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diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst new file mode 100644 index 000000000000..74cdeaed9f7a --- /dev/null +++ b/Documentation/admin-guide/cgroup-v2.rst @@ -0,0 +1,1998 @@ +================ +Control Group v2 +================ + +: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 +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-v1/. + +.. CONTENTS + + 1. Introduction + 1-1. Terminology + 1-2. What is cgroup? + 2. Basic Operations + 2-1. Mounting + 2-2. Organizing Processes and Threads + 2-2-1. Processes + 2-2-2. Threads + 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. Device + 5-6. RDMA + 5-6-1. RDMA Interface Files + 5-7. Misc + 5-7-1. perf_event + 5-N. Non-normative information + 5-N-1. CPU controller root cgroup process behaviour + 5-N-2. IO controller root cgroup process behaviour + 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 +qualifier as in "cgroup controllers". When explicitly referring to +multiple individual control groups, the plural form "cgroups" is used. + + +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. + + +Basic Operations +================ + +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. + +During transition to v2, system management software might still +automount the v1 cgroup filesystem and so hijack all controllers +during boot, before manual intervention is possible. To make testing +and experimenting easier, the kernel parameter cgroup_no_v1= allows +disabling controllers in v1 and make them always available in v2. + +cgroup v2 currently supports the following mount options. + + nsdelegate + + Consider cgroup namespaces as delegation boundaries. This + option is system wide and can only be set on mount or modified + through remount from the init namespace. The mount option is + ignored on non-init namespace mounts. Please refer to the + Delegation section for details. + + +Organizing Processes and Threads +-------------------------------- + +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) + + +Threads +~~~~~~~ + +cgroup v2 supports thread granularity for a subset of controllers to +support use cases requiring hierarchical resource distribution across +the threads of a group of processes. By default, all threads of a +process belong to the same cgroup, which also serves as the resource +domain to host resource consumptions which are not specific to a +process or thread. The thread mode allows threads to be spread across +a subtree while still maintaining the common resource domain for them. + +Controllers which support thread mode are called threaded controllers. +The ones which don't are called domain controllers. + +Marking a cgroup threaded makes it join the resource domain of its +parent as a threaded cgroup. The parent may be another threaded +cgroup whose resource domain is further up in the hierarchy. The root +of a threaded subtree, that is, the nearest ancestor which is not +threaded, is called threaded domain or thread root interchangeably and +serves as the resource domain for the entire subtree. + +Inside a threaded subtree, threads of a process can be put in +different cgroups and are not subject to the no internal process +constraint - threaded controllers can be enabled on non-leaf cgroups +whether they have threads in them or not. + +As the threaded domain cgroup hosts all the domain resource +consumptions of the subtree, it is considered to have internal +resource consumptions whether there are processes in it or not and +can't have populated child cgroups which aren't threaded. Because the +root cgroup is not subject to no internal process constraint, it can +serve both as a threaded domain and a parent to domain cgroups. + +The current operation mode or type of the cgroup is shown in the +"cgroup.type" file which indicates whether the cgroup is a normal +domain, a domain which is serving as the domain of a threaded subtree, +or a threaded cgroup. + +On creation, a cgroup is always a domain cgroup and can be made +threaded by writing "threaded" to the "cgroup.type" file. The +operation is single direction:: + + # echo threaded > cgroup.type + +Once threaded, the cgroup can't be made a domain again. To enable the +thread mode, the following conditions must be met. + +- As the cgroup will join the parent's resource domain. The parent + must either be a valid (threaded) domain or a threaded cgroup. + +- When the parent is an unthreaded domain, it must not have any domain + controllers enabled or populated domain children. The root is + exempt from this requirement. + +Topology-wise, a cgroup can be in an invalid state. Please consider +the following topology:: + + A (threaded domain) - B (threaded) - C (domain, just created) + +C is created as a domain but isn't connected to a parent which can +host child domains. C can't be used until it is turned into a +threaded cgroup. "cgroup.type" file will report "domain (invalid)" in +these cases. Operations which fail due to invalid topology use +EOPNOTSUPP as the errno. + +A domain cgroup is turned into a threaded domain when one of its child +cgroup becomes threaded or threaded controllers are enabled in the +"cgroup.subtree_control" file while there are processes in the cgroup. +A threaded