linux-kernel - Re: [PATCH RESEND v9 00/10] sched: consolidation of CPU capacity and usage

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Message-ID: <CAKfTPtBbGBzo73_59fvLri_wOtNWL830byUUQqJhT3a9soE-cA@mail.gmail.com>
Date:	Fri, 20 Feb 2015 15:13:21 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"kamalesh@...ux.vnet.ibm.com" <kamalesh@...ux.vnet.ibm.com>,
	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH RESEND v9 00/10] sched: consolidation of CPU capacity and usage

On 20 February 2015 at 12:52, Morten Rasmussen <morten.rasmussen@....com> wrote:
> On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
>> On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
>>
>> > Also, it still not clear why patch 10 uses relative capacity reduction
>> > instead of absolute capacity available to CFS tasks.
>>
>> As present in your asymmetric big and small systems? Yes it would be
>> unfortunate to migrate a task to an idle small core when the big core is
>> still faster, even if reduced by rt/irq work.
>
> Yes, exactly. I don't think it would cause any harm for symmetric cases
> to use absolute capacity instead. Am I missing something?

If absolute capacity is used, we will trig an active load balance from
little to big core each time a little has got 1 task and a big core is
idle whereas we only want to trig an active migration is the src_cpu's
capacity that is available for the  cfs task is significantly reduced
by rt tasks.

I can mix absolute and relative tests by 1st testing that the capacity
of the src is reduced and then ensure that the dst_cpu has more
absolute capacity than src_cpu
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