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docs: cgroup-v2: Describe effective CPUs of partition roots

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The cpuset.cpus.effective description says that the effective CPUs are
granted by the parent and are normally a subset of cpuset.cpus. This
does not always hold for a valid non-root partition root. Its effective
CPUs are the active CPUs in cpuset.cpus.exclusive.effective after CPUs
assigned to valid child partition roots are removed.

If cpuset.cpus.exclusive differs from cpuset.cpus, the effective CPUs
may therefore be outside cpuset.cpus. Remote partitions also do not
necessarily get their CPUs from their immediate parent.

Suggested-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Guopeng Zhang 2026-09-22 17:20:34 +08:00 committed by Tejun Heo
parent 7abedcd2ab
commit aa4decd2d9
1 changed files with 48 additions and 9 deletions
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57
Documentation/admin-guide/cgroup-v2.rst
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@ -2536,16 +2536,55 @@ Cpuset Interface Files
A read-only multiple values file which exists on all
cpuset-enabled cgroups.
It lists the onlined CPUs that are actually granted to this
cgroup by its parent. These CPUs are allowed to be used by
tasks within the current cgroup.
It lists the active CPUs available to tasks in this cgroup.
If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows
all the CPUs from the parent cgroup that can be available to
be used by this cgroup. Otherwise, it should be a subset of
"cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus"
can be granted. In this case, it will be treated just like an
empty "cpuset.cpus".
For a cgroup that is not a valid partition root, an empty
"cpuset.cpus" makes "cpuset.cpus.effective" show all CPUs
available from the parent cgroup. Otherwise, it is a subset
of "cpuset.cpus" unless none of the requested CPUs can be
granted. In that case, it is treated like an empty
"cpuset.cpus".
For example::
P (cpuset.cpus.effective=0-3)
|
+-- C0 (member): cpuset.cpus=1-2
| cpuset.cpus.effective=1-2
+-- C1 (invalid root): cpuset.cpus=4-5
| cpuset.cpus.effective=0-3
\-- C2 (member): cpuset.cpus=<empty>
cpuset.cpus.effective=0-3
C0 gets CPUs 1-2 because they are available from P.
None of C1's requested CPUs 4-5 are available from P, so C1 uses P's
effective CPUs 0-3 instead.
C2 has an empty "cpuset.cpus" and therefore also uses P's effective
CPUs 0-3.
For a valid non-root partition root, "cpuset.cpus.effective" contains
the active CPUs from "cpuset.cpus.exclusive.effective" except
those granted to valid child partition roots. If
"cpuset.cpus.exclusive" is set to a value different from
"cpuset.cpus", the effective CPUs need not be a subset of
"cpuset.cpus".
For example, if CPUs 2-5 are allocated to P for exclusive use,
CPUs 2-4 are active, CPU 5 is offline, and none of CPUs 2-5 are
assigned to a valid child partition root::
top (root)
|
\-- P (valid partition root)
cpuset.cpus=0-3
cpuset.cpus.exclusive=2-5
cpuset.cpus.exclusive.effective=2-5
cpuset.cpus.effective=2-4
P has 2-5 in "cpuset.cpus.exclusive.effective", including the
offline CPU 5. CPU 5 is excluded from "cpuset.cpus.effective",
leaving 2-4. The result also shows that "cpuset.cpus.effective"
need not be a subset of "cpuset.cpus", which is 0-3.
Its value will be affected by CPU hotplug events.