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Date:	Thu, 1 Jul 2010 15:23:40 +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 02:55:07PM +0300, Michael S. Tsirkin wrote:
> On Thu, Jul 01, 2010 at 01:43:23PM +0200, Peter Zijlstra wrote:
> > On Thu, 2010-07-01 at 13:19 +0200, Peter Zijlstra wrote:
> > > On Thu, 2010-07-01 at 14:07 +0300, Michael S. Tsirkin wrote:
> > > > Author: Sridhar Samudrala <sri@...ibm.com>
> > > > 
> > > > sched: export sched_set/getaffinity to modules
> > > > 
> > > > vhost-net driver wants to copy the affinity from the
> > > > owner thread to thread it creates. Export
> > > > sched_set/get affinity to modules to make this possible
> > > > when vhost is built as a module.
> > 
> > > Urgh,.. so why again is that a good idea?
> > 
> > In particular:
> >  - who sets the affinity of the task? 
> 
> management tools do this when they start qemu.
> 
> >  - 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.
> 
> >  - 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.
> 
> > So no, I don't think this is a sensible thing to do at all.

The patch using this is here:
http://www.mail-archive.com/kvm@vger.kernel.org/msg35411.html

It simply copies the affinity from the parent when thread is created.

-- 
MST
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