linux-kernel - Re: [Patch v8 4/7] sched/fair: Enable periodic update of average thermal pressure

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Date:   Thu, 30 Jan 2020 10:49:20 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Zhang Rui <rui.zhang@...el.com>,
        Quentin Perret <qperret@...gle.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        viresh kumar <viresh.kumar@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Amit Kachhap <amit.kachhap@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Amit Kucheria <amit.kucheria@...durent.com>
Subject: Re: [Patch v8 4/7] sched/fair: Enable periodic update of average
 thermal pressure

On Wed, 29 Jan 2020 at 16:41, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 27/01/2020 16:15, Vincent Guittot wrote:
> > On Mon, 27 Jan 2020 at 13:09, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>
> >> On 24/01/2020 16:45, Vincent Guittot wrote:
> >>> On Fri, 24 Jan 2020 at 16:37, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>>
> >>>> On 17/01/2020 16:39, Vincent Guittot wrote:
> >>>>> On Fri, 17 Jan 2020 at 15:55, Peter Zijlstra <peterz@...radead.org> wrote:
> >>>>>>
> >>>>>> On Fri, Jan 17, 2020 at 02:22:51PM +0100, Vincent Guittot wrote:
> >>>>>>> On Thu, 16 Jan 2020 at 16:15, Peter Zijlstra <peterz@...radead.org> wrote:
> >>
> >> [...]
> >>
> >>>> The 'now' argument is one thing but why not:
> >>>>
> >>>> -int update_thermal_load_avg(u64 now, struct rq *rq, u64 capacity)
> >>>> +int update_thermal_load_avg(u64 now, struct rq *rq)
> >>>>  {
> >>>> +       u64 capacity = arch_cpu_thermal_pressure(cpu_of(rq));
> >>>> +
> >>>>         if (___update_load_sum(now, &rq->avg_thermal,
> >>>>
> >>>> This would make the call-sites __update_blocked_others() and
> >>>> task_tick(_fair)() cleaner.
> >>>
> >>> I prefer to keep the capacity as argument. This is more aligned with
> >>> others that provides the value of the signal to apply
> >>>
> >>>>
> >>>> I guess the argument is not to pollute pelt.c. But it already contains
> >>>
> >>> you've got it. I don't want to pollute the pelt.c file with things not
> >>> related to pelt but thermal as an example.
> >>>
> >>>> arch_scale_[freq|cpu]_capacity() for irq.
> >>
> >> But isn't arch_cpu_thermal_pressure() not exactly the same as
> >> arch_scale_cpu_capacity() and arch_scale_freq_capacity()?
> >>
> >> All of them are defined by default within the scheduler code
> >> [include/linux/sched/topology.h or kernel/sched/sched.h] and can be
> >> overwritten by arch code with a fast implementation (e.g. returning a
> >> per-cpu variable).
> >>
> >> So why is using arch_scale_freq_capacity() and arch_scale_cpu_capacity()
> >> in update_irq_load_avg [kernel/sched/pelt.c] and update_rq_clock_pelt()
> >
> > As explained previously, update_irq_load_avg is an exception and not
> > the example to follow. update_rt/dl_rq_load_avg are the example to
> > follow and fixing update_irq_load_avg exception is on my todo list
>
> There is already a v9 but I comment here so the thread stays intact.
>
> I guess this thread leads to nowhere. For me the question is do we
> review against existing code or some possible future changes? The
> arguments didn't convince me so far.
> But we're not talking functional issues here so I won't continue to push
> for change on this one here.
>
> >> [kernel/sched/pelt.h] OK but arch_cpu_thermal_pressure() in
> >> update_thermal_load_avg() [kernel/sched/pelt.c] not?
> >>
> >> Shouldn't arch_cpu_thermal_pressure() not be called
> >> arch_scale_thermal_capacity() to highlight the fact that those three
> >
> > Quoted from cover letter https://lkml.org/lkml/2020/1/14/1164:
> > "
> > v6->v7:
> >        - ...
> >         - Renamed arch_scale_thermal_capacity to arch_cpu_thermal_pressure
> >           as per review comments from Peter, Dietmar and Ionela.
> >        -...
> >
> > "
>
> I went back to the v6 review. Peter originally asked for a better name
> (or an additional comment) for arch_scale_thermal_capacity() because the
> return value is not capacity.
>
> So IMHO arch_scale_thermal_pressure() is a good name for keeping this
> aligned w/ the other arch_scale_* functions and to address this review
> comment.
>
> arch_scale_cpu_capacity()     - scale capacity by cpu
> arch_scale_freq_capacity()    - scale capacity by frequency
> arch_scale_thermal_pressure() - scale pressure (1 - capacity) by thermal

If arch_scale_thermal_pressure() is ok for everybody i'm fine too.

I don't have a strong opinion about the name of the function as long
as we don't go back and forth

>
> [...]