-
+================
Control Group v2
+================
-October, 2015 Tejun Heo <tj@kernel.org>
+:Date: October, 2015
+:Author: 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
future changes must be reflected in this document. Documentation for
v1 is available under Documentation/cgroup-v1/.
-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
- 5-4. PID
- 5-4-1. PID Interface Files
- 5-5. RDMA
- 5-5-1. RDMA Interface Files
- 5-6. Misc
- 5-6-1. perf_event
-6. Namespace
- 6-1. Basics
- 6-2. The Root and Views
- 6-3. Migration and setns(2)
- 6-4. Interaction with Other Namespaces
-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
+.. 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
+ 5-4. PID
+ 5-4-1. PID Interface Files
+ 5-5. RDMA
+ 5-5-1. RDMA Interface Files
+ 5-6. Misc
+ 5-6-1. perf_event
+ 6. Namespace
+ 6-1. Basics
+ 6-2. The Root and Views
+ 6-3. Migration and setns(2)
+ 6-4. Interaction with Other Namespaces
+ 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
+
+
+Introduction
+============
+
+Terminology
+-----------
"cgroup" stands for "control group" and is never capitalized. The
singular form is used to designate the whole feature and also as a
multiple individual control groups, the plural form "cgroups" is used.
-1-2. What is cgroup?
+What is cgroup?
+---------------
cgroup is a mechanism to organize processes hierarchically and
distribute system resources along the hierarchy in a controlled and
overridden from further away.
-2. Basic Operations
+Basic Operations
+================
-2-1. Mounting
+Mounting
+--------
Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2
-hierarchy can be mounted with the following mount command.
+hierarchy can be mounted with the following mount command::
# mount -t cgroup2 none $MOUNT_POINT
Delegation section for details.
-2-2. Organizing Processes
+Organizing Processes
+--------------------
Initially, only the root cgroup exists to which all processes belong.
-A child cgroup can be created by creating a sub-directory.
+A child cgroup can be created by creating a sub-directory::
# mkdir $CGROUP_NAME
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.
+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::$PATH".
+format "0::$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.
+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
+[Un]populated Notification
+--------------------------
Each non-root cgroup has a "cgroup.events" file which contains
"populated" field indicating whether the cgroup's sub-hierarchy has
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.
+in each cgroup::
A(4) - B(0) - C(1)
\ D(0)
both cgroups.
-2-4. Controlling Controllers
+Controlling Controllers
+-----------------------
-2-4-1. Enabling and Disabling
+Enabling and Disabling
+~~~~~~~~~~~~~~~~~~~~~~
Each cgroup has a "cgroup.controllers" file which lists all
-controllers available for the cgroup to enable.
+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.
+disabled by writing to the "cgroup.subtree_control" file::
# echo "+cpu +memory -io" > cgroup.subtree_control
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.
+listed in parentheses::
A(cpu,memory) - B(memory) - C()
\ D()
"cgroup." are owned by the parent rather than the cgroup itself.
-2-4-2. Top-down Constraint
+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
disabled if one or more children have it enabled.
-2-4-3. No Internal Process Constraint
+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
file.
-2-5. Delegation
+Delegation
+----------
-2-5-1. Model of Delegation
+Model of Delegation
+~~~~~~~~~~~~~~~~~~~
A cgroup can be delegated in two ways. First, to a less privileged
user by granting write access of the directory and its "cgroup.procs"
this may be limited explicitly in the future.
-2-5-2. Delegation Containment
+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 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.
+all processes under C0 and C1 belong to U0::
~~~~~~~~~~~~~ - C0 - C00
~ cgroup ~ \ C01
is not reachable, the migration is rejected with -ENOENT.
-2-6. Guidelines
+Guidelines
+----------
-2-6-1. Organize Once and Control
+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
the interface files.
-2-6-2. Avoid Name Collisions
+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
user's responsibility to avoid them.
-3. Resource Distribution Models
+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
+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
and is an example of this type.
-3-2. Limits
+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
on an IO device and is an example of this type.
-3-3. Protections
+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
example of this type.
