linux-kernel - Re: [PATCH V2 Resend 3/4] workqueue: Schedule work on non-idle cpu instead of current one

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Date:	Tue, 27 Nov 2012 10:49:54 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Tejun Heo <tj@...nel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	pjt@...gle.com, paul.mckenney@...aro.org, tglx@...utronix.de,
	suresh.b.siddha@...el.com, venki@...gle.com, mingo@...hat.com,
	peterz@...radead.org, rostedt@...dmis.org, Arvind.Chauhan@....com,
	linaro-dev@...ts.linaro.org, patches@...aro.org,
	pdsw-power-team@....com, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH V2 Resend 3/4] workqueue: Schedule work on non-idle cpu
 instead of current one

Hi Tejun,

On 26 November 2012 22:45, Tejun Heo <tj@...nel.org> wrote:
> On Tue, Nov 06, 2012 at 04:08:45PM +0530, Viresh Kumar wrote:

> I'm pretty skeptical about this.  queue_work() w/o explicit CPU
> assignment has always guaranteed that the work item will be executed
> on the same CPU.  I don't think there are too many users depending on
> that but am not sure at all that there are none.  I asked you last
> time that you would at the very least need to audit most users but it
> doesn't seem like there has been any effort there.

My bad. I completely missed/forgot that comment from your earlier mails.
Will do it.

> That said, if the obtained benefit is big enough, sure, we can
> definitely change the behavior, which isn't all that great to begin
> with, and try to shake out the bugs quicky by e.g. forcing all work
> items to execute on different CPUs, but you're presenting a few
> percent of work items being migrated to a different CPU from an
> already active CPU, which doesn't seem like such a big benefit to me
> even if the migration target CPU is somewhat more efficient.  How much
> powersaving are we talking about?

Hmm.. I actually implemented the problem discussed here:
(I know you have seen this earlier :) )

http://www.linuxplumbersconf.org/2012/wp-content/uploads/2012/08/lpc2012-sched-timer-workqueue.pdf

Specifically slides: 12 & 19.

I haven't done much power calculations with it and have tested it more from
functionality point of view.

@Vincent: Can you add some comments here?

--
viresh
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