linux-kernel - Re: [PATCH V6] powercap/drivers/idle_injection: Add an idle injection framework

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Date:   Tue, 12 Jun 2018 16:06:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     viresh.kumar@...aro.org, rjw@...ysocki.net,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Eduardo Valentin <edubezval@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Leo Yan <leo.yan@...aro.org>,
        Kevin Wangtao <kevin.wangtao@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rui Zhang <rui.zhang@...el.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Subject: Re: [PATCH V6] powercap/drivers/idle_injection: Add an idle
 injection framework

On Tue, Jun 12, 2018 at 03:02:14PM +0200, Daniel Lezcano wrote:
> On 12/06/2018 14:52, Peter Zijlstra wrote:
> > In this case, you can do:
> 
> That is what we had before but we change the code to set the count
> before waking up the task, so compute the cpumask_weight of the
> resulting AND right before this loop.
> 
> > +       for_each_cpu_and(cpu, &ii_dev->cpumask, cpu_online_mask) {
> > +               iit = per_cpu_ptr(&idle_injection_thread, cpu);
> > +               iit->should_run = 1;
> > +               wake_up_process(iit->tsk);
> > +       }


Ah, I see, but since you do:

	if (atomic_dec_and_test())
	  last_man()

where that last_man() thing will start a timer, there is no real problem
with doing atomic_inc() with before wake_up_process().

Yes, it allows doing last_man, too often, but repeated hrtimer_start()
will DTRT and reprogram the timer.

Also, last_man() uses @run_duration, but the way I read it, the timer is
for waking things up again, this means it is in fact the sleep duration,
no?

Furthermore, should you not be using hrtimer_forward(&timer,
idle_duration + run_duration) instead? AFAICT the current scheme is
prone to drifting.