-3-4. Allocations
+Allocations
+-----------
A cgroup is exclusively allocated a certain amount of a finite
resource. Allocations can't be over-committed - the sum of the
type.
-4. Interface Files
+Interface Files
+===============
-4-1. Format
+Format
+------
All interface files should be in one of the following formats whenever
-possible.
+possible::
New-line separated values
(when only one value can be written at once)
may be specified in any order and not all pairs have to be specified.
-4-2. Conventions
+Conventions
+-----------
- Settings for a single feature should be contained in a single file.
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.
+ 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
+ The default value can be updated by::
# echo 125 > cgroup-example-interface-file
- or
+ or::
# echo "default 125" > cgroup-example-interface-file
- An override can be set by
+ An override can be set by::
# echo "8:16 170" > cgroup-example-interface-file
- and cleared by
+ and cleared by::
# echo "8:0 default" > cgroup-example-interface-file
# cat cgroup-example-interface-file
generated on the file.
-4-3. Core Interface Files
+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.
should be granted along with the containing directory.
cgroup.controllers
-
A read-only space separated values file which exists on all
cgroups.
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.
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
+Controllers
+===========
-5-1. CPU
+CPU
+---
-[NOTE: The interface for the cpu controller hasn't been merged yet]
+.. 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
realtime scheduling policy.
-5-1-1. CPU Interface Files
+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.
+ It reports the following six stats:
- usage_usec
- user_usec
- system_usec
- nr_periods
- nr_throttled
- throttled_usec
+ - 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.
+ The maximum bandwidth limit. It's in the following format::
$MAX $PERIOD
one number is written, $MAX is updated.
cpu.rt.max
+ .. note::
- [NOTE: The semantics of this file is still under discussion and the
- interface hasn't been merged yet]
+ 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.
+ following format::
$MAX $PERIOD
updated.
-5-2. Memory
+Memory
+------
The "memory" controller regulates distribution of memory. Memory is
stateful and implements both limit and protection models. Due to the
The above list may expand in the future for better coverage.
-5-2-1. Memory Interface Files
+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.
and its descendants.
memory.low
-
A read-write single value file which exists on non-root
cgroups. The default is "0".
protection is discouraged.
memory.high
-
A read-write single value file which exists on non-root
cgroups. The default is "max".
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".
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
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 cgroup goes to OOM state.
oom
-
The number of time the cgroup's memory usage was
reached the limit and allocation was about to fail.
Failed allocation in its turn could be returned into
userspace as -ENOMEM or siletly ignored in cases like
- disk readahead. For now OOM in memory cgroup kills
+ disk readahead. For now OOM in memory cgroup kills
tasks iff shortage has happened inside page fault.
oom_kill
-
The number of processes belonging to this cgroup
killed by any kind of OOM killer.
memory.stat
-
A read-only flat-keyed file which exists on non-root cgroups.
This breaks down the cgroup's memory footprint into different
fixed position; use the keys to look up specific values!
anon
-
Amount of memory used in anonymous mappings such as
brk(), sbrk(), and mmap(MAP_ANONYMOUS)
file
-
Amount of memory used to cache filesystem data,
including tmpfs and shared memory.
kernel_stack
-
Amount of memory allocated to kernel stacks.
slab
-
Amount of memory used for storing in-kernel data
structures.
sock
-
Amount of memory used in network transmission buffers
shmem
-
Amount of cached filesystem data that is swap-backed,
such as tmpfs, shm segments, shared anonymous mmap()s
file_mapped
-
Amount of cached filesystem data mapped with mmap()
file_dirty
-
Amount of cached filesystem data that was modified but
not yet written back to disk
file_writeback
-
Amount of cached filesystem data that was modified and
is currently being written back to disk
- inactive_anon
- active_anon
- inactive_file
- active_file
- unevictable
-
+ inactive_anon, active_anon, inactive_file, active_file, unevictable
Amount of memory, swap-backed and filesystem-backed,
on the internal memory management lists used by the
page reclaim algorithm
slab_reclaimable
-
Part of "slab" that might be reclaimed, such as
dentries and inodes.
slab_unreclaimable
-
Part of "slab" that cannot be reclaimed on memory
pressure.
pgfault
-
Total number of page faults incurred
pgmajfault
-
Number of major page faults incurred
workingset_refault
Amount of reclaimed lazyfree pages
memory.swap.current
-
A read-only single value file which exists on non-root
cgroups.
and its descendants.
memory.swap.max
-
A read-write single value file which exists on non-root
cgroups. The default is "max".
limit, anonymous meomry of the cgroup will not be swapped out.
