linux-kernel - Re: [RFC PATCH v2 0/1] cpuidle: teo: Introduce optional util-awareness

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Message-ID: <2e02056b-24ef-6262-03bd-63fd6f3a5754@arm.com>
Date:   Fri, 28 Oct 2022 08:08:31 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Dietmar.Eggemann@....com, dsmythies@...us.net,
        Kajetan Puchalski <kajetan.puchalski@....com>,
        yu.chen.surf@...il.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [RFC PATCH v2 0/1] cpuidle: teo: Introduce optional
 util-awareness



On 10/20/22 20:52, Daniel Lezcano wrote:
> 
> Hi Kajetan,
> 
> On 20/10/2022 18:20, Kajetan Puchalski wrote:
>> Hi Rafael,
>>
>>> The avg_util value tells us nothing about how much the CPU is going to
>>> be idle this time and it also tells us nothing about the
>>> latency-sensitivity of the workload.
>>>
>>> Yes, it tells us how much idle time there was on the given CPU in the
>>> past, on the average, but there is zero information about the
>>> distribution of that idle time in it.
>>>
>>> So in the first place please tell me why it fundamentally makes sense
>>> to use avg_util in CPU idle time management at all.
>>
>> I have an alternative suggestion that could be a reasonable way forward
>> here. Instead of applying util-awareness on top of TEO where it would
>> have to be reconciled with how TEO is currently expected to work, I just
>> wrote a simple completely new governor which operates only on timer
>> events alongside util values.
> 
> I second the idea. I took a long time to investigate how to improve the 
> governor and reached the conclusion having a dedicated governor for 
> mobile platform makes sense. Also the behavior is very platform dependent.
> 
> Regarding the utilization, one of the issue is the kernel threads 
> preventing a task to wake up on the same CPU and forcing its migration 
> at wake up time. So the prediction is screwed up at that time.
> 
> There is a paper talking this issue [1]
> 
> I've done a 'mobile' governor, including the next interrupt prediction 
> [2]. It is very simple and almost has the same results as the teo on my 
> platform (rock960).
> 
> I'm not planning to upstream it because I don't have spare time to 
> improve the results and take care of the IPIs. part.
> 
> Also the paradigm is radically different and you may be interested in 
> the approach.
> 
> So if you want to rework, improve, test, upstream it, feel free to reuse 
> the code.
> 
>    -- Daniel
> 
> [1] Dynamic workload characterization for power efficient scheduling on 
> CMP systems : https://cseweb.ucsd.edu//~tullsen/islped10.pdf
> 
> [2] 
> https://git.linaro.org/people/daniel.lezcano/linux.git/commit/?h=cpuidle/mobile-governor-v5.1&id=de1edb05e3c342f0738b414aa84263d6555b7462 
> 
> 
> 

Thanks Daniel! I forgot about your work in this area. As I have
responded in some other email in this thread, we might start
from a new small governor and than others can contribute.

Even this small governor that Kajetan showed me performs really
good on pixel6.

Regards,
Lukasz