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

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Message-ID: <CAKfTPtB4wadVkAnonZ=imBOPVHGowL46juoSAfXv4F7Y7wo_zA@mail.gmail.com>
Date:   Mon, 6 Jul 2020 14:53:29 +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, 5 Jul 2020 at 16:19, Valentin Schneider
<valentin.schneider@....com> wrote:
>
>
> Sorry for getting back to this only now;
>
> On 22/06/20 09:37, Vincent Guittot wrote:
> > On Sun, 21 Jun 2020 at 00:28, Valentin Schneider <valentin.schneider@....com> wrote:
> >> On 20/06/20 18:49, Ionela Voinescu wrote:
> >> > On Thursday 18 Jun 2020 at 17:03:24 (+0200), Vincent Guittot wrote:
> >> >> 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.
> >
>
> include/linux/sched/topology.h already defines a stub for
> arch_scale_thermal_pressure(), I suppose we could have one for
> arch_set_thermal_pressure() there.
>
> That would require having something like
>
> #define arch_set_thermal_pressure topology_set_thermal_pressure
>
> in the arm & arm64 include/asm/topology.h headers, with
> topology_set_thermal_pressure() being what arch_set_thermal_pressure()
> currently is in this patchset.

That looks like a better solution IMO. At least everything is gathered
in the same place:
topology_get/set_thermal_pressure are in arch_topology.c
and arch_scale_thermal_pressure/arch_set_thermal_pressure in the
respective topology.h

>
>
> This would set an odd precedent in that so far we only ever had to #define
> getter functions, the setters being either:
> - entirely contained within arch_topology. (for the CPU scale)
> - defined in arch_topology, declared in cpufreq and contained there (for
>   the freq scale).
>
> It made the most sense to me to follow the arch_set_freq_scale() pattern
> and contain the thermal pressure setter within cpufreq_cooling, especially
> since I didn't see a strong point in breaking the current patterns.