update a specific override, "default" can be used as
+ the value to indicate removal of the override. Override entries
+ with "default" as the value must not appear when read.
+
+ For example, a setting which is keyed by major:minor device numbers
+ with integer values may look like the following.
+
+ # cat cgroup-example-interface-file
+ default 150
+ 8:0 300
+
+ The default value can be updated by
+
+ # echo 125 > cgroup-example-interface-file
+
+ or
+
+ # echo "default 125" > cgroup-example-interface-file
+
+ An override can be set by
+
+ # echo "8:16 170" > cgroup-example-interface-file
+
+ and cleared by
+
+ # echo "8:0 default" > cgroup-example-interface-file
+ # cat cgroup-example-interface-file
+ default 125
+ 8:16 170
+
+- For events which are not very high frequency, an interface file
+ "events" should be created which lists event key value pairs.
+ Whenever a notifiable event happens, file modified event should be
+ generated on the file.
+
+
+4-3. Core Interface Files
+
+All cgroup core files are prefixed with "cgroup."
+
+ cgroup.procs
+
+ A read-write new-line separated values file which exists on
+ all cgroups.
+
+ When read, it lists the PIDs of all processes which belong to
+ the cgroup one-per-line. The PIDs are not ordered and the
+ same PID may show up more than once if the process got moved
+ to another cgroup and then back or the PID got recycled while
+ reading.
+
+ A PID can be written to migrate the process associated with
+ the PID to the cgroup. The writer should match all of the
+ following conditions.
+
+ - Its euid is either root or must match either uid or suid of
+ the target process.
+
+ - It must have write access to the "cgroup.procs" file.
+
+ - It must have write access to the "cgroup.procs" file of the
+ common ancestor of the source and destination cgroups.
+
+ When delegating a sub-hierarchy, write access to this file
+ should be granted along with the containing directory.
+
+ cgroup.controllers
+
+ A read-only space separated values file which exists on all
+ cgroups.
+
+ It shows space separated list of all controllers available to
+ the cgroup. The controllers are not ordered.
+
+ cgroup.subtree_control
+
+ A read-write space separated values file which exists on all
+ cgroups. Starts out empty.
+
+ When read, it shows space separated list of the controllers
+ which are enabled to control resource distribution from the
+ cgroup to its children.
+
+ Space separated list of controllers prefixed with '+' or '-'
+ can be written to enable or disable controllers. A controller
+ name prefixed with '+' enables the controller and '-'
+ disables. If a controller appears more than once on the list,
+ the last one is effective. When multiple enable and disable
+ operations are specified, either all succeed or all fail.
+
+ cgroup.events
+
+ A read-only flat-keyed file which exists on non-root cgroups.
+ The following entries are defined. Unless specified
+ otherwise, a value change in this file generates a file
+ modified event.
+
+ populated
+
+ 1 if the cgroup or its descendants contains any live
+ processes; otherwise, 0.
+
+
+5. Controllers
+
+5-1. CPU
+
+[NOTE: The interface for the cpu controller hasn't been merged yet]
+
+The "cpu" controllers regulates distribution of CPU cycles. This
+controller implements weight and absolute bandwidth limit models for
+normal scheduling policy and absolute bandwidth allocation model for
+realtime scheduling policy.
+
+
+5-1-1. CPU Interface Files
+
+All time durations are in microseconds.
+
+ cpu.stat
+
+ A read-only flat-keyed file which exists on non-root cgroups.
+
+ It reports the following six stats.
+
+ usage_usec
+ user_usec
+ system_usec
+ nr_periods
+ nr_throttled
+ throttled_usec
+
+ cpu.weight
+
+ A read-write single value file which exists on non-root
+ cgroups. The default is "100".
+
+ The weight in the range [1, 10000].
+
+ cpu.max
+
+ A read-write two value file which exists on non-root cgroups.
+ The default is "max 100000".
+
+ The maximum bandwidth limit. It's in the following format.
+
+ $MAX $PERIOD
+
+ which indicates that the group may consume upto $MAX in each
+ $PERIOD duration. "max" for $MAX indicates no limit. If only
+ one number is written, $MAX is updated.
+
+ cpu.rt.max
+
+ [NOTE: The semantics of this file is still under discussion and the
+ interface hasn't been merged yet]
+
+ A read-write two value file which exists on all cgroups.
+ The default is "0 100000".
