netdev - Re: [PATCH repost] sched: export sched

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Message-ID: <20100701115507.GA31333@redhat.com>
Date:	Thu, 1 Jul 2010 14:55:07 +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 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.
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