linux-kernel - Re: cgroup scheduling: Adding kthreadd to a non-RT cgroup can deadlock the kernel

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Message-ID: <AANLkTi=FC27TTngERgbS95Z29CYBqLGxR4Uuw9fSTVuO@mail.gmail.com>
Date:	Wed, 5 Jan 2011 10:02:31 -0500
From:	Nelson Elhage <nelhage@...lice.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: cgroup scheduling: Adding kthreadd to a non-RT cgroup can
 deadlock the kernel

Ok. I bothered reporting this here since the kernel /does/ try to prevent you
from shooting yourself in the foot in related ways, by moving existing
SCHED_FIFO tasks into a cgroup without any RT runtime, so I figured it might
make sense to add a check here. libcgroup, for example, will try to move all
processes into a default cgroup, ignoring errors, and relies on the kernel to
prevent it from shooting itself in the foot.

It sounds like you consider that behavior a bug, though, so I'll go report this
bug there.

- Nelson

On Wed, Jan 5, 2011 at 4:44 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2011-01-04 at 23:54 -0500, Nelson Elhage wrote:
>> Hi,
>>
>> I've found a bug where, on CONFIG_RT_GROUP_SCHED systems, adding the kthreadd
>> task to a cgroup with cpu.rt_runtime_us = 0 (as some cgroup configuration
>> scripts do, when they move all processes into a default cgroup), can result in
>> deadlocks in the kernel.
>
> IMHO its a bug to move kthreadd into a cgroup. Simply don't do that.
>
>
>
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