domain reverts to a normal domain when the conditions +clear. + +When read, "cgroup.threads" contains the list of the thread IDs of all +threads in the cgroup. Except that the operations are per-thread +instead of per-process, "cgroup.threads" has the same format and +behaves the same way as "cgroup.procs". While "cgroup.threads" can be +written to in any cgroup, as it can only move threads inside the same +threaded domain, its operations are confined inside each threaded +subtree. + +The threaded domain cgroup serves as the resource domain for the whole +subtree, and, while the threads can be scattered across the subtree, +all the processes are considered to be in the threaded domain cgroup. +"cgroup.procs" in a threaded domain cgroup contains the PIDs of all +processes in the subtree and is not readable in the subtree proper. +However, "cgroup.procs" can be written to from anywhere in the subtree +to migrate all threads of the matching process to the cgroup. + +Only threaded controllers can be enabled in a threaded subtree. When +a threaded controller is enabled inside a threaded subtree, it only +accounts for and controls resource consumptions associated with the +threads in the cgroup and its descendants. All consumptions which +aren't tied to a specific thread belong to the threaded domain cgroup. + +Because a threaded subtree is exempt from no internal process +constraint, a threaded controller must be able to handle competition +between threads in a non-leaf cgroup and its child cgroups. Each +threaded controller defines how such competitions are handled. + + +[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. + + +Controlling Controllers +----------------------- + +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. + + +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. + + +No Internal Process Constraint +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Non-root cgroups can distribute domain resources to their children +only when they don't have any processes of their own. In other words, +only domain cgroups which don't contain any processes can have domain +controllers enabled in their "cgroup.subtree_control" files. + +This guarantees that, when a domain 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 (for more information on this topic please +refer to the Non-normative information section in the Controllers +chapter). + +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. + + +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", +"cgroup.threads" and "cgroup.subtree_control" files to the user. +Second, if the "nsdelegate" mount option is set, automatically to a +cgroup namespace on namespace creation. + +Because the resource control interface files in a given directory +control the distribution of the parent's resources, the delegatee +shouldn't be allowed to write to them. For the first method, this is +achieved by not granting access to these files. For the second, the +kernel rejects writes to all files other than "cgroup.procs" and +"cgroup.subtree_control" on a namespace root from inside the +namespace. + +The end results are equivalent for both delegation types. Once +delegated, the user can build sub-hierarchy under the directory, +organize processes inside it 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. + + +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 delegations to a less privileged user, this is achieved by +requiring the following conditions 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 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 two 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; 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. + +For delegations to namespaces, containment is achieved by requiring +that both the source and destination cgroups are reachable from the +namespace of the process which is attempting the migration. If either +is not reachable, the migration is rejected with -ENOENT. + + +Guidelines +---------- + +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. + + +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. + + +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. + + +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. + + +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. + + +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. + + +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. + + +Interface Files +=============== + +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. + + +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. + + +Core Interface Files +-------------------- + +All cgroup core files are prefixed with "cgroup." + + cgroup.type + + A read-write single value file which exists on non-root + cgroups. + + When read, it indicates the current type of the cgroup, which + can be one of the following values. + + - "domain" : A normal valid domain cgroup. + + - "domain threaded" : A threaded domain cgroup which is + serving as the root of a threaded subtree. + + - "domain invalid" : A cgroup which is in an invalid state. + It can't be populated or have controllers enabled. It may + be allowed to become a threaded cgroup. + + - "threaded" : A threaded cgroup which is a member of a + threaded subtree. + + A cgroup can be turned into a