Re: [PATCH v1 4/4] thermal: gov_bang_bang: Use governor_data to reduce overhead

["Zhang, Rui" <rui.zhang@intel.com>]

On Wed, 2024-08-14 at 19:26 +0200, Rafael J. Wysocki wrote:
> On Wed, Aug 14, 2024 at 8:09 AM Zhang, Rui <rui.zhang@intel.com>
> wrote:
> > 
> > On Tue, 2024-08-13 at 16:29 +0200, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > 
> > > After running once, the for_each_trip_desc() loop in
> > > bang_bang_manage() is pure needless overhead because it is not
> > > going
> > > to
> > > make any changes unless a new cooling device has been bound to
> > > one of
> > > the trips in the thermal zone or the system is resuming from
> > > sleep.
> > > 
> > > For this reason, make bang_bang_manage() set governor_data for
> > > the
> > > thermal zone and check it upfront to decide whether or not it
> > > needs
> > > to
> > > do anything.
> > > 
> > > However, governor_data needs to be reset in some cases to let
> > > bang_bang_manage() know that it should walk the trips again, so
> > > add
> > > an
> > > .update_tz() callback to the governor and make the core
> > > additionally
> > > invoke it during system resume.
> > > 
> > > To avoid affecting the other users of that callback
> > > unnecessarily,
> > > add
> > > a special notification reason for system resume,
> > > THERMAL_TZ_RESUME,
> > > and
> > > also pass it to __thermal_zone_device_update() called during
> > > system
> > > resume for consistency.
> > > 
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > ---
> > >  drivers/thermal/gov_bang_bang.c |   18 ++++++++++++++++++
> > >  drivers/thermal/thermal_core.c  |    3 ++-
> > >  include/linux/thermal.h         |    1 +
> > >  3 files changed, 21 insertions(+), 1 deletion(-)
> > > 
> > > Index: linux-pm/drivers/thermal/gov_bang_bang.c
> > > =================================================================
> > > ==
> > > --- linux-pm.orig/drivers/thermal/gov_bang_bang.c
> > > +++ linux-pm/drivers/thermal/gov_bang_bang.c
> > > @@ -86,6 +86,10 @@ static void bang_bang_manage(struct ther
> > >         const struct thermal_trip_desc *td;
> > >         struct thermal_instance *instance;
> > > 
> > > +       /* If the code below has run already, nothing needs to be
> > > done. */
> > > +       if (tz->governor_data)
> > > +               return;
> > > +
> > >         for_each_trip_desc(tz, td) {
> > >                 const struct thermal_trip *trip = &td->trip;
> > > 
> > > @@ -107,11 +111,25 @@ static void bang_bang_manage(struct ther
> > >                                
> > > bang_bang_set_instance_target(instanc
> > > e, 0);
> > >                 }
> > >         }
> > > +
> > > +       tz->governor_data = (void *)true;
> > > +}
> > > +
> > > +static void bang_bang_update_tz(struct thermal_zone_device *tz,
> > > +                               enum thermal_notify_event reason)
> > > +{
> > > +       /*
> > > +        * Let bang_bang_manage() know that it needs to walk
> > > trips
> > > after binding
> > > +        * a new cdev and after system resume.
> > > +        */
> > > +       if (reason == THERMAL_TZ_BIND_CDEV || reason ==
> > > THERMAL_TZ_RESUME)
> > > +               tz->governor_data = NULL;
> > >  }
> > 
> > can we do the cdev initialization for BIND_CDEV and RESUME
> > notification
> > in .update_tz() directly?
> 
> That would be viable if the zone temperature was known at the time
> .update_tz() runs, but it isn't.  See this message:
> 
> https://lore.kernel.org/linux-pm/CAJZ5v0ji_7Z-24iCO_Xxu4Zm4jgVFmR9jVp8QNiCOxzV9gqSnA@mail.gmail.com/
> 
> As long as the zone temperature is not known, it is not clear which
> way to initialize the cooling devices.

right. Then the patch LGTM.

BTW, what do you think if we add handling for first temperature read in
handle_thermal_trip(), say, some draft code like below,

if (tz->last_temperature == THERMAL_TEMP_INIT) {
	if (tz->temperature < trip->temperature)
		list_add(&td->notify_list_node, way_down_list);
	else
		list_add_tail(&td->notify_list_node, way_up_list);
}

This should handle both the init and the resume case.

thanks,
rui
> 
> Interestingly enough, the zone temperature is first checked by the
> core when the zone is enabled and not when it is registered.
> 
> > Then we don't need .manage() callback. This makes more sense to me
> > because bang_bang governor cares about trip point crossing only.
> 
> That's true, but this is all about a corner case in which no trip
> points are crossed and the cooling devices need to be initialized
> properly regardless.
> 
> > >  static struct thermal_governor thermal_gov_bang_bang = {
> > >         .name           = "bang_bang",
> > >         .trip_crossed   = bang_bang_control,
> > >         .manage         = bang_bang_manage,
> > > +       .update_tz      = bang_bang_update_tz,
> > >  };
> > >  THERMAL_GOVERNOR_DECLARE(thermal_gov_bang_bang);
> > > Index: linux-pm/drivers/thermal/thermal_core.c
> > > =================================================================
> > > ==
> > > --- linux-pm.orig/drivers/thermal/thermal_core.c
> > > +++ linux-pm/drivers/thermal/thermal_core.c
> > > @@ -1692,7 +1692,8 @@ static void thermal_zone_device_resume(s
> > > 
> > >         thermal_debug_tz_resume(tz);
> > >         thermal_zone_device_init(tz);
> > > -       __thermal_zone_device_update(tz,
> > > THERMAL_EVENT_UNSPECIFIED);
> > > +       thermal_governor_update_tz(tz, THERMAL_TZ_RESUME);
> > > +       __thermal_zone_device_update(tz, THERMAL_TZ_RESUME);
> > > 
> > >         complete(&tz->resume);
> > >         tz->resuming = false;
> > > Index: linux-pm/include/linux/thermal.h
> > > =================================================================
> > > ==
> > > --- linux-pm.orig/include/linux/thermal.h
> > > +++ linux-pm/include/linux/thermal.h
> > > @@ -55,6 +55,7 @@ enum thermal_notify_event {
> > >         THERMAL_TZ_BIND_CDEV, /* Cooling dev is bind to the
> > > thermal
> > > zone */
> > >         THERMAL_TZ_UNBIND_CDEV, /* Cooling dev is unbind from the
> > > thermal zone */
> > >         THERMAL_INSTANCE_WEIGHT_CHANGED, /* Thermal instance
> > > weight
> > > changed */
> > > +       THERMAL_TZ_RESUME, /* Thermal zone is resuming after
> > > system
> > > sleep */
> > >  };
> > > 
> > >  /**
> > > 
> > > 
> > > 
> > 
>