+
+ The maximum realtime runtime allocation. Over-committing
+ configurations are disallowed and process migrations are
+ rejected if not enough bandwidth is available. It's in the
+ following format.
+
+ $MAX $PERIOD
+
+ which indicates that the group may consume upto $MAX in each
+ $PERIOD duration. If only one number is written, $MAX is
+ updated.
+
+
+5-2. Memory
+
+The "memory" controller regulates distribution of memory. Memory is
+stateful and implements both limit and protection models. Due to the
+intertwining between memory usage and reclaim pressure and the
+stateful nature of memory, the distribution model is relatively
+complex.
+
+While not completely water-tight, all major memory usages by a given
+cgroup are tracked so that the total memory consumption can be
+accounted and controlled to a reasonable extent. Currently, the
+following types of memory usages are tracked.
+
+- Userland memory - page cache and anonymous memory.
+
+- Kernel data structures such as dentries and inodes.
+
+- TCP socket buffers.
+
+The above list may expand in the future for better coverage.
+
+
+5-2-1. Memory Interface Files
+
+All memory amounts are in bytes. If a value which is not aligned to
+PAGE_SIZE is written, the value may be rounded up to the closest
+PAGE_SIZE multiple when read back.
+
+ memory.current
+
+ A read-only single value file which exists on non-root
+ cgroups.
+
+ The total amount of memory currently being used by the cgroup
+ and its descendants.
+
+ memory.low
+
+ A read-write single value file which exists on non-root
+ cgroups. The default is "0".
+
+ Best-effort memory protection. If the memory usages of a
+ cgroup and all its ancestors are below their low boundaries,
+ the cgroup's memory won't be reclaimed unless memory can be
+ reclaimed from unprotected cgroups.
+
+ Putting more memory than generally available under this
+ protection is discouraged.
+
+ memory.high
+
+ A read-write single value file which exists on non-root
+ cgroups. The default is "max".
+
+ Memory usage throttle limit. This is the main mechanism to
+ control memory usage of a cgroup. If a cgroup's usage goes
+ over the high boundary, the processes of the cgroup are
+ throttled and put under heavy reclaim pressure.
+
+ Going over the high limit never invokes the OOM killer and
+ under extreme conditions the limit may be breached.
+
+ memory.max
+
+ A read-write single value file which exists on non-root
+ cgroups. The default is "max".
+
+ Memory usage hard limit. This is the final protection
+ mechanism. If a cgroup's memory usage reaches this limit and
+ can't be reduced, the OOM killer is invoked in the cgroup.
+ Under certain circumstances, the usage may go over the limit
+ temporarily.
+
+ This is the ultimate protection mechanism. As long as the
+ high limit is used and monitored properly, this limit's
+ utility is limited to providing the final safety net.
+
+ memory.events
+
+ A read-only flat-keyed file which exists on non-root cgroups.
+ The following entries are defined. Unless specified
+ otherwise, a value change in this file generates a file
+ modified event.
+
+ low
+
+ The number of times the cgroup is reclaimed due to
+ high memory pressure even though its usage is under
+ the low boundary. This usually indicates that the low
+ boundary is over-committed.
+
+ high
+
+ The number of times processes of the cgroup are
+ throttled and routed to perform direct memory reclaim
+ because the high memory boundary was exceeded. For a
+ cgroup whose memory usage is capped by the high limit
+ rather than global memory pressure, this event's
+ occurrences are expected.
+
+ max
+
+ The number of times the cgroup's memory usage was
+ about to go over the max boundary. If direct reclaim
+ fails to bring it down, the OOM killer is invoked.
+
+ oom
+
+ The number of times the OOM killer has been invoked in
+ the cgroup. This may not exactly match the number of
+ processes killed but should generally be close.
+
+
+5-2-2. General Usage
+
+"memory.high" is the main mechanism to control memory usage.
+Over-committing on high limit (sum of high limits > available memory)
+and letting global memory pressure to distribute memory according to
+usage is a viable strategy.
+
+Because breach of the high limit doesn't trigger the OOM killer but
+throttles the offending cgroup, a management agent has ample
+opportunities to monitor and take appropriate actions such as granting
+more memory or terminating the workload.
