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SubjectRe: TSC to Mono-raw Drift
Christopher,

Please Cc LKML on such issues in the future.

On Mon, 15 Oct 2018, Christopher Hall wrote:

Leaving context around for new readers:

> Problem Statement:
>
> The TSC clocksource mult/shift values are derived from CPUID[15H], but the
> monotonic raw clock value is not equal to TSC in nominal nanoseconds, i.e.
> the timekeeping code is not accurately transforming TSC ticks to nominal
> nanoseconds based on CPUID[15H}.
>
> The included code calculates the drift between nominal TSC nanoseconds and
> the monotonic raw clock.
>
> Background:
>
> Starting with 6th generation Intel CPUs, the TSC is "phase locked" to the
> Always Running Timer (ART). The relation between TSC and ART is read from
> CPUID[15H]. Details of the TSC-ART relation are in the "Invariant
> Timekeeping" section of the SDM.
>
> CPUID[15H].ECX returns the nominal frequency of ART (or crystal frequency).
> CPU feature TSC_KNOWN_FREQ indicates that tsc_khz (tsc.c) is derived from
> CPUID[15H]. The calculation is in tsc.c:native_calibrate_tsc().
>
> When the TSC clocksource is selected, the timekeeping code uses mult/shift
> values to transform TSC into nanoseconds. The mult/shift value is determined
> using tsc_khz.
>
> Example Output:
>
> Running for 3 seconds trial 1
> Scaled TSC delta: 3000328845
> Monotonic raw delta: 3000329117
> Ran for 3 seconds with 272 ns skew
>
> Running for 3 seconds trial 2
> Scaled TSC delta: 3000295209
> Monotonic raw delta: 3000295482
> Ran for 3 seconds with 273 ns skew
>
> Running for 3 seconds trial 3
> Scaled TSC delta: 3000262870
> Monotonic raw delta: 3000263142
> Ran for 3 seconds with 272 ns skew
>
> Running for 300 seconds trial 4
> Scaled TSC delta: 300000281725
> Monotonic raw delta: 300000308905
> Ran for 300 seconds with 27180 ns skew
>
> The skew between tsc and monotonic raw is about 91 PPB.
>
> System Information:
>
> CPU model string: Intel(R) Core(TM) i5-6600 CPU @ 3.30GHz
> Kernel version tested: 4.14.71-rt44
> NOTE: The skew seems to be insensitive to kernel version after
> introduction of TSC_KNOWN_FREQ capability
>
> >From CPUID[15H]:
> Time Stamp Counter/Core Crystal Clock Information (0x15):
> TSC/clock ratio = 276/2
> nominal core crystal clock = 24000000 Hz (table lookup)
>
> TSC kHz used to calculate mult/shift value: 3312000

Now the most interesting information here would be the resulting mult/shift
value which the kernel uses. Can you please provide that?

Thanks,

Thomas

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