On Fri, Nov 11, 2016 at 12:26:40AM +0100, Thomas Gleixner wrote: > On Thu, 10 Nov 2016, Bin Gao wrote: > > > > @@ -702,6 +702,15 @@ unsigned long native_calibrate_tsc(void) > > > > } > > > > } > > > > > > > > + setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ); > > > > > > I can understand the one below, but this one changes existing behaviour > > > w/o explaining why this is correct and desired. If at all then this wants > > > to be a seperate patch and not just mingled in your goldmont update. > > > > native_calibrate_tsc() implements determining TSC frequency via CPUID. > > The purpose to add X86_FEATURE_TSC_KNOWN_FREQ flag is exactly for this case: > > TSC frequency determined via CPUID or MSR are always correct and the whole > > calibration should be skipped. > > Did you actually verify that this is correct and does not introduce NTP > issues compared to the long term calibration on such platforms? > > We've been burnt before and myself and others wasted enough time already > debugging that crap.
Yes, we had a 24 hours test before on one of the CPUID capable platforms. With PIT calibrated frequency, we got more than 3 seconds drift whereas with CPUID determined frequency we only got less than 0.5 second drift. Another fact is that on MSR capable platforms, PIT/HPET is generally not available so calibration won't work at all.