-5-2-2. Usage Guidelines
+Usage Guidelines
+~~~~~~~~~~~~~~~~
"memory.high" is the main mechanism to control memory usage.
Over-committing on high limit (sum of high limits > available memory)
implemented yet.
-5-2-3. Memory Ownership
+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
belonging to the affected files to ensure correct memory ownership.
-5-3. IO
+IO
+--
The "io" controller regulates the distribution of IO resources. This
controller implements both weight based and absolute bandwidth or IOPS
blk-mq devices.
-5-3-1. IO Interface Files
+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.
+ 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".
$WEIGHT" or simply "$WEIGHT". Overrides can be set by writing
"$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
- An example read output follows.
+ 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.
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
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.
+ 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.
+ Reading returns the following::
8:16 rbps=2097152 wbps=max riops=max wiops=120
- Write IOPS limit can be removed by writing the following.
+ Write IOPS limit can be removed by writing the following::
echo "8:16 wiops=max" > io.max
- Reading now returns the following.
+ Reading now returns the following::
8:16 rbps=2097152 wbps=max riops=max wiops=max
-5-3-2. Writeback
+Writeback
+~~~~~~~~~
Page cache is dirtied through buffered writes and shared mmaps and
written asynchronously to the backing filesystem by the writeback
The sysctl knobs which affect writeback behavior are applied to cgroup
writeback as follows.
- vm.dirty_background_ratio
- vm.dirty_ratio
-
+ 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
-
+ 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.
-5-4. PID
+PID
+---
The process number controller is used to allow a cgroup to stop any
new tasks from being fork()'d or clone()'d after a specified limit is
used by the kernel.
-5-4-1. PID Interface Files
+PID Interface Files
+~~~~~~~~~~~~~~~~~~~
pids.max
-
A read-write single value file which exists on non-root
cgroups. The default is "max".
Hard limit of number of processes.
pids.current
-
A read-only single value file which exists on all cgroups.
The number of processes currently in the cgroup and its
of a new process would cause a cgroup policy to be violated.
-5-5. RDMA
+RDMA
+----
The "rdma" controller regulates the distribution and accounting of
of RDMA resources.
-5-5-1. RDMA Interface Files
+RDMA Interface Files
+~~~~~~~~~~~~~~~~~~~~
rdma.max
A readwrite nested-keyed file that exists for all the cgroups
The following nested keys are defined.
+ ========== =============================
hca_handle Maximum number of HCA Handles
hca_object Maximum number of HCA Objects
+ ========== =============================
- An example for mlx4 and ocrdma device follows.
+ An example for mlx4 and ocrdma device follows::
mlx4_0 hca_handle=2 hca_object=2000
ocrdma1 hca_handle=3 hca_object=max
A read-only file that describes current resource usage.
It exists for all the cgroup except root.
- An example for mlx4 and ocrdma device follows.
+ An example for mlx4 and ocrdma device follows::
mlx4_0 hca_handle=1 hca_object=20
ocrdma1 hca_handle=1 hca_object=23
-5-6. Misc
+Misc
+----
-5-6-1. perf_event
+perf_event
+~~~~~~~~~~
perf_event controller, if not mounted on a legacy hierarchy, is
automatically enabled on the v2 hierarchy so that perf events can
moved to a legacy hierarchy after v2 hierarchy is populated.
