linux-kernel - Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

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Message-ID: <89067f71-9b83-e647-053e-07f7d55b6529@arm.com>
Date:   Thu, 7 Sep 2023 14:06:15 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Qais Yousef <qyousef@...alina.io>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins



On 9/7/23 12:53, Peter Zijlstra wrote:
> On Thu, Sep 07, 2023 at 08:48:08AM +0100, Lukasz Luba wrote:
> 
>>> Hehe. That's because they're not really periodic ;-)
>>
>> They are periodic in a sense, they wake up every 16ms, but sometimes
>> they have more work. It depends what is currently going in the game
>> and/or sometimes the data locality (might not be in cache).
>>
>> Although, that's for games, other workloads like youtube play or this
>> one 'Yahoo browser' (from your example) are more 'predictable' (after
>> the start up period). And I really like the potential energy saving
>> there :)
> 
> So everything media is fundamentally periodic, you're hard tied to the
> framerate / audio-buffer size etc..

Agree

> 
> Also note that the traditional periodic task model from the real-time
> community has the notion of WCET, which completely covers this
> fluctuation in frame-to-frame work, it only considers the absolute worst
> case.

That's good point, the WCET here. IMO shorter PELT e.g. 8ms allows us
to 'see' a bit more that information: the worst case in fluctuation of
a particular task. Then this 'seen' value is maintained in util_est
for a while. That's why (probably) I see a better 95-, 99-percentile
numbers for frames rendering time.

> 
> Now, practically, that stinks, esp. when you care about batteries, but
> it does not mean these tasks are not periodic.

Totally agree they are periodic.

> 
> Many extentions to the periodic task model are possible, including
> things like average runtime with bursts etc.. all have their trade-offs.

Was that maybe proposed somewhere on LKML (the other models)?

I can recall one idea - WALT.
IIRC ~2016/2017 the WALT proposal and some discussion/conferences, it
didn't get positive feedback [2].

I don't know if you remember those numbers back than, e.g. video 1080p
playback was using ~10% less energy... Those 10%-15% are still important
for us ;)

Regards,
Lukasz

[1] 
https://lore.kernel.org/all/1477638642-17428-1-git-send-email-markivx@codeaurora.org/