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

Stephane Gasparini
2016-04-01T09:30:51Z

my comment was about your comment that MSR have wrapped however many times

> On Apr 1, 2016, at 10:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> That is; if userspace doesn't request a freq reading we can go without
> reading this for a very long time.
>
>> +
>> + rdmsrl(MSR_IA32_APERF, aperf);
>> + rdmsrl(MSR_IA32_MPERF, mperf);
>> +
>> + aperf_delta = aperf - s->aperf;
>> + mperf_delta = mperf - s->mperf;
>
> That means these delta's can be arbitrarily large, in fact the MSRs can
> have wrapped however many times.

The MSRs will not wrap that often.

—
Steph

> On Apr 1, 2016, at 10:29 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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.
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