Date:   Tue, 13 Oct 2020 13:04:48 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     amitk@...nel.org, Dietmar.Eggemann@....com
Subject: Re: [PATCH 1/2] thermal: power allocator: change the 'k_i'
 coefficient estimation



On 10/13/20 12:22 PM, Daniel Lezcano wrote:
> On 13/10/2020 12:59, Lukasz Luba wrote:
>> Hi Daniel,
>>
>> On 10/13/20 11:21 AM, Daniel Lezcano wrote:
>>>
>>> Hi Lukasz,
>>>
>>> On 02/10/2020 14:24, Lukasz Luba wrote:
>>>> Intelligent Power Allocation (IPA) is built around the PID controller
>>>> concept. The initialization code tries to setup the environment based on
>>>> the information available in DT or estimate the value based on minimum
>>>> power reported by each of the cooling device. The estimation will
>>>> have an
>>>> impact on the PID controller behaviour via the related 'k_po', 'k_pu',
>>>> 'k_i' coefficients and also on the power budget calculation.
>>>>
>>>> This change prevents the situation when 'k_i' is relatively big compared
>>>> to 'k_po' and 'k_pu' values. This might happen when the estimation for
>>>> 'sustainable_power' returned small value, thus 'k_po' and 'k_pu' are
>>>> small.
>>>>
>>>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>>>> ---
>>>>    drivers/thermal/gov_power_allocator.c | 8 ++++++--
>>>>    1 file changed, 6 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/thermal/gov_power_allocator.c
>>>> b/drivers/thermal/gov_power_allocator.c
>>>> index 5cb518d8f156..f69fafe486a5 100644
>>>> --- a/drivers/thermal/gov_power_allocator.c
>>>> +++ b/drivers/thermal/gov_power_allocator.c
>>>> @@ -131,6 +131,7 @@ static void estimate_pid_constants(struct
>>>> thermal_zone_device *tz,
>>>>        int ret;
>>>>        int switch_on_temp;
>>>>        u32 temperature_threshold;
>>>> +    s32 k_i;
>>>>          ret = tz->ops->get_trip_temp(tz, trip_switch_on,
>>>> &switch_on_temp);
>>>>        if (ret)
>>>> @@ -156,8 +157,11 @@ static void estimate_pid_constants(struct
>>>> thermal_zone_device *tz,
>>>>            tz->tzp->k_pu = int_to_frac(2 * sustainable_power) /
>>>>                temperature_threshold;
>>>>    -    if (!tz->tzp->k_i || force)
>>>> -        tz->tzp->k_i = int_to_frac(10) / 1000;
>>>> +    if (!tz->tzp->k_i || force) {
>>>> +        k_i = tz->tzp->k_pu / 10;
>>>> +        tz->tzp->k_i = k_i > 0 ? k_i : 1;
>>>> +    }
>>>
>>> I do not understand the rational behind this change.
>>
>> This is the unfortunate impact of the EM abstract scale of power values.
>> IPA didn't have to deal with it, because we always had milli-Watts.
>> Because the EM allows the bogoWatts and some vendors already have
>> them I have to re-evaluate the IPA.
>>
>>>
>>> Do you have some values to share describing what would be the impact of
>>> this change?
>>
>> Yes, here is an example:
>> EM has 3 devices with abstract scale power values, where minimum power
>> is 25 and max is 200. The minimum power is used by
>> estimate_sustainable_power()
>> as a sum of all devices' min power. Sustainable power is going to be
>> estimated to 75.
>>
>> Then in the code we have 'temperature_threshold' which is in
>> milli-Celcius, thus 15degC is 15000.
>>
>> We estimate 'k_po' according to:
>> int_to_frac(sustainable_power) / temperature_threshold;
>>
>> which is:
>> (75 << 10) / 15000 = ~75000 / 15000 = 5 <-- 'k_po'
>>
>> then k_pu:
>> ((2*75) << 10) / 15000 = ~150000 / 15000 = 10
>>
>> Then the old 'k_i' is just hard-coded 10, which is
>> the same order of magnitude to what is in 'k_pu'.
>> It should be 1 order of magnitude smaller than 'k_pu'.
>>
>> I did some experiments and the bigger 'k_i' slows down a lot
>> the rising temp. That's why this change.
>>
>> It was OK to have k_i=10 when we were in milliWatts world,
>> when the min power value was bigger, thus 'k_pu' was also bigger
>> than our hard-coded 'k_i'.
>>
>>>
>>> Depending on the thermal behavior of a board, these coefficients could
>>> be very different, no ?
>>>
>>
>> Yes, I strongly believe that vendor engineers will make experiments with
>> these values and not go with default. Then they will store the k_pu,
>> k_po, k_i via sysfs interface, with also sustainable_power.
> 
> IMHO it is the opposite. For what I've seen, the IPA is not used or the
> k_* are misunderstood, thus not changed. The PID regulation loop
> technique is not quite used and known by everyone.

There is quite a few DT entries of 'sustainable-power' so I assumed
it is known, but you might be right.

> 
>> But I have to also fix the hard-coded k_i in the estimation. As
>> described above, when we have small power values from abstract scale,
>> the k_i stays too big.
> 
> May be it is preferable to adjust the k_* dynamically given the
> undershot and overshot results? And then add a set of less opaque
> parameters for the user, like the time or watts, no?
> 

Hmmmm, this is interesting, I haven't thought about it. Thank you
for this idea.
That would require a re-design of current IPA. IPA trying to figure
out better k_* values... I will discuss it internally.

It would take time, definitely more than the proposed small fix
addressing abstract scale and hard-coded 'k_i'.
Do you think that this fix can be applied and then I can experiment
on what you suggested?

There is v3 reviewed by Ionela [1].

Thank you for your comments.

Regards,
Lukasz

[1] 
https://lore.kernel.org/linux-pm/20201009135850.14727-1-lukasz.luba@arm.com/

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