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

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Date:	Tue, 10 Jan 2012 09:08:06 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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,
	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

On 10 January 2012 02:54, Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> On Mon, 2012-01-09 at 15:29 +0100, Vincent Guittot wrote:
>> Hi Peter,
>>
>> I'm also using sched_mc level for doing powersaving load balance on
>> ARM platform and we have real benefits.
>
> hi Vincent, Can you elaborate on your platform topology where you see
> the benefits?
>

Hi Suresh,

I'm using dual/quad cores ARM platform. When sched_mc is different
from 0, the topology is changed from 1 socket/cluster to 2 virtual
sockets/clusters with 1 or 2 cores in each cluster.  For low cpu load
situation, the tasks are gathered in 1 virtual socket and let the
cores in the other socket to enter idle state thanks to cpuidle. On
ARM platform, cores in a cluster can enter interesting idle state even
if the complete cluster can't reach a deep idle state.
With dual cores platform, i'm increasing the cpu_power to gather tasks
on 1 core when cores are at lowest frequency. In fact i'm detecting a
low cpu load (only small tasks are running) and ensure that these
small tasks are kept on 1 core.

Regards,
Vincent

> on x86, 'sched_mc' was designed for the case of consolidating load into
> one socket before using another idle socket.
>
> Just wondering if this is your use case too or if you are consolidating
> at a different topological level.
>
> thanks,
> suresh
>
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