threaded cgroup by writing + "threaded" to this file. + + 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. + + - 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. + + In a threaded cgroup, reading this file fails with EOPNOTSUPP + as all the processes belong to the thread root. Writing is + supported and moves every thread of the process to the cgroup. + + cgroup.threads + A read-write new-line separated values file which exists on + all cgroups. + + When read, it lists the TIDs of all threads which belong to + the cgroup one-per-line. The TIDs are not ordered and the + same TID may show up more than once if the thread got moved to + another cgroup and then back or the TID got recycled while + reading. + + A TID can be written to migrate the thread associated with the + TID to the cgroup. The writer should match all of the + following conditions. + + - It must have write access to the "cgroup.threads" file. + + - The cgroup that the thread is currently in must be in the + same resource domain as the destination cgroup. + + - 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. + + cgroup.max.descendants + A read-write single value files. The default is "max". + + Maximum allowed number of descent cgroups. + If the actual number of descendants is equal or larger, + an attempt to create a new cgroup in the hierarchy will fail. + + cgroup.max.depth + A read-write single value files. The default is "max". + + Maximum allowed descent depth below the current cgroup. + If the actual descent depth is equal or larger, + an attempt to create a new child cgroup will fail. + + cgroup.stat + A read-only flat-keyed file with the following entries: + + nr_descendants + Total number of visible descendant cgroups. + + nr_dying_descendants + Total number of dying descendant cgroups. A cgroup becomes + dying after being deleted by a user. The cgroup will remain + in dying state for some time undefined time (which can depend + on system load) before being completely destroyed. + + A process can't enter a dying cgroup under any circumstances, + a dying cgroup can't revive. + + A dying cgroup can consume system resources not exceeding + limits, which were active at the moment of cgroup deletion. + + +Controllers +=========== + +CPU +--- + +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. + +WARNING: cgroup2 doesn't yet support control of realtime processes and +the cpu controller can only be enabled when all RT processes are in +the root cgroup. Be aware that system management software may already +have placed RT processes into nonroot cgroups during the system boot +process, and these processes may need to be moved to the root cgroup +before the cpu controller can be enabled. + + +CPU Interface Files +~~~~~~~~~~~~~~~~~~~ + +All time durations are in microseconds. + + cpu.stat + A read-only flat-keyed file which exists on non-root cgroups. + This file exists whether the controller is enabled or not. + + It always reports the following three stats: + + - usage_usec + - user_usec + - system_usec + + and the following three when the controller is enabled: + + - 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.weight.nice + A read-write single value file which exists on non-root + cgroups. The default is "0". + + The nice value is in the range [-20, 19]. + + This interface file is an alternative interface for + "cpu.weight" and allows reading and setting weight using the + same values used by nice(2). Because the range is smaller and + granularity is coarser for the nice values, the read value is + the closest approximation of the current weight. + + 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. + + +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. + + +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 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. + + Depending on context result could be invocation of OOM + killer and retrying allocation or failing allocation. + + Failed allocation in its turn could be returned into + userspace as -ENOMEM or silently ignored in cases like + 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 + types of memory, type-specific details, and other information + on the state and past events of the memory management system. + + All memory amounts are in bytes. + + The entries are ordered to be human readable, and new entries + can show up in the middle. Don't rely on items remaining in a + 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 + 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 + + Number of refaults of previously evicted pages + + workingset_activate + + Number of refaulted pages that were immediately activated + + workingset_nodereclaim + + Number of times a shadow node has been reclaimed + + pgrefill + + Amount of scanned pages (in an active LRU list) + + pgscan + + Amount of scanned pages (in an inactive LRU list) + + pgsteal + + Amount of reclaimed pages + + pgactivate + + Amount of pages moved to the active LRU list + + pgdeactivate + + Amount of pages moved to the inactive LRU lis + + pglazyfree + + Amount of pages postponed to be freed under memory pressure + + pglazyfreed + + Amount of reclaimed lazyfree pages + + memory.swap.current + A read-only single value file which exists on non-root + cgroups. + + The total