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Date:	Thu, 1 Jul 2010 15:50:38 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Sridhar Samudrala <sri@...ibm.com>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitri Vorobiev <dmitri.vorobiev@...ial.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH repost] sched: export sched_set/getaffinity to modules

On Thu, Jul 01, 2010 at 02:32:43PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-07-01 at 14:55 +0300, Michael S. Tsirkin wrote:
> 
> > >  - why can't it set the kernel thread's affinity too?
> > 
> > It can. However: the threads are started internally by the driver
> > when qemu does an ioctl.  What we want to do is give it a sensible
> > default affinity. management tool can later tweak it if it wants to.
> 
> So have that ioctl return the tid of that new fancy thread and then set
> its affinity, stuff it in cgroup, whatever you fancy.
> 
> > >  - what happens if someone changes the tasks' affinity?
> > 
> > We would normally create a cgroup including all internal
> > tasks, making it easy to find and change affinity for
> > them all if necessary. 
> 
> And to stuff them in a cgroup you also need the tid, at which point it
> might as well set the affinity from userspace, right?

We also put it in a cgroup transparently. I think that it's actually
important to do it on thread creation: if we don't, malicious userspace
can create large amount of work exceeding the cgroup limits.

And the same applies so the affinity, right? If the qemu process
is limited to a set of CPUs, isn't it important to make
the kernel thread that does work our behalf limited to the same
set of CPUs?

-- 
MST
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