On Fri, Oct 19, 2018 at 11:34 AM, John Stultz <john.stultz@xxxxxxxxxx> wrote: > On Fri, Oct 19, 2018 at 8:25 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: >> 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 > > So, just to understand, your saying the problem that we calculate a > tsc_khz value before calculating the mult/shift and the intermediate > step is losing some precision? > > Or is the cause from something else? The other potential cause here might be just that when we calculate the mult/shift pair, we select a shift small enough that avoids the multiplication from overflowing if we have a long timerval. So there is liklely always some granularity error converting to mult/shift pair. thanks -john
