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

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Message-ID: <d57cc0aa-01e4-e3b2-c591-be7e54f95780@arm.com>
Date:   Thu, 8 Oct 2020 13:34:40 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Ionela Voinescu <ionela.voinescu@....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 Ionela,

On 10/8/20 11:14 AM, Ionela Voinescu wrote:
> 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.

The problem is that we don't expose the estimated value in the sysfs.
This is the case when there was no DT entry with 'sustainable-power'.
If someone is going to write the values via sysfs, we assume he/she
knows the consequences and also what other values to write and where,
to make it working optimally.

> 
>   - 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.
>     """

Good catch, that comment left. I will remove it.

>     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.

We can also call estimate_sustainable_power() inside the
estimate_pid_constants() if sust. power was 0, then set the
tz->tzp->sustainable_power = sustainable_power

Thank you for your comments.

Regards,
Lukasz

> 
> Regards,
> Ionela.
>