-6. Namespace
+Namespace
+=========
-6-1. Basics
+Basics
+------
cgroup namespace provides a mechanism to virtualize the view of the
"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone
complete path of the cgroup of a process. In a container setup where
a set of cgroups and namespaces are intended to isolate processes the
"/proc/$PID/cgroup" file may leak potential system level information
-to the isolated processes. For Example:
+to the isolated processes. For Example::
# cat /proc/self/cgroup
0::/batchjobs/container_id1
The path '/batchjobs/container_id1' can be considered as system-data
and undesirable to expose to the isolated processes. cgroup namespace
can be used to restrict visibility of this path. For example, before
-creating a cgroup namespace, one would see:
+creating a cgroup namespace, one would see::
# ls -l /proc/self/ns/cgroup
lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
# cat /proc/self/cgroup
0::/batchjobs/container_id1
-After unsharing a new namespace, the view changes.
+After unsharing a new namespace, the view changes::
# ls -l /proc/self/ns/cgroup
lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
remain.
-6-2. The Root and Views
+The Root and Views
+------------------
The 'cgroupns root' for a cgroup namespace is the cgroup in which the
process calling unshare(2) is running. For example, if a process in
init_cgroup_ns, this is the real root ('/') cgroup.
The cgroupns root cgroup does not change even if the namespace creator
-process later moves to a different cgroup.
+process later moves to a different cgroup::
# ~/unshare -c # unshare cgroupns in some cgroup
# cat /proc/self/cgroup
Processes running inside the cgroup namespace will be able to see
cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
-From within an unshared cgroupns:
+From within an unshared cgroupns::
# sleep 100000 &
[1] 7353
0::/sub_cgrp_1
From the initial cgroup namespace, the real cgroup path will be
-visible:
+visible::
$ cat /proc/7353/cgroup
0::/batchjobs/container_id1/sub_cgrp_1
From a sibling cgroup namespace (that is, a namespace rooted at a
different cgroup), the cgroup path relative to its own cgroup
namespace root will be shown. For instance, if PID 7353's cgroup
-namespace root is at '/batchjobs/container_id2', then it will see
+namespace root is at '/batchjobs/container_id2', then it will see::
# cat /proc/7353/cgroup
0::/../container_id2/sub_cgrp_1
its relative to the cgroup namespace root of the caller.
-6-3. Migration and setns(2)
+Migration and setns(2)
+----------------------
Processes inside a cgroup namespace can move into and out of the
namespace root if they have proper access to external cgroups. For
example, from inside a namespace with cgroupns root at
/batchjobs/container_id1, and assuming that the global hierarchy is
-still accessible inside cgroupns:
+still accessible inside cgroupns::
# cat /proc/7353/cgroup
0::/sub_cgrp_1
process under the target cgroup namespace root.
-6-4. Interaction with Other Namespaces
+Interaction with Other Namespaces
+---------------------------------
Namespace specific cgroup hierarchy can be mounted by a process
-running inside a non-init cgroup namespace.
+running inside a non-init cgroup namespace::
# mount -t cgroup2 none $MOUNT_POINT
provides a properly isolated cgroup view inside the container.
-P. Information on Kernel Programming
+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
+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
directly.
-D. Deprecated v1 Core Features
+Deprecated v1 Core Features
+===========================
- Multiple hierarchies including named ones are not supported.
at the root instead.
-R. Issues with v1 and Rationales for v2
+Issues with v1 and Rationales for v2
+====================================
-R-1. Multiple Hierarchies
+Multiple Hierarchies
+--------------------
cgroup v1 allowed an arbitrary number of hierarchies and each
hierarchy could host any number of controllers. While this seemed to
to control how CPU cycles are distributed.
-R-2. Thread Granularity
+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
locked into constructs inadvertently.
-R-3. Competition Between Inner Nodes and Threads
+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
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
+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
in a uniform way.
-R-4. Other Interface Issues
+Other Interface Issues
+----------------------
cgroup v1 grew without oversight and developed a large number of
idiosyncrasies and inconsistencies. One issue on the cgroup core side
controllers so that they expose minimal and consistent interfaces.
-R-5. Controller Issues and Remedies
+Controller Issues and Remedies
+------------------------------
-R-5-1. Memory
+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
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