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 11:13:34 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.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>
Subject: Re: [RFC/RFT][PATCH v3] cpuidle: New timer events oriented governor
 for tickless systems

On 07/11/2018 00:39, 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.
> 
>>> 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.

I agree the current irq next event can add an overhead and actually it
works well if there are interrupts with regular interrupts but with
irregular timings interval we need to search a pattern.

The first version called 'anova' I shared with you (Peter and Rafael)
privately is pretty good to detect such intervals but the algorithm has
a cost in terms of computation in the self-learning phase. I dropped
this approach as it could be overkill.

I'm working on a different algorithm to detect such patterns which
should be faster.

That said, the pattern detection process shouldn't be happening every
time we enter idle but when the pattern is invalidated. Until the
pattern gives a successful prediction, we keep it and the repeating
period is used to give the next event.

So IMO, the cost to detect the pattern should be compensated by the
computation saving we do if the pattern is found successfully.

These are all assumptions, hopefully I can provide a patch series and
numbers in January ...



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