linux-kernel - Re: [PATCH 2/2] thermal: power allocator: estimate sustainable power only once

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Message-ID: <20201008101434.GA23491@arm.com>
Date:   Thu, 8 Oct 2020 11:14:34 +0100
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        daniel.lezcano@...aro.org, amitk@...nel.org,
        Dietmar.Eggemann@....com
Subject: Re: [PATCH 2/2] thermal: power allocator: estimate sustainable power
 only once

Hi Lukasz,

On Friday 02 Oct 2020 at 13:24:16 (+0100), Lukasz Luba wrote:
> The sustainable power value might come from the Device Tree or can be
> estimated in run time. There is no need to estimate every time when the
> governor is called and temperature is high. Instead, store the estimated
> value and make it available via standard sysfs interface so it can be
> checked from the user-space.
> 
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  drivers/thermal/gov_power_allocator.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/thermal/gov_power_allocator.c b/drivers/thermal/gov_power_allocator.c
> index f69fafe486a5..dd59085f38f5 100644
> --- a/drivers/thermal/gov_power_allocator.c
> +++ b/drivers/thermal/gov_power_allocator.c
> @@ -204,6 +204,8 @@ static u32 pid_controller(struct thermal_zone_device *tz,
>  		estimate_pid_constants(tz, sustainable_power,
>  				       params->trip_switch_on, control_temp,
>  				       true);
> +		/* Do the estimation only once and make available in sysfs */
> +		tz->tzp->sustainable_power = sustainable_power;

After looking over the code, it does seems mostly useless to do the
estimation every time the controller kicks in.

But I have two comments in this regard:

 - The estimation is dependent on the temperature we control for which
   can be changed from sysfs. While I don't see that as a big worry,
   (sustainable power is an estimation anyway), it might be worth a
   more detailed comment on why we don't expect this to be a problem,
   or what we expect the consequences of computing sustainable power
   only once could be.

 - In the function comment for estimate_pid_constants() there is a
   mention of sustainable power:
   """
    * Sustainable power is provided in case it was estimated.  The
    * estimated sustainable_power should not be stored in the
    * thermal_zone_parameters so it has to be passed explicitly to this
    * function.
   """
   If we are going to compute the sustainable power estimation only once,
   this comment should be removed, the estimated value should be added to
   the trip point parameters before estimate_pid_constants(), and the
   sustainable_power argument should be removed.
   Otherwise we end up with conflicting information in the code.

Regards,
Ionela.

>  	}
>  
>  	err = control_temp - tz->temperature;
> -- 
> 2.17.1
>