linux-kernel - Re: [PATCH v3 07/12] sched: test the cpu's capacity in wake affine

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Message-ID: <CAKfTPtB0hPyfzMpF_=34y=AyrEPXztbZCVvpqHZtEeR56_Qyiw@mail.gmail.com>
Date:	Thu, 10 Jul 2014 15:58:20 +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>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v3 07/12] sched: test the cpu's capacity in wake affine

On 10 July 2014 13:06, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Jun 30, 2014 at 06:05:38PM +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/or 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 cache.
>
> The above, doesn't seem to explain:
>
>> +     } else if (!(sd->flags & SD_SHARE_PKG_RESOURCES)) {
>> +             this_eff_load = 0;
>> +     }
>> +
>> +     balanced = this_eff_load <= prev_eff_load;
>
> this. Why would you unconditionally allow wake_affine across cache
> domains?

The current policy is to use this_cpu if this_load <= 0. I want to
keep the current wake affine policy for all sched_domain that doesn't
share cache so if the involved CPUs don't share cache, I clear
this_eff_load to force balanced to be true. But when CPUs share their
cache,  we not only look at idleness but also at available capacity of
prev and local CPUs.
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