amount of swap currently being used by the cgroup + and its descendants. + + memory.swap.max + A read-write single value file which exists on non-root + cgroups. The default is "max". + + Swap usage hard limit. If a cgroup's swap usage reaches this + limit, anonymous memory of the cgroup will not be swapped out. + + +Usage Guidelines +~~~~~~~~~~~~~~~~ + +"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. + + +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. + + +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. + + +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 + + +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. + + +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 +reached. + +The number of tasks in a cgroup can be exhausted in ways which other +controllers cannot prevent, thus warranting its own controller. For +example, a fork bomb is likely to exhaust the number of tasks before +hitting memory restrictions. + +Note that PIDs used in this controller refer to TIDs, process IDs as +used by the kernel. + + +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 + descendants. + +Organisational operations are not blocked by cgroup policies, so it is +possible to have pids.current > pids.max. This can be done by either +setting the limit to be smaller than pids.current, or attaching enough +processes to the cgroup such that pids.current is larger than +pids.max. However, it is not possible to violate a cgroup PID policy +through fork() or clone(). These will return -EAGAIN if the creation +of a new process would cause a cgroup policy to be violated. + + +Device controller +----------------- + +Device controller manages access to device files. It includes both +creation of new device files (using mknod), and access to the +existing device files. + +Cgroup v2 device controller has no interface files and is implemented +on top of cgroup BPF. To control access to device files, a user may +create bpf programs of the BPF_CGROUP_DEVICE type and attach them +to cgroups. On an attempt to access a device file, corresponding +BPF programs will be executed, and depending on the return value +the attempt will succeed or fail with -EPERM. + +A BPF_CGROUP_DEVICE program takes a pointer to the bpf_cgroup_dev_ctx +structure, which describes the device access attempt: access type +(mknod/read/write) and device (type, major and minor numbers). +If the program returns 0, the attempt fails with -EPERM, otherwise +it succeeds. + +An example of BPF_CGROUP_DEVICE program may be found in the kernel +source tree in the tools/testing/selftests/bpf/dev_cgroup.c file. + + +RDMA +---- + +The "rdma" controller regulates the distribution and accounting of +of RDMA resources. + +RDMA Interface Files +~~~~~~~~~~~~~~~~~~~~ + + rdma.max + A readwrite nested-keyed file that exists for all the cgroups + except root that describes current configured resource limit + for a RDMA/IB device. + + Lines are keyed by device name and are not ordered. + Each line contains space separated resource name and its configured + limit that can be distributed. + + 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:: + + mlx4_0 hca_handle=2 hca_object=2000 + ocrdma1 hca_handle=3 hca_object=max + + rdma.current + 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:: + + mlx4_0 hca_handle=1 hca_object=20 + ocrdma1 hca_handle=1 hca_object=23 + + +Misc +---- + +perf_event +~~~~~~~~~~ + +perf_event controller, if not mounted on a legacy hierarchy, is +automatically enabled on the v2 hierarchy so that perf events can +always be filtered by cgroup v2 path. The controller can still be +moved to a legacy hierarchy after v2 hierarchy is populated. + + +Non-normative information +------------------------- + +This section contains information that isn't considered to be a part of +the stable kernel API and so is subject to change. + + +CPU controller root cgroup process behaviour +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +When distributing CPU cycles in the root cgroup each thread in this +cgroup is treated as if it was hosted in a separate child cgroup of the +root cgroup. This child cgroup weight is dependent on its thread nice +level. + +For details of this mapping see sched_prio_to_weight array in +kernel/sched/core.c file (values from this array should be scaled +appropriately so the neutral - nice 0 - value is 100 instead of 1024). + + +IO controller root cgroup process behaviour +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Root cgroup processes are hosted in an implicit leaf child node. +When distributing IO resources this implicit child node is taken into +account as if it was a normal child cgroup of the root cgroup with a +weight value of 200. + + +Namespace +========= + +Basics +------ + +cgroup namespace provides a mechanism to virtualize the view of the +"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone +flag can be used with clone(2) and unshare(2) to create a new cgroup +namespace. The process running inside the cgroup namespace will have +its "/proc/$PID/cgroup" output restricted to cgroupns root. The +cgroupns root is the cgroup of the process at the time of creation of +the cgroup namespace. + +Without cgroup namespace, the "/proc/$PID/cgroup" file shows the +complete path of