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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>,
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Date: Thu, 20 Oct 2011 12:32:07 +0200
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On Wed, 2011-10-19 at 12:47 -0700, David Rientjes wrote:
> 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,

And yet they're called: Control Groups..

>  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.

I guess you could, but does it really make sense? Also, you could sort
this by extending the cgroup interface to explicitly distinct between
controllers and !controllers.

> 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().

Yeah, and I'm saying we want to exclude _all_ controllers from placing
constraints on kthreadd, even those that might not immediately break
stuff.