linux-kernel - Re: [patch] sched: prevent bound kthreads from changing cpus

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Message-ID: <20080610170005.GA6038@tv-sign.ru>
Date:	Tue, 10 Jun 2008 21:00:05 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Max Krasnyansky <maxk@...lcomm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Paul Jackson <pj@....com>, mingo@...e.hu, menage@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] sched: prevent bound kthreads from changing	cpus_allowed

On 06/10, Max Krasnyansky wrote:
> 
> Peter Zijlstra wrote:
> >
> > Per cpu workqueues should stay on their cpu.
> >
> > What you're really looking for is a more fine grained alternative to
> > flush_workqueue().
> Actually I had a discussion on that with Oleg Nesterov. If you remember my
> original solution (ie centralized cpu_isolate_map) was to completely redirect
> work onto other cpus. Then you pointed out that it's the flush_() that really
> makes the box stuck. So I started thinking about redoing the flush. While
> looking at the code I realized that if I only change the flush_() then queued
> work can get stale so to speak. ie Machine does not get stuck but some work
> submitted on the isolated cpus will sit there for a long time. Oleg pointed
> out exact same thing. So the simplest solution that does not require any
> surgery to the workqueue is to just move the threads to other cpus.

Cough... I'd like to mention that I _personally agree with Peter, cwq->thread's
should stay on their cpu.

I just meant that from the workqueue.c pov it is (afaics) OK to move cwq->thread
to other CPUs, in a sense that this shouldn't add races or hotplug problems, etc.
But still this doesn't look right to me.

Oleg.

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