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

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Date:	Mon, 1 Oct 2012 11:12:17 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	pjt@...gle.com, paul.mckenney@...aro.org, tglx@...utronix.de,
	suresh.b.siddha@...el.com, venki@...gle.com, mingo@...hat.com,
	peterz@...radead.org, robin.randhawa@....com,
	Steve.Bannister@....com, Arvind.Chauhan@....com,
	amit.kucheria@...aro.org, linaro-dev@...ts.linaro.org,
	patches@...aro.org
Subject: Re: [PATCH V2 3/3] workqueue: Schedule work on non-idle cpu instead
 of current one

On 1 October 2012 05:47, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 1 October 2012 06:02, Tejun Heo <tj@...nel.org> wrote:
>> It isn't about the CPU being actually idle?
>
> No. Being idle only from scheduler's perspective. :)
>
>> Also, if it's only about timers, shouldn't it be enough to implement
>> it for timer and delayed_work?
>
> What if we need a timer, which must re-arm itself + schedule a work?
> delayed_work will not be sufficient in that case, and we would need
> to use normal work.
>
> If i am not wrong, there can be other future users of this routine too.
> @Vincent: Can you please comment on this?

The goal is to consolidate the place, where the selection of a target
CPU for running something is done. The scheduler should select the CPU
in order to have coherent behaviour of all framework.

A delayed work can be delayed for a long period and the target CPU
that have been selected for the timer could not be the best place to
queue a work any more.

The goal is to be able to migrate a work if the scheduler thinks that'
is necessary.

Vincent

>
>> It would be great if you explain what you're trying to achieve how.  I
>> can't tell what you're aiming for and why that would be beneficial
>> from these submissions.
>
> Following slides are implemented by Vincent and presented during LPC.
> Please have a look at them, they explain the problem statement well:
>
> http://www.linuxplumbersconf.org/2012/wp-content/uploads/2012/08/lpc2012-sched-timer-workqueue.pdf
>
> Specifically slides: 12 & 19.
>
> There aren't too many users with this behavior, but even a single user
> will be sufficient not to let the cpu get idle at all. And that will result in
> less power saving.
>
> --
> viresh
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