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cgroup: replace unified-hierarchy.txt with a proper cgroup v2 documentation 

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>
This commit is contained in:
Tejun Heo 2015-11-16 11:13:34 -05:00
parent 0d94276645
commit 6c2920926b
3 changed files with 1293 additions and 724 deletions
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79
Documentation/cgroup-legacy/blkio-controller.txt
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@ -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.

645
Documentation/cgroup-legacy/unified-hierarchy.txt
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@ -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.

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