+
+Determining whether a cgroup has enough memory is not trivial as
+memory usage doesn't indicate whether the workload can benefit from
+more memory. For example, a workload which writes data received from
+network to a file can use all available memory but can also operate as
+performant with a small amount of memory. A measure of memory
+pressure - how much the workload is being impacted due to lack of
+memory - is necessary to determine whether a workload needs more
+memory; unfortunately, memory pressure monitoring mechanism isn't
+implemented yet.
+
+
+5-2-3. Memory Ownership
+
+A memory area is charged to the cgroup which instantiated it and stays
+charged to the cgroup until the area is released. Migrating a process
+to a different cgroup doesn't move the memory usages that it
+instantiated while in the previous cgroup to the new cgroup.
+
+A memory area may be used by processes belonging to different cgroups.
+To which cgroup the area will be charged is in-deterministic; however,
+over time, the memory area is likely to end up in a cgroup which has
+enough memory allowance to avoid high reclaim pressure.
+
+If a cgroup sweeps a considerable amount of memory which is expected
+to be accessed repeatedly by other cgroups, it may make sense to use
+POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
+belonging to the affected files to ensure correct memory ownership.
+
+
+5-3. IO
+
+The "io" controller regulates the distribution of IO resources. This
+controller implements both weight based and absolute bandwidth or IOPS
+limit distribution; however, weight based distribution is available
+only if cfq-iosched is in use and neither scheme is available for
+blk-mq devices.
+
+
+5-3-1. IO Interface Files
+
+ io.stat
+
+ A read-only nested-keyed file which exists on non-root
+ cgroups.
+
+ Lines are keyed by $MAJ:$MIN device numbers and not ordered.
+ The following nested keys are defined.
+
+ rbytes Bytes read
+ wbytes Bytes written
+ rios Number of read IOs
+ wios Number of write IOs
+
+ An example read output follows.
+
+ 8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353
+ 8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252
+
+ io.weight
+
+ A read-write flat-keyed file which exists on non-root cgroups.
+ The default is "default 100".
+
+ The first line is the default weight applied to devices
+ without specific override. The rest are overrides keyed by
+ $MAJ:$MIN device numbers and not ordered. The weights are in
+ the range [1, 10000] and specifies the relative amount IO time
+ the cgroup can use in relation to its siblings.
+
+ The default weight can be updated by writing either "default
+ $WEIGHT" or simply "$WEIGHT". Overrides can be set by writing
+ "$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
+
+ An example read output follows.
+
+ default 100
+ 8:16 200
+ 8:0 50
+
+ io.max
+
+ A read-write nested-keyed file which exists on non-root
+ cgroups.
+
+ BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN
+ device numbers and not ordered. The following nested keys are
+ defined.
+
+ rbps Max read bytes per second
+ wbps Max write bytes per second
+ riops Max read IO operations per second
+ wiops Max write IO operations per second
+
+ When writing, any number of nested key-value pairs can be
+ specified in any order. "max" can be specified as the value
+ to remove a specific limit. If the same key is specified
+ multiple times, the outcome is undefined.
+
+ BPS and IOPS are measured in each IO direction and IOs are
+ delayed if limit is reached. Temporary bursts are allowed.
+
+ Setting read limit at 2M BPS and write at 120 IOPS for 8:16.
+
+ echo "8:16 rbps=2097152 wiops=120" > io.max
+
+ Reading returns the following.
+
+ 8:16 rbps=2097152 wbps=max riops=max wiops=120
+
+ Write IOPS limit can be removed by writing the following.
+
+ echo "8:16 wiops=max" > io.max
+
+ Reading now returns the following.
+
+ 8:16 rbps=2097152 wbps=max riops=max wiops=max
+
+
+5-3-2. Writeback
+
+Page cache is dirtied through buffered writes and shared mmaps and
+written asynchronously to the backing filesystem by the writeback
+mechanism. Writeback sits between the memory and IO domains and
+regulates the proportion of dirty memory by balancing dirtying and
+write IOs.
+
+The io controller, in conjunction with the memory controller,
+implements control of page cache writeback IOs. The memory controller
+defines the memory domain that dirty memory ratio is calculated and
+maintained for and the io controller defines the io domain which
+writes out dirty pages for the memory domain. Both system-wide and
+per-cgroup dirty memory states are examined and the more restrictive
+of the two is enforced.
+
+cgroup writeback requires explicit support from the underlying
+filesystem. Currently, cgroup writeback is implemented on ext2, ext4
+and btrfs. On other filesystems, all writeback IOs are attributed to
+the root cgroup.
