linux-kernel - Re: [PATCH v1 06/13] thermal: gov_fair_share: Rearrange get_trip

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Date:   Wed, 27 Sep 2023 17:06:14 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Lukasz Luba <lukasz.luba@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v1 06/13] thermal: gov_fair_share: Rearrange get_trip_level()

On Wed, Sep 27, 2023 at 5:00 PM Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
>
> On 21/09/2023 19:54, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Make get_trip_level() access the thermal zone's trip table directly
> > instead of using __thermal_zone_get_trip() which adds overhead related
> > to the unnecessary bounds checking and copying the trip point data.
> >
> > Also rearrange the code in it to make it somewhat easier to follow.
> >
> > The general functionality is not expected to be changed.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >   drivers/thermal/gov_fair_share.c |   22 ++++++++++------------
> >   1 file changed, 10 insertions(+), 12 deletions(-)
> >
> > Index: linux-pm/drivers/thermal/gov_fair_share.c
> > ===================================================================
> > --- linux-pm.orig/drivers/thermal/gov_fair_share.c
> > +++ linux-pm/drivers/thermal/gov_fair_share.c
> > @@ -21,23 +21,21 @@
> >    */
> >   static int get_trip_level(struct thermal_zone_device *tz)
> >   {
> > -     struct thermal_trip trip;
> > -     int count;
> > +     const struct thermal_trip *trip = tz->trips;
> > +     int i;
> >
> > -     for (count = 0; count < tz->num_trips; count++) {
> > -             __thermal_zone_get_trip(tz, count, &trip);
> > -             if (tz->temperature < trip.temperature)
> > +     if (tz->temperature < trip->temperature)
> > +             return 0;
> > +
> > +     for (i = 0; i < tz->num_trips - 1; i++) {
> > +             trip++;
> > +             if (tz->temperature < trip->temperature)
> >                       break;
> >       }
>
> Is it possible to use for_each_thermal_trip() instead ? That would make
> the code more self-encapsulate

It is possible in principle, but this is a governor which is regarded
as part of the core, isn't it?

So is an extra overhead related to using a callback (which may be
subject to retpolines and such) really justified in this case?

>
> > -     /*
> > -      * count > 0 only if temperature is greater than first trip
> > -      * point, in which case, trip_point = count - 1
> > -      */
> > -     if (count > 0)
> > -             trace_thermal_zone_trip(tz, count - 1, trip.type);
> > +     trace_thermal_zone_trip(tz, i, tz->trips[i].type);
> >
> > -     return count;
> > +     return i;
> >   }
> >
> >   static long get_target_state(struct thermal_zone_device *tz,
> >
> >
> >
>
> --