linux-kernel - Re: [RFC 0/3] Experimental patchset for CPPC

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Message-ID: <20140815144124.GK19379@twins.programming.kicks-ass.net>
Date:	Fri, 15 Aug 2014 16:41:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Mike Turquette <mike.turquette@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Arjan van de Ven <arjan@...ux.intel.com>, mingo@...nel.org,
	len.brown@...el.com, rjw@...ysocki.net,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Arnd Bergmann <arnd@...db.de>, linux-acpi@...r.kernel.org,
	cpufreq@...r.kernel.org, Patch Tracking <patches@...aro.org>,
	Dirk Brandewie <dirk.brandewie@...il.com>
Subject: Re: [RFC 0/3] Experimental patchset for CPPC

On Fri, Aug 15, 2014 at 10:37:32AM -0400, Ashwin Chaugule wrote:
> Hi Peter,
> 
> On 15 August 2014 10:07, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Fri, Aug 15, 2014 at 09:08:50AM -0400, Ashwin Chaugule wrote:
> >> If the OS only looks at Highest, Lowest, Delivered registers and only
> >> writes to Desired, then we're not really any different than how we do
> >> things today in the CPUFreq layer.
> >
> > The thing is; we're already struggling to make 'sense' of x86 as it
> > stands today. And it looks like this CPPC stuff makes the behaviour even
> > less certain.
> 
> I think its still better than the "p-state" thing we have going today,
> where the algorithms are making their decisions based on the incorrect
> assumption that the CPU got what it requested for. (among other things
> listed earlier.) CPPC at least gives you a guarantee that the
> delivered performance will be within a range you requested. It can
> even force the platform to deliver a specific performance value if you
> choose over a specific time window.

Maybe; the guarantee and interrupt on change might be useful indeed. But
which ever way we need aperf/mperf ratios somewhere.

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