+
+There are inherent differences in memory and writeback management
+which affects how cgroup ownership is tracked. Memory is tracked per
+page while writeback per inode. For the purpose of writeback, an
+inode is assigned to a cgroup and all IO requests to write dirty pages
+from the inode are attributed to that cgroup.
+
+As cgroup ownership for memory is tracked per page, there can be pages
+which are associated with different cgroups than the one the inode is
+associated with. These are called foreign pages. The writeback
+constantly keeps track of foreign pages and, if a particular foreign
+cgroup becomes the majority over a certain period of time, switches
+the ownership of the inode to that cgroup.
+
+While this model is enough for most use cases where a given inode is
+mostly dirtied by a single cgroup even when the main writing cgroup
+changes over time, use cases where multiple cgroups write to a single
+inode simultaneously are not supported well. In such circumstances, a
+significant portion of IOs are likely to be attributed incorrectly.
+As memory controller assigns page ownership on the first use and
+doesn't update it until the page is released, even if writeback
+strictly follows page ownership, multiple cgroups dirtying overlapping
+areas wouldn't work as expected. It's recommended to avoid such usage
+patterns.
+
+The sysctl knobs which affect writeback behavior are applied to cgroup
+writeback as follows.
+
+ vm.dirty_background_ratio
+ vm.dirty_ratio
+
+ These ratios apply the same to cgroup writeback with the
+ amount of available memory capped by limits imposed by the
+ memory controller and system-wide clean memory.
+
+ vm.dirty_background_bytes
+ vm.dirty_bytes
+
+ For cgroup writeback, this is calculated into ratio against
+ total available memory and applied the same way as
+ vm.dirty[_background]_ratio.
+
+
+P. Information on Kernel Programming
+
+This section contains kernel programming information in the areas
+where interacting with cgroup is necessary. cgroup core and
+controllers are not covered.
+
+
+P-1. Filesystem Support for Writeback
+
+A filesystem can support cgroup writeback by updating
+address_space_operations->writepage[s]() to annotate bio's using the
+following two functions.
+
+ wbc_init_bio(@wbc, @bio)
+
+ Should be called for each bio carrying writeback data and
+ associates the bio with the inode's owner cgroup. Can be
+ called anytime between bio allocation and submission.
+
+ wbc_account_io(@wbc, @page, @bytes)
+
+ Should be called for each data segment being written out.
+ While this function doesn't care exactly when it's called
+ during the writeback session, it's the easiest and most
+ natural to call it as data segments are added to a bio.
+
+With writeback bio's annotated, cgroup support can be enabled per
+super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for
+selective disabling of cgroup writeback support which is helpful when
+certain filesystem features, e.g. journaled data mode, are
+incompatible.
+
+wbc_init_bio() binds the specified bio to its cgroup. Depending on
+the configuration, the bio may be executed at a lower priority and if
+the writeback session is holding shared resources, e.g. a journal
+entry, may lead to priority inversion. There is no one easy solution
+for the problem. Filesystems can try to work around specific problem
+cases by skipping wbc_init_bio() or using bio_associate_blkcg()
+directly.
+
+
+D. Deprecated v1 Core Features
+
+- Multiple hierarchies including named ones are not supported.
+
+- All mount options and remounting are not supported.
+
+- The "tasks" file is removed and "cgroup.procs" is not sorted.
+
+- "cgroup.clone_children" is removed.
+
+- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file
+ at the root instead.
+
+
+R. Issues with v1 and Rationales for v2
+
+R-1. Multiple Hierarchies
+
+cgroup v1 allowed an arbitrary number of hierarchies and each
+hierarchy could host any number of controllers. While this seemed to
+provide a high level of flexibility, it wasn't useful in practice.
+
+For example, as there is only one instance of each controller, utility
+type controllers such as freezer which can be useful in all
+hierarchies could only be used in one. The issue is exacerbated by
+the fact that controllers couldn't be moved to another hierarchy once
+hierarchies were populated. Another issue was that all controllers
+bound to a hierarchy were forced to have exactly the same view of the
+hierarchy. It wasn't possible to vary the granularity depending on
+the specific controller.
+
+In practice, these issues heavily limited which controllers could be
+put on the same hierarchy and most configurations resorted to putting
+each controller on its own hierarchy. Only closely related ones, such
+as the cpu and cpuacct controllers, made sense to be put on the same
+hierarchy. This often meant that userland ended up managing multiple
+similar hierarchies repeating the same steps on each hierarchy
+whenever a hierarchy management operation was necessary.
