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cgroup, docs: Be explicit about independence of RT_GROUP_SCHED and non-cpu controllers

The cgroup v2 cpu controller has a limitation that if
CONFIG_RT_GROUP_SCHED is enabled, the cpu controller can be enabled only
if all the realtime processes are in the root cgroup. The other
controllers have no such restriction. They can be used for the resource
control of realtime processes irrespective of whether
CONFIG_RT_GROUP_SCHED is enabled or not.

Signed-off-by: Shashank Balaji <shashank.mahadasyam@sony.com>
Acked-by: Waiman Long <longman@redhat.com>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

authored by

Shashank Balaji and committed by
Tejun Heo c7461cca c4af66a9

+14 -9
+14 -9
Documentation/admin-guide/cgroup-v2.rst
··· 1075 1075 provided by a CPU, as well as the maximum desired frequency, which should not 1076 1076 be exceeded by a CPU. 1077 1077 1078 - WARNING: cgroup2 doesn't yet support control of realtime processes. For 1079 - a kernel built with the CONFIG_RT_GROUP_SCHED option enabled for group 1080 - scheduling of realtime processes, the cpu controller can only be enabled 1081 - when all RT processes are in the root cgroup. This limitation does 1082 - not apply if CONFIG_RT_GROUP_SCHED is disabled. Be aware that system 1083 - management software may already have placed RT processes into nonroot 1084 - cgroups during the system boot process, and these processes may need 1085 - to be moved to the root cgroup before the cpu controller can be enabled 1086 - with a CONFIG_RT_GROUP_SCHED enabled kernel. 1078 + WARNING: cgroup2 cpu controller doesn't yet fully support the control of 1079 + realtime processes. For a kernel built with the CONFIG_RT_GROUP_SCHED option 1080 + enabled for group scheduling of realtime processes, the cpu controller can only 1081 + be enabled when all RT processes are in the root cgroup. Be aware that system 1082 + management software may already have placed RT processes into non-root cgroups 1083 + during the system boot process, and these processes may need to be moved to the 1084 + root cgroup before the cpu controller can be enabled with a 1085 + CONFIG_RT_GROUP_SCHED enabled kernel. 1086 + 1087 + With CONFIG_RT_GROUP_SCHED disabled, this limitation does not apply and some of 1088 + the interface files either affect realtime processes or account for them. See 1089 + the following section for details. Only the cpu controller is affected by 1090 + CONFIG_RT_GROUP_SCHED. Other controllers can be used for the resource control of 1091 + realtime processes irrespective of CONFIG_RT_GROUP_SCHED. 1087 1092 1088 1093 1089 1094 CPU Interface Files