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

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Message-ID: <CAKgNAkjm9wh-XWfMHV1gjY0VEVAkq9Rc-mmSYDpCChwpAyVN7w@mail.gmail.com>
Date:   Tue, 5 Dec 2017 19:24:08 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH cgroup/for-4.15-fixes] cgroup: add warning about RT not
 being supported on cgroup2

Hello Tejun,

On 5 December 2017 at 18:13, Tejun Heo <tj@...nel.org> wrote:
> From: Tejun Heo <tj@...nel.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.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
> ---
>  Documentation/cgroup-v2.txt | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
> index 779211f..55a28f4 100644
> --- a/Documentation/cgroup-v2.txt
> +++ b/Documentation/cgroup-v2.txt
> @@ -898,6 +898,10 @@ 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.
> +

The documentation file already carries this useful text:

[[
During transition to v2, system management software might still
automount the v1 cgroup filesystem and so hijack all controllers
during boot, before manual intervention is possible. To make testing
and experimenting easier, the kernel parameter cgroup_no_v1= allows
disabling controllers in v1 and make them always available in v2.
]]

In that spirit, it may be worth alerting the reader that "system
management software" might already have put some RT processes in
nonroot cgroups. Maybe something like this:

[[
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.
]]

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/