cgroup: add warning about RT not being supported on cgroup2

We haven't yet figured out what to do with RT threads on cgroup2.
Document the limitation.

v2: Included the warning about system management software behavior as
    suggested by Michael.

Signed-off-by: Tejun Heo <[email protected]>
Reported-by: "Michael Kerrisk (man-pages)" <[email protected]>

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index 779211fbb69ffac450f22b0ad6864c7c6c2bd98f..2cddab7efb20df0dcf07c4d7d64a5611138a8d7c 100644 (file)
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -898,6 +898,13 @@ controller implements weight and absolute bandwidth limit models for
 normal scheduling policy and absolute bandwidth allocation model for
 realtime scheduling policy.
 
 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
 ~~~~~~~~~~~~~~~~~~~
 
 CPU Interface Files
 ~~~~~~~~~~~~~~~~~~~