linux-kernel - Re: [RFCv3 PATCH 45/48] sched: Skip cpu as lb src which has one task and capacity gte the dst cpu

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Date:	Tue, 05 May 2015 11:01:25 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>
CC:	Juri Lelli <Juri.Lelli@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-kernel-owner@...r.kernel.org" 
	<linux-kernel-owner@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>
Subject: Re: [RFCv3 PATCH 45/48] sched: Skip cpu as lb src which has one task
 and capacity gte the dst cpu

On 30/04/15 08:46, pang.xunlei@....com.cn wrote:
> linux-kernel-owner@...r.kernel.org wrote 2015-03-26 AM 02:44:48:
> 
>> Dietmar Eggemann <dietmar.eggemann@....com>
>>
>> Re: [RFCv3 PATCH 45/48] sched: Skip cpu as lb src which has one task
>> and capacity gte the dst cpu
>>
>> On 24/03/15 15:27, Peter Zijlstra wrote:
>> > On Wed, Feb 04, 2015 at 06:31:22PM +0000, Morten Rasmussen wrote:
>> >> From: Dietmar Eggemann <dietmar.eggemann@....com>
>> >>
>> >> Skip cpu as a potential src (costliest) in case it has only one task
>> >> running and its original capacity is greater than or equal to the
>> >> original capacity of the dst cpu.
>> >
>> > Again, that's what, but is lacking a why.
>> >
>>
>> You're right, the 'why' is completely missing.
>>
>> This is one of our heterogeneous (big.LITTLE) cpu related patches. We
>> don't want to end up migrating this single task from a big to a little
>> cpu, hence the use of capacity_orig_of(cpu). Our cpu topology makes sure
>> that this rule is only active on DIE sd level.
> 
> Hi Dietmar,
> 
> Could you tell me the reason why don't want to end up migrating this single
> task from a big to a little cpu?
> 
> Like what I just replied to "[RFCv3 PATCH 47/48]", if the task is a
> small one,
> why couldn't we migrate it to the little cpu to save energy especially when
> the cluster has shared freq, the saving may be appreciable?

If it's a big (always running) task, it should stay on the cpu with the
higher capacity. If it is a small task it will eventually go to sleep
and the wakeup path will take care of placing it onto the right cpu.

[...]

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