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cgroup, docs: Clarify limitation of RT processes with cgroup v2 cpu controller 

The limitation that all RT processes have to be in the root cgroup
before enabling cpu controller only applies if the CONFIG_RT_GROUP_SCHED
option is enabled in the running kernel. If a kernel does not have
CONFIG_RT_GROUP_SCHED enabled, RT processes can exist in a non-root
cgroup even when cpu controller is enabled. CPU sharing of RT processes
will not be under cgroup control, but other resources like memory can be.

Clarify this limitation to avoid confusion to users that are using
cgroup v2.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Waiman Long 2024-03-20 10:23:02 -04:00 committed by Tejun Heo
parent 4cece76496
commit 20d46283f5
1 changed files with 9 additions and 6 deletions
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15
Documentation/admin-guide/cgroup-v2.rst
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@ -1058,12 +1058,15 @@ cpufreq governor about the minimum desired frequency which should always be
provided by a CPU, as well as the maximum desired frequency, which should not provided by a CPU, as well as the maximum desired frequency, which should not
be exceeded by a CPU. be exceeded by a CPU.
WARNING: cgroup2 doesn't yet support control of realtime processes and WARNING: cgroup2 doesn't yet support control of realtime processes. For
the cpu controller can only be enabled when all RT processes are in a kernel built with the CONFIG_RT_GROUP_SCHED option enabled for group
the root cgroup. Be aware that system management software may already scheduling of realtime processes, the cpu controller can only be enabled
have placed RT processes into nonroot cgroups during the system boot when all RT processes are in the root cgroup. This limitation does
process, and these processes may need to be moved to the root cgroup not apply if CONFIG_RT_GROUP_SCHED is disabled. Be aware that system
before the cpu controller can be enabled. 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
with a CONFIG_RT_GROUP_SCHED enabled kernel.
CPU Interface Files CPU Interface Files