Re: [PATCH v2 15/17] Documentation: EM: Add a runtime modifiable EM design description

[Lukasz Luba <lukasz.luba@arm.com>]

On 5/30/23 11:42, Dietmar Eggemann wrote:
> On 12/05/2023 11:57, Lukasz Luba wrote:
>> Document the new runtime modifiable EM design and how it can be used.
>> Change the last section layout and allow to provide another example
>> how to use this new API in a driver code.
>>
>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
>> ---
>>   Documentation/power/energy-model.rst | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
>> diff --git a/Documentation/power/energy-model.rst b/Documentation/power/energy-model.rst
>> index e97c7f18d8bd..64c2462dc9a6 100644
>> --- a/Documentation/power/energy-model.rst
>> +++ b/Documentation/power/energy-model.rst
>> @@ -89,6 +89,26 @@ due to the workload instruction mix and data set is not modeled.
>>   Also static power which can change during runtime due to variation of SOC
>>   temperature is not modeled in EM.
>>   
>> +2.2 Runtime modifiable EM
>> +^^^^^^^^^^^^^^^^^^^^^^^^^
>> +
>> +To better reflect power variation due to static power (leakage) the EM
>> +supports runtime modifications of the power values. The mechanism relies on
>> +RCU to free the modifiable EM perf_state table memory. Its user, the task
>> +scheduler, also uses RCU to access this memory. The EM framework is
>> +responsible for allocating the new memory for the modifiable EM perf_state
>> +table. The old memory is freed automatically using RCU callback mechanism.
>> +This design decision is made based on task scheduler using that data and
>> +to prevent wrong usage of kernel modules if they would be responsible for the
>> +memory management.
>> +The drivers which want to modify the EM values are protected from concurrent
>> +access using a mutex. Therefore, the drivers must use sleeping context when
>> +they want to modify the EM. The runtime modifiable EM might also be used for
>> +better reflecting real workload scenarios, e.g. when they pop-up on the screen
>> +and will run for longer period, such as: games, video recoding or playing,
>> +video calls, etc. It is up to the platform engineers to experiment and choose
>> +the right approach for their device.
> 
> IMHO, there are a lot of design aspects missing here.
> 
> E.g.
> 
> Why 2 tables, modifiable (a) and default (b)?
> 
> Why does only EAS use (a)?
> 
> (a) and (b) being the same performance state table until first call to
> modify (a) ()
> 

I'll add that explanation in the v3. I wanted to avoid such detailed
description about e.g. 'being the same performance state table until 
first call to > modify (a)' since it's a memory optimization
bit. Although, I will add that reason to the doc as well.