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 14:34:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.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 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.