linux-kernel - Re: [PATCH v4 2/3] sched/fair: Take thermal pressure into account while estimating energy

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Date:   Wed, 16 Jun 2021 21:25:45 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Lukasz Luba <lukasz.luba@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Quentin Perret <qperret@...gle.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Beata Michalska <Beata.Michalska@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>, segall@...gle.com,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Amit Kachhap <amit.kachhap@...il.com>,
        Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v4 2/3] sched/fair: Take thermal pressure into account
 while estimating energy

On Wed, 16 Jun 2021 at 19:24, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 15/06/2021 18:09, Lukasz Luba wrote:
> >
> > On 6/15/21 4:31 PM, Dietmar Eggemann wrote:
> >> On 14/06/2021 21:11, Lukasz Luba wrote:
>
> [...]
>
> >> It's important to highlight that this will only fix this issue between
> >> schedutil and EAS when it's due to `thermal pressure` (today only via
> >> CPU cooling). There are other places which could restrict policy->max
> >> via freq_qos_update_request() and EAS will be unaware of it.
> >
> > True, but for this I have some other plans.
>
> As long as people are aware of the fact that this was developed to be
> beneficial for `EAS - IPA` integration, I'm fine with this.

I don't think it's only for EAS - IPA. Thermal_pressure can be used by
HW throttling like here:
https://lkml.org/lkml/2021/6/8/1791

EAS is involved but not IPA

>
> [...]
>
> >> IMHO, this means that this is catered for the IPA governor then. I'm not
> >> sure if this would be beneficial when another thermal governor is used?
> >
> > Yes, it will be, the cpufreq_set_cur_state() is called by
> > thermal exported function:
> > thermal_cdev_update()
> >   __thermal_cdev_update()
> >     thermal_cdev_set_cur_state()
> >       cdev->ops->set_cur_state(cdev, target)
> >
> > So it can be called not only by IPA. All governors call it, because
> > that's the default mechanism.
>
> True, but I'm still not convinced that it is useful outside `EAS - IPA`.
>
> >> The mechanical side of the code would allow for such benefits, I just
> >> don't know if their CPU cooling device + thermal zone setups would cater
> >> for this?
> >
> > Yes, it's possible. Even for custom vendor governors (modified clones
> > of IPA)
>
> Let's stick to mainline here ;-) It's complicated enough ...
>
> [...]
>
> >> Maybe shorter?
> >>
> >>          struct cpumask *pd_mask = perf_domain_span(pd);
> >> -       unsigned long cpu_cap =
> >> arch_scale_cpu_capacity(cpumask_first(pd_mask));
> >> +       int cpu = cpumask_first(pd_mask);
> >> +       unsigned long cpu_cap = arch_scale_cpu_capacity(cpu);
> >> +       unsigned long _cpu_cap = cpu_cap -
> >> arch_scale_thermal_pressure(cpu);
> >>          unsigned long max_util = 0, sum_util = 0;
> >> -       unsigned long _cpu_cap = cpu_cap;
> >> -       int cpu;
> >> -
> >> -       _cpu_cap -= arch_scale_thermal_pressure(cpumask_first(pd_mask));
> >
> > Could be, but still, the definitions should be sorted from longest on
> > top, to shortest at the bottom. I wanted to avoid modifying too many
> > lines with this simple patch.
>
> Only if there are no dependencies, but here we have already `cpu_cap ->
> pd_mask`. OK, not a big deal.
>
> [...]
>
> >> There is IPA specific code in cpufreq_set_cur_state() ->
> >> get_state_freq() which accesses the EM:
> >>
> >>      ...
> >>      return cpufreq_cdev->em->table[idx].frequency;
> >>      ...
> >>
> >> Has it been discussed that the `per-PD max (allowed) CPU capacity` (1)
> >> could be stored in the EM from there so that code like the EAS wakeup
> >> code (compute_energy()) could retrieve this information from the EM?
> >
> > No, we haven't think about this approach in these patch sets.
> > The EM structure given to the cpufreq_cooling device and stored in:
> > cpufreq_cdev->em should not be modified. There are a few places which
> > receive the EM, but they all should not touch it. For those clients
> > it's a read-only data structure.
> >
> >> And there wouldn't be any need to pass (1) into the EM (like now via
> >> em_cpu_energy()).
> >> This would be signalling within the EM compared to external signalling
> >> via `CPU cooling -> thermal pressure <- EAS wakeup -> EM`.
> >
> > I see what you mean, but this might cause some issues in the design
> > (per-cpu scmi cpu perf control). Let's use this EM pointer gently ;)
>
> OK, with the requirement that clients see the EM as ro:
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>