linux-kernel - Re: Stopping the tick on a fully loaded system

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Message-ID: <CAJZ5v0iZhEcRLM7yRNffYSSfNxQH+MyuF2rMsqqX5O0jOSVG7Q@mail.gmail.com>
Date:   Wed, 26 Jul 2023 17:07:27 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Gautham R. Shenoy" <gautham.shenoy@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: Stopping the tick on a fully loaded system

On Wed, Jul 26, 2023 at 12:59 PM Frederic Weisbecker
<frederic@...nel.org> wrote:
>
> Le Tue, Jul 25, 2023 at 04:27:56PM +0200, Rafael J. Wysocki a écrit :
> > On Tue, Jul 25, 2023 at 3:07 PM Anna-Maria Behnsen
> > I'll let Frederic respond to the above, but from my perspective it all
> > just means that the idle governors in use today are not perfect.
> >
> > However, they will never be perfect, because they only have a little
> > time to make a decision, so it's a matter of balancing that with
> > precision.
>
> So, even without considering Anna-Maria's patchset, sparing
> the next timer lookup if we already know we won't stop it would show
> quite a benefit because that lookup involves locking and quite some
> overhead walking through all levels.

You are right.

In principle, this can be done by using TICK_NSEC as the initial
estimation of the idle duration and only calling
tick_nohz_get_sleep_length() if the candidate state ends up in the
higher bins.  Interesting.