linux-kernel - Re: [discussion]sched: a rough proposal to enable power saving in scheduler

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Message-ID: <502D3FB2.7000008@redhat.com>
Date:	Thu, 16 Aug 2012 14:45:06 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
CC:	preeti <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Alex Shi <alex.shi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	vincent.guittot@...aro.org, svaidy@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [discussion]sched: a rough proposal to enable power saving in
 scheduler

On 08/16/2012 10:01 AM, Arjan van de Ven wrote:
>> *Power policy*:
>>
>> So how is power policy different? As Peter says,'pack more than spread
>> more'.
>
> this is ... a dubiously general statement.
>
> for good power, at least on Intel cpus, you want to spread. Parallelism is efficient.
>
> the only thing you do not want to do, is wake cpus up for
> tasks that only run extremely briefly (think "100 usec" or less).
>
> so maybe the balance interval is slightly different, or more, you don't balance tasks that
> historically ran only for brief periods

This makes me think that maybe, in addition to tracking
the idle residency time in the c-state governor, we may
also want to track the average run times in the scheduler.

The c-state governor can call the scheduler code before
putting a CPU to sleep, to indicate (1) the wakeup latency
of the CPU, and (2) whether TLB and/or cache get invalidated.

At wakeup time, the scheduler can check whether the CPU
the to-be-woken process ran on is in a deeper sleep state,
and whether the typical run time for the process significantly
exceeds the wakeup latency of the CPU it last ran on.

If the process typically runs for a short interval, and/or
the process's CPU lost its cached state, it may be better
to run the just-woken task on the CPU that is doing the
waking up, instead of on the CPU where it used to run.

Does that make sense?

Am I overlooking any factors?

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