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

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Message-ID: <CAKohponQP7+fUJtTysDiaDNQA0fTCcErb97gpKCxXpshSOuYfg@mail.gmail.com>
Date:	Mon, 7 Jan 2013 23:37:22 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	rostedt@...dmis.org, Vincent Guittot <vincent.guittot@...aro.org>,
	pjt@...gle.com, paul.mckenney@...aro.org, tglx@...utronix.de,
	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

[Removed Suresh and Venki from discussion, they switched their companies
probably]

On 7 January 2013 20:34, Tejun Heo <tj@...nel.org> wrote:
> The latter part "not using idle cpu just for processing work" does
> apply to homogeneous systems too but as I wrote earlier work items
> don't spontaneously happen on an idle core.  Something, including
> timer, needs to kick it.  So, by definition, a CPU already isn't idle
> when a work item starts execution on it.  What am I missing here?

We are talking about a core being idle from schedulers perspective :)

>> I have another idea that we can try:
>>
>> queue_work_on_any_cpu().
>>
>> 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?
>
> Yeah, this could be a better solution, I think.  Plus, it's not like
> finding the optimal cpu is free.

Thanks for the first part (When i shared this idea with Vincent and Amit, i
wasn't sure at all about the feedback i will get from you and others, but i
am very happy now :) ).

I couldn't understand the second part. We still need to search for a free cpu
for this new routine. And the implementation would almost be same as the
implementation of queue_work() in my initial patch

>> 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.
>
> Let's start simple for now.  If we really need it, we can always add
> more later.

:)
Agreed. But i liked the idea from steven, we can have two routines:
queue_work_on_any_cpu() and queue_work_on_cpus()
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