linux-kernel - Re: [PATCH] sched/core: Fix cpu controller for !RT_GROUP

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Message-ID: <20190605142003.GD4255@blackbody.suse.cz>
Date:   Wed, 5 Jun 2019 16:20:03 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     peterz@...radead.org, mingo@...hat.com, rostedt@...dmis.org,
        lizefan@...wei.com, tj@...nel.org, bristot@...hat.com,
        luca.abeni@...tannapisa.it, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/core: Fix cpu controller for !RT_GROUP_SCHED

On Wed, Jun 05, 2019 at 01:49:35PM +0200, Juri Lelli <juri.lelli@...hat.com> wrote:
> Existing code comes with a comment saying the "we don't support RT-tasks
> being in separate groups".
I'm also inclined to this check not being completely correct.

This guard also prevents enabling cpu controller on unified hierarchy
with !CONFIG_RT_GROUP_SCHED. (If there are any kernel RT threads in root
cgroup, they can't be migrated to the newly create cpu controller's root
in cgroup_update_dfl_csses().)

I considered relaxing the check to non-root cgroups only, however, as
your example shows, it doesn't prevent reaching the avoided state by
other paths. I'm not that familiar with RT sched to tell whether
RT-priority tasks in different task_groups break any assumptions.

Michal