Messages in this thread | | | Date | Thu, 05 Jun 2014 08:03:15 -0700 | From | Dirk Brandewie <> | Subject | Re: [RFC PATCH 06/16] arm: topology: Define TC2 sched energy and provide it to scheduler |
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On 06/04/2014 11:52 PM, Peter Zijlstra wrote: > On Wed, Jun 04, 2014 at 11:56:55PM +0200, Rafael J. Wysocki wrote: >> On Wednesday, June 04, 2014 07:27:12 PM Peter Zijlstra wrote: > >>> Well, we eventually want to go there I think. Although we still needed >>> to come up with something for Intel, because I'm not at all sure how all >>> that works. >> >> Do you mean power numbers or how P-states work on Intel in general? > > P-states, I'm still not at all sure how all that works on Intel and what > we can sanely do with them. > > Supposedly Intel has a means of setting P-states (there's a driver after > all), but then is completely free to totally ignore it and do something > entirely different anyhow.
You can request a P state per core but the package does coordination at a package level for the P state that will be used based on all requests. This is due to the fact that most SKUs have a single VR and PLL. So the highest P state wins. When a core goes idle it loses it's vote for the current package P state and that cores clock it turned off.
> > And while APERF/MPERF allows observing what it did, its afaik, nigh on > impossible to predict wtf its going to do, and therefore any such energy > computation is going to be a PRNG at best. > > Now, given all that I'm not sure what we need that P-state driver for, > so supposedly I'm missing something.
intel_pstate tries to keep the core P state as low as possible to satisfy the given load, so when various cores go idle the package P state can be as low as possible. The big power win is a core going idle.
> > Ideally Len (or someone equally in-the-know) would explain to me how > exactly all that works and what we can rely upon. All I've gotten so far > is, you can't rely on anything, and magik. Which is entirely useless. > The only thing you can rely on is that you will get "at least" the P state requested in the presence of hardware coordination.
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