Re: [PATCH 1/3] powernow-k6: disable cache when changing frequency

From: Mikulas Patocka
Date: Thu Jan 02 2014 - 12:38:34 EST

On Tue, 17 Dec 2013, Viresh Kumar wrote:

> On 12 December 2013 06:08, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> > During frequency transition the processor is in "EPM Stop Grant" state.
> > The documentation says that the processor doesn't respond to inquiry
> > requests in this state. Consequently, coherency of processor caches and
> > bus master devices is not maintained, causing the system instability.
> >
> > This patch flushes the cache during frequency transition. It fixes the
> > instability.
> I don't have much idea of these systems but wouldn't this affect performance
> badly?

Flushing the cache and changing frequency takes approximatelly 500us. The
patch increases policy->cpuinfo.transition_latency to that value.

I suppose that the governors should be smart enough to not do frequency
transitions so often so that majority of time is spent doing the

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