+
+Furthermore, support for multiple hierarchies came at a steep cost.
+It greatly complicated cgroup core implementation but more importantly
+the support for multiple hierarchies restricted how cgroup could be
+used in general and what controllers was able to do.
+
+There was no limit on how many hierarchies there might be, which meant
+that a thread's cgroup membership couldn't be described in finite
+length. The key might contain any number of entries and was unlimited
+in length, which made it highly awkward to manipulate and led to
+addition of controllers which existed only to identify membership,
+which in turn exacerbated the original problem of proliferating number
+of hierarchies.
+
+Also, as a controller couldn't have any expectation regarding the
+topologies of hierarchies other controllers might be on, each
+controller had to assume that all other controllers were attached to
+completely orthogonal hierarchies. This made it impossible, or at
+least very cumbersome, for controllers to cooperate with each other.
+
+In most use cases, putting controllers on hierarchies which are
+completely orthogonal to each other isn't necessary. What usually is
+called for is the ability to have differing levels of granularity
+depending on the specific controller. In other words, hierarchy may
+be collapsed from leaf towards root when viewed from specific
+controllers. For example, a given configuration might not care about
+how memory is distributed beyond a certain level while still wanting
+to control how CPU cycles are distributed.
+
+
+R-2. Thread Granularity
+
+cgroup v1 allowed threads of a process to belong to different cgroups.
+This didn't make sense for some controllers and those controllers
+ended up implementing different ways to ignore such situations but
+much more importantly it blurred the line between API exposed to
+individual applications and system management interface.
+
+Generally, in-process knowledge is available only to the process
+itself; thus, unlike service-level organization of processes,
+categorizing threads of a process requires active participation from
+the application which owns the target process.
+
+cgroup v1 had an ambiguously defined delegation model which got abused
+in combination with thread granularity. cgroups were delegated to
+individual applications so that they can create and manage their own
+sub-hierarchies and control resource distributions along them. This
+effectively raised cgroup to the status of a syscall-like API exposed
+to lay programs.
+
+First of all, cgroup has a fundamentally inadequate interface to be
+exposed this way. For a process to access its own knobs, it has to
+extract the path on the target hierarchy from /proc/self/cgroup,
+construct the path by appending the name of the knob to the path, open
+and then read and/or write to it. This is not only extremely clunky
+and unusual but also inherently racy. There is no conventional way to
+define transaction across the required steps and nothing can guarantee
+that the process would actually be operating on its own sub-hierarchy.
+
+cgroup controllers implemented a number of knobs which would never be
+accepted as public APIs because they were just adding control knobs to
+system-management pseudo filesystem. cgroup ended up with interface
+knobs which were not properly abstracted or refined and directly
+revealed kernel internal details. These knobs got exposed to
+individual applications through the ill-defined delegation mechanism
+effectively abusing cgroup as a shortcut to implementing public APIs
+without going through the required scrutiny.
+
+This was painful for both userland and kernel. Userland ended up with
+misbehaving and poorly abstracted interfaces and kernel exposing and
+locked into constructs inadvertently.
+
+
+R-3. Competition Between Inner Nodes and Threads
+
+cgroup v1 allowed threads to be in any cgroups which created an
+interesting problem where threads belonging to a parent cgroup and its
+children cgroups competed for resources. This was nasty as two
+different types of entities competed and there was no obvious way to
+settle it. Different controllers did different things.
+
+The cpu controller considered threads and cgroups as equivalents and
+mapped nice levels to cgroup weights. This worked for some cases but
+fell flat when children wanted to be allocated specific ratios of CPU
+cycles and the number of internal threads fluctuated - the ratios
+constantly changed as the number of competing entities fluctuated.
+There also were other issues. The mapping from nice level to weight
+wasn't obvious or universal, and there were various other knobs which
+simply weren't available for threads.
+
+The io controller implicitly created a hidden leaf node for each
+cgroup to host the threads. The hidden leaf had its own copies of all
+the knobs with "leaf_" prefixed. While this allowed equivalent
+control over internal threads, it was with serious drawbacks. It
+always added an extra layer of nesting which wouldn't be necessary
+otherwise, made the interface messy and significantly complicated the
+implementation.
