Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional frequency invariant accounting

[Rafael J. Wysocki]

On Thu, May 17, 2018 at 8:28 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, May 17, 2018 at 06:56:37PM +0200, Rafael J. Wysocki wrote:
>> On Thu, May 17, 2018 at 6:42 PM, Srinivas Pandruvada
>
>> > What will happen if we look at all core turbo as max and cap any
>> > utilization above this to 1024?
>>
>> I was going to suggest that.
>
> To the basic premise behind all our frequency scaling is that there's a
> linear relation between utilization and frequency, where u=1 gets us the
> fastest.
>
> Now, we all know this is fairly crude, but it is what we work with.
>
> OTOH, the whole premise of turbo is that you don't in fact know what the
> fastest is, and in that respect setting u=1 at the guaranteed or
> sustainable frequency makes sense.
>
> The direct concequence of allowing clipping is that u=1 doesn't select
> the highest frequency, but since we don't select anything anyway
> (p-code does that for us) all we really need is to have u=1 above that
> turbo activation point you mentioned.
>
> For parts where we have to directly select frequency this obviously
> comes apart.
>
> However; what happens when the sustainable freq drops below our initial
> 'max'? Imagine us dropping below the all-core-turbo because of AVX. Then
> we're back to running at u<1 at full tilt.
>
> Or for mobile parts, the sustainable frequency could drop because of
> severe thermal limits. Now I _think_ we have the possibility for getting
> interrupts and reading the new guaranteed frequency, so we could
> re-guage.
>
> So in theory I think it works, in practise we need to always be able to
> find the actual max -- be it all-core turbo, AVX or thermal constrained
> frequency. Can we do that in all cases?

We should be, but unfortunately that's a dynamic thing.

For example, the AVX limit only kicks in when AVX instructions are executed.

> I need to go back to see what the complains against Vincent's proposal
> were, because I really liked the fact that it did away with all this.

That would be the best way to deal with this mess, I agree.