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

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Date:	Tue, 10 Jan 2012 16:32:44 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Youquan Song <youquan.song@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, hpa@...or.com,
	akpm@...ux-foundation.org, stable@...r.kernel.org,
	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

On 10 January 2012 15:32, Arjan van de Ven <arjan@...ux.intel.com> wrote:
> On 1/10/2012 1:18 AM, Ingo Molnar wrote:
>>
>> * Suresh Siddha <suresh.b.siddha@...el.com> wrote:
>>
>>> On Tue, 2012-01-10 at 00:58 -0500, Youquan Song wrote:
>>>> Thanks Peter! Here is the patch.
>>>
>>> Youquan, As far as I know both the
>>> sched_smt_power_savings/sched_mc_power_savings are broken for atleast an
>>> year.
>>
>> We want a single knob, sched_power_savings - with the mc_ and
>> smt_ ones still kept and aliased to sched_power_savings, for
>> compatibility reasons.
>>
>> As Peter said, the other reasonable option is to have no knob at
>> all and restart this code from scratch.
>>
>> The other thing we should do is to add sane defaults: to turn on
>> sched_power_savings *AUTOMATICALLY* when a system is obviously
>> battery driven and turn it off when the system is obviously AC
>> driven. User-space can still implement policy and override the
>> kernel's default, but there's absolutely no excuse to not offer
>> this default ourselves.
>
> a very good default would be to keep all tasks on one package until half
> the cores in the package are busy, and then start spreading out.
>

The choice of spreading or not tasks is clearly architecture or even
platform dependent. Can't we get such optimal threshold  information
from architecture code ?

> I suspect that'll be the 90% case coverage.
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