linux-kernel - Re: [PATCH v2 09/11] sched: test the cpu's capacity in wake affine

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Message-ID: <CAKfTPtAvFEmAmPMQsEN-=ftudQwd0fHOxq4MRs3HDyHYjLPGZQ@mail.gmail.com>
Date:	Wed, 28 May 2014 13:15:01 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 09/11] sched: test the cpu's capacity in wake affine

On 28 May 2014 12:58, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, May 23, 2014 at 05:53:03PM +0200, Vincent Guittot wrote:
>> Currently the task always wakes affine on this_cpu if the latter is idle.
>> Before waking up the task on this_cpu, we check that this_cpu capacity is not
>> significantly reduced because of RT tasks or irq activity.
>>
>> Use case where the number of irq and the time spent under irq is important
>> will take benefit of this because the task that is woken up by irq or softirq
>> will not use the same CPU than irq (and softirq) but a idle one which share
>> its LLC.
>
> OK, so I'm having a terrible time parsing the above.
>
> So looking at the patch you make balance false even when this_load==0
> when the effective power/capacity (nico's patches haven't fully sunk in
> yet) of this cpu is less than that of the previous cpu.
>
> Is that right?

yes,

>
> Now I'm only struggling to understand the rationale for this, its got
> LLC in there somewhere, but I'm failing to comprehend.

Ah.. i have probably overestimated the fact that wake_affine was only
done at MC or SMT level but after reading more deeply the glags
configuration of all  sched_domain level , my assumption is not true.
So I need to test sd flag to make sure that their share their cache at
this level
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