Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur_freq

Peter Zijlstra
2016-04-01T08:29:23Z

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.