linux-kernel - Re: [patch] cpusets, cgroups: disallow attaching kthreadd

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Date:	Tue, 11 Oct 2011 16:08:07 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Tejun Heo <htejun@...il.com>
Cc:	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Paul Menage <paul@...lmenage.org>,
	David Rientjes <rientjes@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch] cpusets, cgroups: disallow attaching kthreadd

On Mon, 2011-10-10 at 09:43 -0700, Tejun Heo wrote:

> Yes, I think we need something like this.  wq workers were using
> PF_THREAD_BOUND to prevent diddling from userland which made some
> unhappy. 

But that can be properly fixed.

>  Maybe we need a flag to properly indicate "don't diddle with
> this thread from userland"?  But, then, mainline kernel wouldn't need
> the current PF_THREAD_BOUND at all.  Peter, Steven, what do you think?

Strict per-cpu affinity that is needed for correctness and disallows
sched_setaffinity() is something entirely different from not being
allowed to put something in a cgroup.

As to not allowing to put in a cgroup thing, is there anything other
than kthreadd for which we need to enforce that? So far I've mostly
treated it like: root can do stupid things, this is one of them, don't
do that then.

I don't think its horribly bad to change the affinity mask of kthreadd,
if a sibling kthread needs a specific affinity it should set that and
override whatever it inherits. If it runs with the default, its a neat
way of setting that.

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