linux-kernel - Re: [PATCH] x86,sched: Fix sched_smt_power

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Date:	Mon, 9 Jan 2012 15:29:36 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Youquan Song <youquan.song@...el.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
	hpa@...or.com, akpm@...ux-foundation.org, stable@...r.kernel.org,
	suresh.b.siddha@...el.com, arjan@...ux.intel.com,
	len.brown@...el.com, anhua.xu@...el.com, chaohong.guo@...el.com,
	Youquan Song <youquan.song@...ux.intel.com>
Subject: Re: [PATCH] x86,sched: Fix sched_smt_power_savings totally broken

Hi Peter,

I'm also using sched_mc level for doing powersaving load balance on
ARM platform and we have real benefits.

On 9 January 2012 12:12, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Mon, 2012-01-09 at 19:14 -0500, Youquan Song wrote:
>> > Yes it is.. also that knob should die! Like i've been saying for way too
>> > long. I'm >< close to committing a patch removing all the power_saving
>> > magic from the scheduler.
>> Sorry. I do not notice it.
>
> I hadn't posted it before, but just about every time people post
> something related to smt/mc power balancing I say this needs to get
> cleaned up. Since telling people doesn't seem to have any effect what so
> ever, stronger measures are needed.
>
>> But currently in many real tests, the knob prove to save power in semi-idle system.
>> They are useful in many user scenarios currently.
>
> I'm not saying the stuff is without merit, I'm just saying that a) the
> interface is utter crap and b) nobody seems to spend enough time on it
> to 1) understand the load-balancer so that 2) he can integrate the power
> saving stuff properly.
>
> Instead I get one sporadic band-aid after another.
>

I can also help to on the topic

> Also, fundamentally, the split between {smt/mc/numa/book}_power_savings
> is completely and fundamentally broken, nobody cares about the actual
> topology.

We might modify the way we choose between power or performance mode
because it's not always a matter of gathering or spreading tasks on
cpus but until we found a best interface it's the way to enable
powersaving mode

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