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SubjectRe: [PATCH] x86/tsc: Don't use cpuid 0x16 leaf to determine cpu speed.
Thank you for your reply.

I experience 2% tsc clock-drift (671s ahead of my local NTP server after 9.5h)
on my machine with Supermicro's Hyperspeed turned on. There's no clock
drift when I turn Hyperspeed off.

As far as I know Hyperspeed increases base clock frequency[1].

That's what CPUID says about my overclocked Intel Xeon Gold 6146:

Time Stamp Counter/Core Crystal Clock Information (0x15):
TSC/clock ratio = 256/2
nominal core crystal clock = 0 Hz
Processor Frequency Information (0x16):
Core Base Frequency (MHz) = 0xc80 (3200)
Core Maximum Frequency (MHz) = 0x1068 (4200)
Bus (Reference) Frequency (MHz) = 0x64 (100)

tsc_refine_calibration_work never corrects the early calibration
because it calculates a tsc frequency beyond 1% tolerance.

I've bumped the tsc_refine_calibration_work's tolerance to 3% and made it work:

[ 8.571471] tsc: Refined TSC clocksource calibration: 3264.012 MHz
No hyperspeed:
[ 8.506009] tsc: Refined TSC clocksource calibration: 3200.013 MHz

Increasing the tolerance to 3% would work in my case but apparently some
servers can increase the base-clock frequency to 6%. [2]
At this point in order to completely eliminate this bug we would need to
significantly increase the tolerance which might introduce other bugs.


Kind regards,
Krzysztof Piecuch

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