linux-kernel - Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur

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Message-ID: <20160401082923.GH11035@twins.programming.kicks-ass.net>
Date:	Fri, 1 Apr 2016 10:29:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Gasparini <stephane.gasparini@...ux.intel.com>
Cc:	Len Brown <lenb@...nel.org>, x86@...nel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and
 scaling_cur_freq

On Fri, Apr 01, 2016 at 10:23:23AM +0200, Peter Zijlstra wrote:
> 
> Trim your emails
> 
> On Fri, Apr 01, 2016 at 10:16:42AM +0200, Stephane Gasparini wrote:
> 
> > > That means these delta's can be arbitrarily large, in fact the MSRs can
> > > have wrapped however many times.
> > 
> > 64 bits is 18 446 744 073 709 551 615
> > 
> > so even assuming a 10 GHz frequency if my math are good this is more than
> > 58 years before the MSR wrap around, assuming the device ran always at max
> > freq.
> 
> fair enough.. but going with 10Ghz, cpu_khz would be 10e6 ~ 33 bits,

I can't do maths this morning; 23 bits

> which effectively reduces the wrap/overflow time to just 31 bits, which
> per that frequency is just ~1/4th of a second.

41 giving lots more, but a reasonable time to wrap/overflow.