the cgroup of a process. In a container setup where +a set of cgroups and namespaces are intended to isolate processes the +"/proc/$PID/cgroup" file may leak potential system level information +to the isolated processes. For Example:: + + # 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:: + + # 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:: + + # ls -l /proc/self/ns/cgroup + lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183] + # cat /proc/self/cgroup + 0::/ + +When some thread from a multi-threaded process unshares its cgroup +namespace, the new cgroupns gets applied to the entire process (all +the threads). This is natural for the v2 hierarchy; however, for the +legacy hierarchies, this may be unexpected. + +A cgroup namespace is alive as long as there are processes inside or +mounts pinning it. When the last usage goes away, the cgroup +namespace is destroyed. The cgroupns root and the actual cgroups +remain. + + +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 +/batchjobs/container_id1 cgroup calls unshare, cgroup +/batchjobs/container_id1 becomes the cgroupns root. For the +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:: + + # ~/unshare -c # unshare cgroupns in some cgroup + # cat /proc/self/cgroup + 0::/ + # mkdir sub_cgrp_1 + # echo 0 > sub_cgrp_1/cgroup.procs + # cat /proc/self/cgroup + 0::/sub_cgrp_1 + +Each process gets its namespace-specific view of "/proc/$PID/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:: + + # sleep 100000 & + [1] 7353 + # echo 7353 > sub_cgrp_1/cgroup.procs + # cat /proc/7353/cgroup + 0::/sub_cgrp_1 + +From the initial cgroup namespace, the real cgroup path will be +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:: + + # cat /proc/7353/cgroup + 0::/../container_id2/sub_cgrp_1 + +Note that the relative path always starts with '/' to indicate that +its relative to the cgroup namespace root of the caller. + + +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:: + + # cat /proc/7353/cgroup + 0::/sub_cgrp_1 + # echo 7353 > batchjobs/container_id2/cgroup.procs + # cat /proc/7353/cgroup + 0::/../container_id2 + +Note that this kind of setup is not encouraged. A task inside cgroup +namespace should only be exposed to its own cgroupns hierarchy. + +setns(2) to another cgroup namespace is allowed when: + +(a) the process has CAP_SYS_ADMIN against its current user namespace +(b) the process has CAP_SYS_ADMIN against the target cgroup + namespace's userns + +No implicit cgroup changes happen with attaching to another cgroup +namespace. It is expected that the someone moves the attaching +process under the target cgroup namespace root. + + +Interaction with Other Namespaces +--------------------------------- + +Namespace specific cgroup hierarchy can be mounted by a process +running inside a non-init cgroup namespace:: + + # mount -t cgroup2 none $MOUNT_POINT + +This will mount the unified cgroup hierarchy with cgroupns root as the +filesystem root. The process needs CAP_SYS_ADMIN against its user and +mount namespaces. + +The virtualization of /proc/self/cgroup file combined with restricting +the view of cgroup hierarchy by namespace-private cgroupfs mount +provides a properly isolated cgroup view inside the container. + + +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. + + +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. + + +Deprecated v1 Core Features +=========================== + +- Multiple hierarchies including named ones are not supported. + +- All v1 mount options 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. + + +Issues with v1 and Rationales for v2 +==================================== + +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. + + +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. + + +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. + + +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. + + +Controller Issues and Remedies +------------------------------ + +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. + +Setting the original memory.limit_in_bytes below the current usage was +subject to a race condition, where concurrent charges could cause the +limit setting to fail. memory.max on the other hand will first set the +limit to prevent new charges, and then reclaim and OOM kill until the +new limit is met - or the task writing to memory.max is killed. + +The combined memory+swap accounting and limiting is replaced by real +control over swap space. + +The main argument for a combined memory+swap facility in the original +cgroup design was that global or parental pressure would always be +able to swap all anonymous memory of a child group, regardless of the +child's own (possibly untrusted) configuration. However, untrusted +groups can sabotage swapping by other means - such as referencing its +anonymous memory in a tight loop - and an admin can not assume full +swappability when overcommitting untrusted jobs. + +For trusted jobs, on the other hand, a combined counter is not an +intuitive userspace interface, and it flies in the face of the idea +that cgroup controllers should account and limit specific physical +resources. Swap space is a resource like all others in the system, +and that's why unified hierarchy allows distributing it separately. |
