linux-kernel - Re: [Patch v8 4/7] sched/fair: Enable periodic update of average thermal pressure

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Date:   Fri, 17 Jan 2020 16:39:37 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thara Gopinath <thara.gopinath@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Zhang Rui <rui.zhang@...el.com>,
        Quentin Perret <qperret@...gle.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        viresh kumar <viresh.kumar@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Amit Kachhap <amit.kachhap@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Amit Kucheria <amit.kucheria@...durent.com>
Subject: Re: [Patch v8 4/7] sched/fair: Enable periodic update of average
 thermal pressure

On Fri, 17 Jan 2020 at 15:55, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Jan 17, 2020 at 02:22:51PM +0100, Vincent Guittot wrote:
> > On Thu, 16 Jan 2020 at 16:15, Peter Zijlstra <peterz@...radead.org> wrote:
>
> > >
> > > That there indentation trainwreck is a reason to rename the function.
> > >
> > >         decayed = update_rt_rq_load_avg(now, rq, curr_class == &rt_sched_class) |
> > >                   update_dl_rq_load_avg(now, rq, curr_class == &dl_sched_class) |
> > >                   update_thermal_load_avg(rq_clock_task(rq), rq, thermal_pressure) |
> > >                   update_irq_load_avg(rq, 0);
> > >
> > > Is much better.
> > >
> > > But now that you made me look at that, I noticed it's using a different
> > > clock -- it is _NOT_ using now/rq_clock_pelt(), which means it'll not be
> > > in sync with the other averages.
> > >
> > > Is there a good reason for that?
> >
> > We don't need to apply frequency and cpu capacity invariance on the
> > thermal capping signal which is  what rq_clock_pelt does
>
> Hmm, I suppose that is true, and that really could've done with a
> comment. Now clock_pelt is sort-of in sync with clock_task, but won't it
> still give weird artifacts by having it on a slightly different basis?

No we should not. Weird artifacts happens when we
add/subtract/propagate signals between each other and then apply pelt
algorithm on the results. In the case of thermal signal, we only add
it to others to update cpu_capacity but pelt algo is then not applied
on it. The error because of some signals being at segment boundaries
whereas others are not, is limited to 2% and doesn't accumulate over
time.

>
> Anyway, looking at this, would it make sense to remove the @now argument
> from update_*_load_avg()? All those functions already take @rq, and
> rq_clock_*() are fairly trivial inlines.

TBH I was thinking of doing the opposite for update_irq_load_avg which
hides the clock that is used for irq_avg. This helps to easily
identify which signals use the exact same clock and can be mixed to
create a new pelt signal and which can't