Re: [BISECTED] Lots of "rescheduling IPIs" in powertop

Ingo Molnar
2008-05-16T08:41:35Z

* Andreas Herrmann <andreas.herrmann3@amd.com> wrote:

> > > Recap: powertop shows between 200-400 wakeups/second with the
> > > description "<kernel IPI>: Rescheduling interrupts" when all
> > > processors have load (e.g. I need to run two busy-loops on my
> > > 2-CPU system for this to show up).
> >
> > ok, could you try the fix below? It was a mistake to make mwait use
> > dependent on power considerations - on a desktop CPU it is unlikely
> > to use more power than a simple HLT - and the IPIs are extra
> > scheduling latency and extra power used.
>
> It depends on the CPU. For AMD CPUs that support MWAIT this is wrong.
> Family 0x10 and 0x11 CPUs will enter C1 on HLT. Powersavings then
> depend on a clock divisor and current Pstate of the core.
>
> If all cores of a processor are in halt state (C1) the processor can
> enter the C1E (C1 enhanced) state. If mwait is used this will never
> happen.
>
> Thus HLT saves more power than MWAIT here.
>
> It might be best to switch off the mwait flag for these AMD CPU
> families like it was introduced with commit
> f039b754714a422959027cb18bb33760eb8153f0 (x86: Don't use MWAIT on AMD
> Family 10)

agreed, that looks like the cleanest solution. Could you please send a
patch for that, ontop of my patch?

Ingo