linux-kernel - Re: [RFC/RFT][PATCH v3] cpuidle: New timer events oriented governor for tickless systems

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Date:   Wed, 7 Nov 2018 00:39:31 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Doug Smythies <dsmythies@...us.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [RFC/RFT][PATCH v3] cpuidle: New timer events oriented governor
 for tickless systems

On Tue, Nov 6, 2018 at 8:51 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Nov 06, 2018 at 07:19:24PM +0100, Rafael J. Wysocki wrote:
> > On Tue, Nov 6, 2018 at 6:04 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > Instead of this detector; why haven't you used the code from
> > > kernel/irq/timings.c ?
> >
> > Because it doesn't help much AFAICS.
> >
> > Wakeups need not be interrupts in particular
>
> You're alluding to the MWAIT wakeup through the MONITOR address ?

Yes.

> > and interrupt patterns that show up when the CPU is busy may not be
> > relevant for when it is idle.
>
> I think that is not always true; consider things like the periodic
> interrupt from frame rendering or audio; if there is nothing more going
> on in the system than say playing your favourite tune, it gets the
> 'need more data soon' interrupt from the audio card, wakes up, does a little
> mp3/flac/ogg/whatever decode to fill up the buffer and goes back to
> sleep. Same for video playback I assume, the vsync interrupt for buffer
> flips is fairly predictable.
>
> The interrupt predictor we have in kernel/irq/timings.c should be very
> accurate in predicting those interrupts.

In the above case the interrupts should produce a detectable pattern
of wakeups anyway.

In general, however, I need to be convinced that interrupts that
didn't wake up the CPU from idle are relevant for next wakeup
prediction.  I see that this may be the case, but to what extent is
rather unclear to me and it looks like calling
irq_timings_next_event() would add considerable overhead.