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

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Message-ID: <alpine.LRH.2.02.1401021233280.16351@file01.intranet.prod.int.rdu2.redhat.com>
Date:	Thu, 2 Jan 2014 12:38:08 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] powernow-k6: disable cache when changing frequency



On Tue, 17 Dec 2013, Viresh Kumar wrote:

> On 12 December 2013 06:08, Mikulas Patocka <mpatocka@...hat.com> 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 
transition.

Mikulas
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