Re: [PATCH v2] x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined calibration

[Bin Gao <bin.gao@linux.intel.com>]

On Fri, Aug 26, 2016 at 12:14:58PM +0200, Thomas Gleixner wrote:
> On Fri, 26 Aug 2016, Thomas Gleixner wrote:
> > On Thu, 25 Aug 2016, Bin Gao wrote:
> > > On Wed, Aug 24, 2016 at 10:51:20AM +0200, Thomas Gleixner wrote:
> > > > On Tue, 16 Aug 2016, Bin Gao wrote:
> > > > > On some newer Intel x86 processors/SoCs the TSC frequency can be directly
> > > > > calculated by factors read from specific MSR registers or from a cpuid
> > > > > leaf (0x15). TSC frequency calculated by native msr/cpuid is absolutely
> > > > > accurate so we should always skip calibrating TSC aginst another clock,
> > > > > e.g. PIT, HPET, etc. So we want to skip the refined calibration by setting
> > > > > the X86_FEATURE_TSC_RELIABLE flag. Existing code setting the flag by
> > > > > set_cpu_cap() doesn't work as the flag is cleared later in identify_cpu().
> > > > > A cpu caps flag is not cleared only if it's set by setup_force_cpu_cap().
> > > > > This patch converted set_cpu_cap() to setup_force_cpu_cap() to ensure
> > > > > refined calibration is skipped.
> > > > > 
> > > > > We had a test on Intel CherryTrail platform: the 24 hours time drift is
> > > > > 3.6 seconds if refined calibration was not skipped while the drift is less
> > > > > than 0.6 second when refined calibration was skipped.
> > > > > 
> > > > > Correctly setting the X86_FEATURE_TSC_RELIABLE flag also guarantees TSC is
> > > > > not monitored by timekeeping watchdog because on most of these system TSC
> > > > > is the only reliable clocksource. HPET, for instance, works but may not
> > > > > be reliable. So kernel may report a physically reliable TSC is not reliable
> > > > > just because a physically not reliable HPET is acting as timekeeping
> > > > > watchdog.
> > > > 
> > > > What about non SoC systems where the MSR is available, but we still see that
> > > > cross socket TSC wreckage? This change will prevent the watchdog from
> > > > detecting that.
> > > 
> > > MSR is only available on Intel Atom SoCs. There is no such a multi-socket system.
> > 
> > Fair enough.
> 
> Second thoughts. We should seperate the calibration aspect from the reliablity
> aspect.
> 
> If a MSR/CPUID readout provides reliable calibration then this does not tell
> us about the reliablity (i.e. no watchdog required). So having two flags for
> this - and sure you can set both on those SoCs is the proper solution.
> 
> Thanks,
> 
> 	tglx


Hi Thomas,

The Linux kernel does think a reliable calibration implies the reliability (i.e.
no watchdog required). I'm posting some code pieces to explain.

X86_FEATURE_TSC_RELIABLE is referred only in two places as shown below.

As you can see from init_tsc_clocksource(), X86_FEATURE_TSC_RELIABLE acts
as a switch to launch the delayed calibration work. The delayed calibration
is skipped if X86_FEATURE_TSC_RELIABLE is set, else not.

In check_system_tsc_reliable(), X86_FEATURE_TSC_RELIABLE helps to set
tsc_clocksource_reliable which in turn enables TSC as a clocksource
watchdog, i.e. watching others instead of being watched by others.

So X86_FEATURE_TSC_RELIABLE really means two things:
1) Calibrated(or directly calculated) result is trustable so delayed
   calibration is skipped.
2) TSC is reliable so clocksource framework won't monitor it, instead
   TSC acts as a watchdog monitoring other clocksources.

X86_FEATURE_TSC_RELIABLE is set also only from two places:
arch/x86/platform/intel-mid/mrfld.c and arch/x86/platform/intel-mid/mrfld.c
which are for Intel Atom SoCs with MSR based TSC frequency calculation.

The patch I'm doing is to set this flag (X86_FEATURE_TSC_RELIABLE) for
another case: Intel processors/SoCs with CPUID based TSC frequency calculation.

arch/x86/kernel/tsc.c:
satic void __init check_system_tsc_reliable(void)
{
        ...... (lines ignored)

        if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
                /* This flag is used by init_tsc_clocksource(), see below. */
                tsc_clocksource_reliable = 1;
}

arch/x86/kernel/tsc.c:
static int __init init_tsc_clocksource(void)
{
        ...... (lines ignored)

        if (tsc_clocksource_reliable)
                clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;

        ...... (lines ignored)

        /*
         * Trust the results of the earlier calibration on systems
         * exporting a reliable TSC.
         */
        if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) {
                clocksource_register_khz(&clocksource_tsc, tsc_khz);
                return 0;
        }
        schedule_delayed_work(&tsc_irqwork, 0);
        return 0;
}

Thanks,
Bin