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Date:   Thu, 25 Apr 2019 11:45:07 +0100
From:   Quentin Perret <quentin.perret@arm.com>
To:     Thara Gopinath <thara.gopinath@linaro.org>
Cc:     mingo@redhat.com, peterz@infradead.org, rui.zhang@intel.com,
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        edubezval@gmail.com, daniel.lezcano@linaro.org,
        vincent.guittot@linaro.org, nicolas.dechesne@linaro.org,
        bjorn.andersson@linaro.org, dietmar.eggemann@arm.com
Subject: Re: [PATCH V3 3/3] thermal/cpu-cooling: Update thermal pressure in
 case of a maximum frequency capping
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On Tuesday 23 Apr 2019 at 18:38:46 (-0400), Thara Gopinath wrote:
> I think there is one major difference between user-defined frequency
> constraints and frequency constraints due to thermal events in terms of
> the time period the system spends in the the constraint state.
> Typically, a user constraint lasts for seconds if not minutes and I
> think in this case cpu_capacity_orig should reflect this constraint and
> not cpu_capacity like this patch set.

That might not always be true I think. There's tons of userspace thermal
deamons out there, and I wouldn't be suprised if they were writing into
the cpufreq sysfs files, although I'm not sure.

Another thing is, if you want to change the capacity_orig value, you'll
need to rebuild the sched domains and all I believe. Otherwise there is
a risk to 'break' the sd_asym flags. So we need to make sure we're happy
to pay that price.

> Also, in case of the user
> constraint, there is possibly no need to accumulate and average the
> capacity constraints and instantaneous values can be directly applied to
> cpu_capacity_orig. On the other hand thermal pressure is more spiky and
> sometimes in the order of ms and us requiring the accumulating and
> averaging.
> > 
> > Perhaps the Intel boost stuff could be factored in there ? That is,
> > at times when the boost freq is not reachable capacity_of() would appear
> > smaller ... Unless this wants to be reflected instantaneously ?
> Again, do you think intel boost is more applicable to be reflected in
> cpu_capacity_orig and not cpu_capacity?

I'm not even sure if we want to reflect it at all TBH, but I'd be
interested to see what Intel folks think :-)

Thanks,
Quentin