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

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Date:	Mon, 07 Jan 2013 08:28:23 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Tejun Heo <tj@...nel.org>,
	Vincent Guittot <vincent.guittot@...aro.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, 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

On Mon, 2013-01-07 at 15:28 +0530, Viresh Kumar wrote:
> Hi Tejun,
> 
> On 4 January 2013 20:39, Tejun Heo <tj@...nel.org> wrote:
> > I don't know either.  Changing behavior subtly like this is hard.  I
> > usually try to spot some problem cases and try to identify patterns
> > there.  Once you identify a few of them, understanding and detecting
> > other problem cases get a lot easier.  In this case, maybe there are
> > too many places to audit and the problems are too subtle, and, if we
> > *have* to do it, the only thing we can do is implementing a debug
> > option to make such problems more visible - say, always schedule to a
> > different cpu on queue_work().
> >
> > That said, at this point, the patchset doesn't seem all that
> > convincing to me and if I'm comprehending responses from others
> > correctly that seems to be the consensus.  It might be a better
> > approach to identify the specific offending workqueue users and make
> > them handle the situation more intelligently than trying to impose the
> > behavior on all workqueue users.  At any rate, we need way more data
> > showing this actually helps and if so why.
> 
> I understand your concerns and believe me, even i feel the same :)
> I had another idea, that i wanted to share.
> 
> Firstly the root cause of this patchset.
> 
> Myself and some others in Linaro are working on ARM future cores:
> big.LITTLE systems.
> Here we have few very powerful, high power consuming cores (big,
> currently A15's) and
> few very efficient ones (LITTLE, currently A7's).
> 
> The ultimate goal is to save as much power as possible without compromising
> much with performance. For, that we wanted most of the stuff to run on LITTLE
> cores and some performance-demanding stuff on big Cores. There are
> multiple things
> going around in this direction. Now, we thought A15's or big cores
> shouldn't be used
> for running small tasks like timers/workqueues and hence this patch is
> an attempt
> towards reaching that goal.
> 
> Over that we can do some load balancing of works within multiple alive
> cpus, so that
> it can get done quickly. Also, we shouldn't start using an idle cpu
> just for processing
> work :)
> 
> I have another idea that we can try:
> 
> queue_work_on_any_cpu().

I think this is a good idea.

> 
> With this we would not break any existing code and can try to migrate
> old users to
> this new infrastructure (atleast the ones which are rearming works from their
> work_handlers). What do you say?
> 
> To take care of the cache locality issue, we can pass an argument to
> this routine,
> that can provide
> - the mask of cpus to schedule this work on
>   OR
> - Sched Level (SD_LEVEL) of cpus to run it.

I wouldn't give a mask. If one is needed, we could have a
queue_work_on_mask_cpus(), or something. I think the "any" in the name
should be good enough to let developers know that this will not be on
the CPU that is called. By default, I would suggest for cache locality,
that we try to keep it on the same CPU. But if there's a better CPU to
run on, it runs there. Also, we could still add a debug option that
makes it always run on other CPUs to slap developers that don't read.

-- Steve

> 
> Waiting for your view on it :)
> 
> --
> viresh


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