linux-kernel - Re: [PATCH 1/3] thermal/cpu-cooling, sched/core: Cleanup thermal pressure definition

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Message-ID: <CAKfTPtDG26Y9s4c+MbdmbxJaiCv6s6WTqmzztcoFsm2SnRL=vQ@mail.gmail.com>
Date:   Mon, 22 Jun 2020 10:37:23 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ionela Voinescu <ionela.voinescu@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@...il.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Russell King <linux@...linux.org.uk>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] thermal/cpu-cooling, sched/core: Cleanup thermal
 pressure definition

On Sun, 21 Jun 2020 at 00:28, Valentin Schneider
<valentin.schneider@....com> wrote:
>
>
> On 20/06/20 18:49, Ionela Voinescu wrote:
> > Hi Vincent,
> >
> > On Thursday 18 Jun 2020 at 17:03:24 (+0200), Vincent Guittot wrote:
> >> On Sun, 14 Jun 2020 at 03:10, Valentin Schneider
> >> <valentin.schneider@....com> wrote:
> > [..]
> >> > diff --git a/drivers/thermal/cpufreq_cooling.c b/drivers/thermal/cpufreq_cooling.c
> >> > index e297e135c031..a1efd379b683 100644
> >> > --- a/drivers/thermal/cpufreq_cooling.c
> >> > +++ b/drivers/thermal/cpufreq_cooling.c
> >> > @@ -417,6 +417,11 @@ static int cpufreq_get_cur_state(struct thermal_cooling_device *cdev,
> >> >         return 0;
> >> >  }
> >> >
> >> > +__weak void
> >> > +arch_set_thermal_pressure(const struct cpumask *cpus, unsigned long th_pressure)
> >> > +{
> >> > +}
> >>
> >> Having this weak function declared in cpufreq_cooling is weird. This
> >> means that we will have to do so for each one that wants to use it.
> >>
> >> Can't you declare an empty function in a common header file ?
> >
> > Do we expect anyone other than cpufreq_cooling to call
> > arch_set_thermal_pressure()?
> >
> > I'm not against any of the options, either having it here as a week
> > default definition (same as done for arch_set_freq_scale() in cpufreq.c)
> > or in a common header (as done for arch_scale_freq_capacity() in sched.h).
> >
>
> Same thoughts here; I was going for the arch_set_freq_scale() way.
>
> > But for me, Valentin's implementation seems more natural as setters are
> > usually only called from within the framework that does the control
> > (throttling for thermal or frequency setting for cpufreq) and we
> > probably want to think twice if we want to call them from other places.
> >
>
> Well TBH I was tempted to go the other way and keep the definition in
> core.c, given a simple per-cpu value is fairly generic. More precisely, it

Having all definitions in the same place is my main concern here.
If topology.c defines arch_set_thermal_pressure it should also provide
the empty function when the feature is not available or possible
instead of relying of each user of the interface to define a weak
function just in case.

> seems somewhat awkward that architectures have to redefine those interfaces
> when, given what cpufreq_cooling is doing, they'll have to go for per-cpu
> storage in some way or another.
>
> I ultimately decided against it, seeing as it isn't too difficult to come
> up with other drivers of thermal pressure. There was that TDP-bound thing
> [1], where IIUC you could end up with throttling not because of thermal but
> because of power constraints. And then there's always FW that can cap stuff
> as a last resort, and some architectures will want to inform the scheduler
> of that when/if they'll be able to query FW for that.
>
> [1]: 20200428032258.2518-1-currojerez@...eup.net
>
> > Thanks,
> > Ionela.