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

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Message-ID: <83b8c8d7-d53f-bea7-4ca3-5730d5c80b30@arm.com>
Date:   Tue, 12 Sep 2023 12:57:32 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
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>,
        Peter Zijlstra <peterz@...radead.org>,
        Viresh Kumar <viresh.kumar@...aro.org>, juri.lelli@...hat.com
Subject: Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

Hi Daniel,

On 9/8/23 13:51, Daniel Bristot de Oliveira wrote:
> On 9/7/23 15:45, Lukasz Luba wrote:
>>>>> RT literatur mostly methinks. Replacing WCET with a statistical model of
>>>>> sorts is not uncommon, the argument goes that not everybody will have
>>>>> their worst case at the same time and lows and highs can commonly cancel
>>>>> out and this way we can cram a little more on the system.
>>>>>
>>>>> Typically this is proposed in the context of soft-realtime systems.
>>>>
>>>> Thanks Peter, I will dive into some books...
>>>
>>> I would look at academic papers, not sure any of that ever made it to
>>> books, Daniel would know I suppose.
>>
>> Good hint, thanks!
> 
> The key-words that came to my mind are:
> 
> 	- mk-firm, where you accept m tasks will make their deadline
> 	           every k execution - like, because you run too long.
> 	- mixed criticality with pWCET (probabilistic execution time) or
> 		  average execution time + an sporadic tail execution time for
> 		  the low criticality part.
> 
> mk-firm smells like 2005's.. mixed criticality as 2015's..present.
> 
> You will probably find more papers than books. Read the papers
> as a source for inspiration... not necessarily as a definitive
> solution. They generally proposed too restrictive task models.
> 
> -- Daniel
> 

Thanks for describing this context! That would save my time and avoid
maybe sinking in this unknown water. As you said I might tread that
as inspiration, since I don't fight with life-critical system,
but a phone which needs 'nice user experience' (hopefully there are
no people who disagree) ;)

Regards,
Lukasz