+
+The memory controller didn't have a way to control what happened
+between internal tasks and child cgroups and the behavior was not
+clearly defined. There were attempts to add ad-hoc behaviors and
+knobs to tailor the behavior to specific workloads which would have
+led to problems extremely difficult to resolve in the long term.
+
+Multiple controllers struggled with internal tasks and came up with
+different ways to deal with it; unfortunately, all the approaches were
+severely flawed and, furthermore, the widely different behaviors
+made cgroup as a whole highly inconsistent.
+
+This clearly is a problem which needs to be addressed from cgroup core
+in a uniform way.
+
+
+R-4. Other Interface Issues
+
+cgroup v1 grew without oversight and developed a large number of
+idiosyncrasies and inconsistencies. One issue on the cgroup core side
+was how an empty cgroup was notified - a userland helper binary was
+forked and executed for each event. The event delivery wasn't
+recursive or delegatable. The limitations of the mechanism also led
+to in-kernel event delivery filtering mechanism further complicating
+the interface.
+
+Controller interfaces were problematic too. An extreme example is
+controllers completely ignoring hierarchical organization and treating
+all cgroups as if they were all located directly under the root
+cgroup. Some controllers exposed a large amount of inconsistent
+implementation details to userland.
+
+There also was no consistency across controllers. When a new cgroup
+was created, some controllers defaulted to not imposing extra
+restrictions while others disallowed any resource usage until
+explicitly configured. Configuration knobs for the same type of
+control used widely differing naming schemes and formats. Statistics
+and information knobs were named arbitrarily and used different
+formats and units even in the same controller.
+
+cgroup v2 establishes common conventions where appropriate and updates
+controllers so that they expose minimal and consistent interfaces.
+
+
+R-5. Controller Issues and Remedies
+
+R-5-1. Memory
+
+The original lower boundary, the soft limit, is defined as a limit
+that is per default unset. As a result, the set of cgroups that
+global reclaim prefers is opt-in, rather than opt-out. The costs for
+optimizing these mostly negative lookups are so high that the
+implementation, despite its enormous size, does not even provide the
+basic desirable behavior. First off, the soft limit has no
+hierarchical meaning. All configured groups are organized in a global
+rbtree and treated like equal peers, regardless where they are located
+in the hierarchy. This makes subtree delegation impossible. Second,
+the soft limit reclaim pass is so aggressive that it not just
+introduces high allocation latencies into the system, but also impacts
+system performance due to overreclaim, to the point where the feature
+becomes self-defeating.
+
+The memory.low boundary on the other hand is a top-down allocated
+reserve. A cgroup enjoys reclaim protection when it and all its
+ancestors are below their low boundaries, which makes delegation of
+subtrees possible. Secondly, new cgroups have no reserve per default
+and in the common case most cgroups are eligible for the preferred
+reclaim pass. This allows the new low boundary to be efficiently
+implemented with just a minor addition to the generic reclaim code,
+without the need for out-of-band data structures and reclaim passes.
+Because the generic reclaim code considers all cgroups except for the
+ones running low in the preferred first reclaim pass, overreclaim of
+individual groups is eliminated as well, resulting in much better
+overall workload performance.
+
+The original high boundary, the hard limit, is defined as a strict
+limit that can not budge, even if the OOM killer has to be called.
+But this generally goes against the goal of making the most out of the
+available memory. The memory consumption of workloads varies during
+runtime, and that requires users to overcommit. But doing that with a
+strict upper limit requires either a fairly accurate prediction of the
+working set size or adding slack to the limit. Since working set size
+estimation is hard and error prone, and getting it wrong results in
+OOM kills, most users tend to err on the side of a looser limit and
+end up wasting precious resources.
+
+The memory.high boundary on the other hand can be set much more
+conservatively. When hit, it throttles allocations by forcing them
+into direct reclaim to work off the excess, but it never invokes the
+OOM killer. As a result, a high boundary that is chosen too
+aggressively will not terminate the processes, but instead it will
+lead to gradual performance degradation. The user can monitor this
+and make corrections until the minimal memory footprint that still
+gives acceptable performance is found.
+
+In extreme cases, with many concurrent allocations and a complete
+breakdown of reclaim progress within the group, the high boundary can
+be exceeded. But even then it's mostly better to satisfy the
+allocation from the slack available in other groups or the rest of the
+system than killing the group. Otherwise, memory.max is there to
+limit this type of spillover and ultimately contain buggy or even
+malicious applications.