On Thu, May 17, 2018 at 8:28 PM, Peter Zijlstra <pet...@infradead.org> 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.
