linux-kernel - Re: [Patch v4 6/6] sched: thermal: Enable tuning of decay period

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Date:   Tue, 5 Nov 2019 21:29:01 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Thara Gopinath <thara.gopinath@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        rui.zhang@...el.com, edubezval@...il.com, qperret@...gle.com,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, daniel.lezcano@...aro.org
Subject: Re: [Patch v4 6/6] sched: thermal: Enable tuning of decay period

On Tuesday 05 Nov 2019 at 15:26:06 (-0500), Thara Gopinath wrote:
> On 11/04/2019 11:12 AM, Ionela Voinescu wrote:
> > Hi Thara,
> > 
> > On Tuesday 22 Oct 2019 at 16:34:25 (-0400), Thara Gopinath wrote:
> >> Thermal pressure follows pelt signas which means the
> >> decay period for thermal pressure is the default pelt
> >> decay period. Depending on soc charecteristics and thermal
> >> activity, it might be beneficial to decay thermal pressure
> >> slower, but still in-tune with the pelt signals.
> > 
> > I wonder if it can be beneficial to decay thermal pressure faster as
> > well.
> > 
> > This implementation makes 32 (LOAD_AVG_PERIOD) the minimum half-life
> > of the thermal pressure samples. This results in more than 100ms for a
> > sample to decay significantly and therefore let's say it can take more
> > than 100ms for capacity to return to (close to) max when the CPU is no
> > longer capped. This value seems high to me considering that a minimum
> > value should result in close to 'instantaneous' behaviour, when there
> > are thermal capping mechanisms that can react in ~20ms (hikey960 has a
> > polling delay of 25ms, if I'm remembering correctly).
> > 
> > I agree 32ms seems like a good default but given that you've made this
> > configurable as to give users options, I'm wondering if it would be
> > better to cover a wider range.
> >

[...]

> 
> Hi Ionela,
>

[...]

> 
> Regarding a slower decay, we need a strong case for it.
> 
>

I think you mean faster decay, if you refer to my comment above.

To be blunt, I'm not sure there is a strong case for either kind of
dacay, if we look at the test results only. There is a theoretical case
for both, in my opinion and given that the purpose of this patch is to
give options to platform with different thermal characteristics, I do
believe it's worth providing a good range of options for the decay
period.

Thanks,
Ionela.

> 
> -- 
> Warm Regards
> Thara