linux-kernel - Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load

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Date:	Thu, 31 Mar 2016 14:50:54 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg()

On 31 March 2016 at 14:34, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Mar 31, 2016 at 02:14:50PM +0200, Vincent Guittot wrote:
>> In fact, I looks for the sequence where the utilization of a rq is not
>> updated until the next tick but i can't find it.
>
> No, util it always updated, however..
>
>> If cpu doesn't share cache, task is added to wake list and an ipi is
>> sent and the utilization.
>
> Here we run:
>
>  ttwu_do_activate()
>    ttwu_activate()
>      activate_task()
>        enqueue_task()
>          p->sched_class->enqueue_task() := enqueue_task_fair()
>            update_load_avg()
>              update_cfs_rq_load_avg()
>                cfs_rq_util_change()
>
> On the local cpu, and we can indeed call out to have the frequency
> changed.
>
>> Otherwise, we directly enqueue the task on
>> the rq and the utilization is updated
>
> But here we run it on a remote cpu, so we cannot call out and the
> frequency remains the same.
>
> So if a remote wakeup on the same LLC domain happens, utilization will
> increase but we will not observe until the next tick.

ok. I forgot that we have the condition cpu == smp_processor_id() in
cfs_rq_util_change.