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 09:59:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "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 Wed, Nov 07, 2018 at 12:39:31AM +0100, Rafael J. Wysocki wrote:
> 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.

Right, those will not be accounted for and will need something else.

> > > 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.

Ah, not so. Suppose you have both the audio and video interrupt going at
a steady rate but different rate, then the combined pattern isn't
trivial at all.

> 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.

How about we add a (debug) knob so that people can play with it for now?
If it turns out to be useful, we'll learn.