Re: [PATCH 3/3] PM: EM: Increase energy calculation precision

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

On 7/5/21 1:45 PM, Dietmar Eggemann wrote:
> On 25/06/2021 17:26, Lukasz Luba wrote:
>> The Energy Model (EM) provides useful information about device power in
>> each performance state to other subsystems like: Energy Aware Scheduler
>> (EAS). The energy calculation in EAS does arithmetic operation based on
>> the EM em_cpu_energy(). Current implementation of that function uses
>> em_perf_state::cost as a pre-computed cost coefficient equal to:
>> cost = power * max_frequency / frequency.
>> The 'power' is expressed in milli-Watts (or in abstract scale).
>>
>> There are corner cases then the EAS energy calculation for two Performance
>              ^^^^^^^^^^^^
> 
> Again, an easy to understand example to describe in which situation this
> change would bring a benefit would help.
> 
>> Domains (PDs) return the same value, e.g. 10mW. The EAS compares these
>> values to choose smaller one. It might happen that this values are equal
>> due to rounding error. In such scenario, we need better precision, e.g.
>> 10000 times better. To provide this possibility increase the precision on
>> the em_perf_state::cost.
>>
>> This patch allows to avoid the rounding to milli-Watt errors, which might
>> occur in EAS energy estimation for each Performance Domains (PD). The
>> rounding error is common for small tasks which have small utilization
>> values.
> 
> What's the influence of the CPU utilization 'cpu_util_next()' here?
> 
> compute_energy()
>      em_cpu_energy()
>              return ps->cost * sum_util / scale_cpu
>                                ^^^^^^^^

This is the place where the rounding error triggers. If sum_util is
small and scale_cpu is e.g. 1024, then we have a small fraction here.
It depends on the EM 'cost', but for most platforms we have small
power and cost values, so we suffer this rounding.
The example that I gave in my response in patch 2/3 shows this.

>> The rest of the EM code doesn't change, em_perf_state::power is still
>> expressed in milli-Watts (or in abstract scale). Thus, all existing
>> platforms don't have to change their reported power. The same applies to
> 
> Not only existing platforms since there are no changes. So why
> highlighting `existing` here.?

I just wanted to be clear that it doesn't affect existing platforms
at all. We don't require to report power in better resolution e.g.
micro-Watts.
Also, the clients in the kernel won't be affected, since they use
EM 'power' filed, not 'cost'.