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
