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Date: Wed, 19 Oct 2011 12:47:32 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
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>
Subject: Re: patch] cpusets, cgroups: disallow attaching kthreadd
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On Wed, 19 Oct 2011, Peter Zijlstra wrote:

> 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.
> 

Cgroups don't necessarily imply constraints, though, you could devise a 
cgroup to just do monitoring or statistics tracking for an aggregate of 
tasks and placing kthreadd in such a cgroup would make perfect sense 
because then, since children are forked in the same cgroup, you can 
monitor or gather statistics for all kthreads.  This can be your only 
cgroup on the system.

Cpusets, though, does imply cpu constraints, which is why we decline 
PF_THREAD_BOUND threads from moving in the first place, which is the 
source of Mike's issue.  It's can_attach() function can explicitly decline 
kthreadd as well since the cpu constraints of both types of threads should 
never be altered by either cpusets or sched_setaffinity().