linux-kernel - Re: [PATCH 04/40] sched: implement __set_cpus

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Message-ID: <1263977401.4283.819.camel@laptop>
Date:	Wed, 20 Jan 2010 09:50:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	torvalds@...ux-foundation.org, mingo@...e.hu, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 04/40] sched: implement __set_cpus_allowed()

On Wed, 2010-01-20 at 17:35 +0900, Tejun Heo wrote:
> Hello,
> 
> On 01/19/2010 05:37 PM, Peter Zijlstra wrote:
> > On Tue, 2010-01-19 at 10:07 +0900, Tejun Heo wrote:
> >>
> >> It's also necessary to guarantee forward progress during CPU_DOWN.
> >> The problem with kthread_bind() is that it's not synchronized against
> >> CPU hotplug operations.  It needs outer synchronization like calling
> >> it directly from CPU_DOWN_PREP.  I guess it's doable but I think it
> >> would be better to simply share the backend implementation between
> >> set_cpus_allowed_ptr() and kthread_bind().
> > 
> > OK, so you're saying you need to migrate the rescue thread during
> > cpu-down. That thread is guaranteed sleeping right,
> 
> No, it's not.  It might have been tasked to process works from other
> CPUs.

OK, so maybe I'm confused, but in general the workqueue thing needs to
ensure work affinity, right? So why can we move the rescue thread while
its processing another CPU's works?

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