linux-kernel - Re: [PATCH v2 3/3] thermal: gov_step_wise: Allow cooling level to be reduced earlier

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Message-ID: <CAJZ5v0jPJbL+Ume2q1d6aoZ-PgpY-jqVfPDnwS8KbezLcK5bgg@mail.gmail.com>
Date: Thu, 4 Sep 2025 15:25:09 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Lukasz Luba <lukasz.luba@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] thermal: gov_step_wise: Allow cooling level to be
 reduced earlier

On Thu, Sep 4, 2025 at 9:32 AM Lukasz Luba <lukasz.luba@....com> wrote:
>
>
>
> On 8/25/25 14:31, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > The current behavior of the Step-wise thermal governor is to increase
> > the cooling level one step at a time after trip point threshold passing
> > by thermal zone temperature until the temperature stops to rise and then
> > do nothing until it falls down below the (possibly new) trip point
> > threshold, at which point the cooling level is reduced straight to the
> > applicable minimum.
>
> Quite long single sentence to describe these stuff...

Yes, it is long.  I'll try to rearrange it when applying.

> >
> > While this generally works, it is not in agreement with the throttling
> > logic description comment in step_wise_manage() any more after some
> > relatively recent changes, and in the case of passive cooling, it may
> > lead to undesirable performance oscillations between high and low
> > levels.
> >
> > For this reason, modify the governor's cooling device state selection
> > function, get_target_state(), to reduce cooling by one level even if
> > the temperature is still above the thermal zone threshold, but the
> > temperature has started to fall down.  However, ensure that the cooling
> > level will remain above the applicable minimum in that case to pull
> > the zone temperature further down, possibly until it falls below the
> > trip threshold (which may now be equal to the low temperature of the
> > trip).
> >
> > Doing so should help higher performance to be restored earlier in some
> > cases which is desirable especially for passive trip points with
> > relatively high hysteresis values.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >   drivers/thermal/gov_step_wise.c |   15 ++++++++++++++-
> >   1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > --- a/drivers/thermal/gov_step_wise.c
> > +++ b/drivers/thermal/gov_step_wise.c
> > @@ -20,7 +20,9 @@
> >    * If the temperature is higher than a trip point,
> >    *    a. if the trend is THERMAL_TREND_RAISING, use higher cooling
> >    *       state for this trip point
> > - *    b. if the trend is THERMAL_TREND_DROPPING, do nothing
> > + *    b. if the trend is THERMAL_TREND_DROPPING, use a lower cooling state
> > + *       for this trip point, but keep the cooling state above the applicable
> > + *       minimum
> >    * If the temperature is lower than a trip point,
> >    *    a. if the trend is THERMAL_TREND_RAISING, do nothing
> >    *    b. if the trend is THERMAL_TREND_DROPPING, use the minimum applicable
> > @@ -51,6 +53,17 @@
> >       if (throttle) {
> >               if (trend == THERMAL_TREND_RAISING)
> >                       return clamp(cur_state + 1, instance->lower, instance->upper);
> > +
> > +             /*
> > +              * If the zone temperature is falling, the cooling level can
> > +              * be reduced, but it should still be above the lower state of
> > +              * the given thermal instance to pull the temperature further
> > +              * down.
> > +              */
> > +             if (trend == THERMAL_TREND_DROPPING)
> > +                     return clamp(cur_state - 1,
> > +                                  min(instance->lower + 1, instance->upper),
> > +                                  instance->upper);
> >       } else if (trend == THERMAL_TREND_DROPPING) {
> >               if (cur_state <= instance->lower)
> >                       return THERMAL_NO_TARGET;
> >
> >
> >
>
> That make sense
>
> Reviewed-by: Lukasz Luba <lukasz.luba@....com>