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Diffstat (limited to 'Documentation/cgroup-v2.txt')
| -rw-r--r-- | Documentation/cgroup-v2.txt | 185 |
1 files changed, 170 insertions, 15 deletions
diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt index f01f831a3b11..cb9ea281ab72 100644 --- a/Documentation/cgroup-v2.txt +++ b/Documentation/cgroup-v2.txt @@ -18,7 +18,9 @@ v1 is available under Documentation/cgroup-v1/. 1-2. What is cgroup? 2. Basic Operations 2-1. Mounting - 2-2. Organizing Processes + 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 @@ -167,8 +169,11 @@ cgroup v2 currently supports the following mount options. Delegation section for details. -Organizing Processes --------------------- +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:: @@ -219,6 +224,104 @@ is removed subsequently, " (deleted)" is appended to the path:: 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. + +- The cgroup must be empty. No enabled controllers, child cgroups or + processes. + +Topology-wise, a cgroup can be in an invalid state. Please consider +the following toplogy:: + + 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 -------------------------- @@ -302,15 +405,15 @@ disabled if one or more children have it enabled. No Internal Process Constraint ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Non-root cgroups can only distribute resources to their children when -they don't have any processes of their own. In other words, only -cgroups which don't contain any processes can have controllers enabled -in their "cgroup.subtree_control" files. +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 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. +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 @@ -334,10 +437,10 @@ 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" -and "cgroup.subtree_control" files to the user. Second, if the -"nsdelegate" mount option is set, automatically to a cgroup namespace -on namespace creation. +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 @@ -644,6 +747,29 @@ 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. @@ -666,6 +792,35 @@ All cgroup core files are prefixed with "cgroup." 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. |