Date: Thu, 31 Mar 2016 14:34:24 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Steve Muckle <steve.muckle@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Michael Turquette <mturquette@baylibre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to
 update_cfs_rq_load_avg()
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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.