Re: TSC to Mono-raw Drift

[Thomas Gleixner]

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