linux-kernel - Re: [PATCH v3 4/7] thermal/drivers/tegra: Add driver for Tegra30 thermal sensor

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Date:   Wed, 16 Jun 2021 10:51:45 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Amit Kucheria <amitk@...nel.org>,
        Andreas Westman Dorcsak <hedmoo@...oo.com>,
        Maxim Schwalm <maxim.schwalm@...il.com>,
        Svyatoslav Ryhel <clamor95@...il.com>,
        Ihor Didenko <tailormoon@...bler.ru>,
        Ion Agorria <ion@...rria.com>,
        Matt Merhar <mattmerhar@...tonmail.com>,
        Peter Geis <pgwipeout@...il.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-tegra@...r.kernel.org,
        "open list:THERMAL" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v3 4/7] thermal/drivers/tegra: Add driver for Tegra30
 thermal sensor

On Wed, 16 Jun 2021 at 10:39, Dmitry Osipenko <digetx@...il.com> wrote:
>
> 16.06.2021 11:30, Vincent Guittot пишет:
> > On Wed, 16 Jun 2021 at 10:03, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >>
> >> +Vincent.
> >>
> >> On 15-06-21, 22:32, Dmitry Osipenko wrote:
> >>> IIUC, the cpufreq already should be prepared for the case where firmware
> >>> may override frequency. Viresh, could you please clarify what are the
> >>> possible implications of the frequency overriding?
> >>
> >> The only implication is software would think hardware is running at
> >> some other frequency, while it is not. Not sure if something may break
> >> as a result of this.
> >>
> >> The scheduler's view of CPUs will not be same though, i.e. scheduler
> >> will see capacity as X, while in reality it has changed to Y.
> >
> > thermal_pressure is used by scheduler to balance the load between CPUs
> > according to the actual max frequency. If the thermal pressure doesn't
> > reflect reality, scheduler will end up enqueuing too many  tasks on a
> > throttle CPU.
>
> What if all CPUs are throttled equally and running on the same
> frequency, will throttling have any effect on the scheduler decisions?

Yes, the capacity is also used to detect when CPUs have spare capacity
or are already overloaded. We usually try to fill the spare capacity
of a CPU (CPU's max capacity - current utilization) but he max
capacity is reduced when the CPU is throttled, and the spare capacity
doesn't exist but scheduler could try to it