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Subject: Re: patch] cpusets, cgroups: disallow attaching kthreadd
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: David Rientjes <rientjes@google.com>
Cc: Mike Galbraith <efault@gmx.de>, LKML <linux-kernel@vger.kernel.org>,
        Tejun Heo <htejun@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>,
        Paul Menage <paul@paulmenage.org>
Date: Wed, 19 Oct 2011 09:50:59 +0200
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On Tue, 2011-10-18 at 13:38 -0700, David Rientjes wrote:
> 
> Eek, this seems like complete overkill, there may be cgroups now or in the 
> future that it would be perfectly acceptable to move kthreadd into without 
> any negative effects.  There's no reason to restrict it this broadly, 
> that's why I thought your change to cpusets was fine and acked it.  It's 
> the perfect candidate for what the ->can_attach() pointer for a cgroup can 
> identify is problematic for _that_ cgroup. 

My thinking is that kthreadd is the mother of all kernel threads and the
kernel assumes it can spawn kthreads without constraints, a valid
assumption IMO.

Therefore the kthreadd thread should live in the root cgroup at all
times, irrespective of whatever controllers are or aren't actually safe.