linux-kernel - Re: [PATCH V3 3/3] thermal/cpu-cooling: Update thermal pressure in case of a maximum frequency capping

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Date:   Thu, 25 Apr 2019 14:04:10 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Thara Gopinath <thara.gopinath@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Zhang Rui <rui.zhang@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Amit Kachhap <amit.kachhap@...il.com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Javi Merino <javi.merino@...nel.org>,
        Eduardo Valentin <edubezval@...il.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Nicolas Dechesne <nicolas.dechesne@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH V3 3/3] thermal/cpu-cooling: Update thermal pressure in
 case of a maximum frequency capping

On Thu, 25 Apr 2019 at 12:45, Quentin Perret <quentin.perret@....com> wrote:
>
> 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.

They would better use the sysfs set_target interface of cpu_cooling
device in this case.

>
> 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.

That would be the goal, if userspace uses the sysfs interface of
cpufreq to set a new max frequency, it should be considered as a long
change in regards to the scheduling rate and in this case it should be
interesting to update cpacity_orig and rebuild sched_domain.

>
> > 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