Re: Re: [PATCH 2/2] x86: use KNOWN_FREQ and RELIABLE TSC flags on certain processors/SoCs

[Bin Gao]

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.