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Date:   Wed, 5 Jun 2019 16:20:03 +0200
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     peterz@infradead.org, mingo@redhat.com, rostedt@goodmis.org,
        lizefan@huawei.com, tj@kernel.org, bristot@redhat.com,
        luca.abeni@santannapisa.it, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/core: Fix cpu controller for !RT_GROUP_SCHED
Message-ID: <20190605142003.GD4255@blackbody.suse.cz>
References: <20190605114935.7683-1-juri.lelli@redhat.com>
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On Wed, Jun 05, 2019 at 01:49:35PM +0200, Juri Lelli <juri.lelli@redhat.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