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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Message-ID: <55130220.7020700@arm.com>
Date:	Wed, 25 Mar 2015 18:44:48 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: 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.

We could replace the use of capacity_orig_of(cpu) w/ capacity_of(cpu) to 
take big.LITTLE and RT and/or IRQ work into consideration which then 
would effect the MC sd level too.

Will add the 'why' in the next version.

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