linux-kernel - Re: [PATCH V3 6/7] block: queue work on any cpu

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Message-ID: <CAKohpon7SD666o=7sH9QpOt2dbiU2Jy1OE8E__y3+=Q-o33Xnw@mail.gmail.com>
Date:	Sat, 23 Mar 2013 12:14:44 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	pjt@...gle.com, paul.mckenney@...aro.org, tglx@...utronix.de,
	tj@...nel.org, Ingo Molnar <mingo@...hat.com>,
	peterz@...radead.org, rostedt@...dmis.org,
	linaro-kernel@...ts.linaro.org, robin.randhawa@....com,
	Steve.Bannister@....com, Liviu.Dudau@....com,
	charles.garcia-tobin@....com, Arvind.Chauhan@....com,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 6/7] block: queue work on any cpu

On 22 March 2013 20:35, Jens Axboe <axboe@...nel.dk> wrote:

Hi Jens,

First of all, please have a look at:

1. https://lkml.org/lkml/2013/3/18/364

This is cover-letter of my mail where i have shown power savings achieved
with this patchset.

2. https://lkml.org/lkml/2013/3/21/172

This is the link for discussion i had with Tejun which answers some of these
questions.

> What are the performance implications of this? From that perspective,
> scheduling on a local core is the better choice. Hopefully it's already
> running on the local socket of the device, keeping it on that node would
> be preferable.

If the local cpu is busy or all cpus are idle then we don't migrate, and so
for performance shouldn't be affected with it.

> For the delayed functions, the timer is typically added on the current
> CPU.

This is the code that executes here:

	if (unlikely(cpu != WORK_CPU_UNBOUND))
		add_timer_on(timer, cpu);
	else
		add_timer(timer);

Clearly for queue_delayed_work() call we don't use local cpu but we use
the same sched_select_non_idle_cpu() routine to give us the right cpu.

> It's hard to know what the state of idleness of CPUs would be when
> the delay has expired. How are you handling this?

Correct, and we are not deciding that right now, but when the timer expires.

Thanks for your feedback.
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