[PATCH v2 cgroup/for-4.15-fixes] cgroup: add warning about RT not being supported on cgroup2

[Tejun Heo <tj@kernel.org>]

>From f0a821260f24ea9378ed0e0c149e3bbc1a1bd008 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Tue, 5 Dec 2017 09:10:17 -0800

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 <tj@kernel.org>
Reported-by: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
---
 Documentation/cgroup-v2.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index 779211f..c5e3c21 100644
--- 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.
 
+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 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
 ~~~~~~~~~~~~~~~~~~~
-- 
2.9.5