Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibrationvariance

[john stultz]

On Fri, 2009-05-08 at 14:15 +0200, Ingo Molnar wrote:
> * john stultz <johnstul@xxxxxxxxxx> wrote:
> 
> > All, 
> > 
> > Despite recent tweaking, TSC calibration variance is still biting 
> > users who care about keeping close sync with NTP servers over 
> > reboots.
> > 
> > Here's a recent example: 
> > http://lkml.indiana.edu/hypermail/linux/kernel/0905.0/02061.html
> > 
> > The problem is, each reboot, we have to calibrate the TSC, and any 
> > error, regardless of how small, in the calibrated freq has to be 
> > corrected for by NTP. Assuming the error is within 500ppm NTP can 
> > correct this, but until it finds the proper correction value for 
> > the new TSC freq, users may see time offsets from the NTP server.
> > 
> > In my experience, its fairly easy to see 100khz variance from 
> > reboot to reboot with 2.6.30-rc.
> > 
> > While I think its worth trying to improve the calibration further, 
> > there will likely be a trade-off between very accurate calibration 
> > and fast boot times.
> > 
> > To mitigate this, I wanted to provide a tsc_khz= boot option. This 
> > would allow users to set the tsc_khz value at boot-up, assuming 
> > they are within 1Mhz of the calibrated value (to protect against 
> > bad values). Once the tsc_khz value is set in grub, the box will 
> > always boot with the same value, so the NTP drift value prior to 
> > reboot will still be correct after rebooting.
> > 
> > Thanks to George Spelvin for the idea:
> > 	http://lkml.indiana.edu/hypermail/linux/kernel/0905.0/02807.html
> > 
> > Thoughts or feedback?
> > 
> > Signed-off-by: John Stultz <johnstul@xxxxxxxxxx>
> 
> Wouldnt it be a lot more flexible to have a sysctl for this, which 
> would be set before ntpd is started? (or which would be set by ntpd)

The difficulty is that once we've initialized the TSC clocksource and
its in use, its difficult to just override the existing freq. This is
partially why we disqualify the TSC if it changes freq from power
management.

> The mechanism and semantics would be similar: we would _not_ expose 
> cpu_khz directly, we'd have a boot_cpu_khz value saved for sure, and 
> we'd allow the sysctl to set the cpu_khz to within 1MHz of cpu_khz - 
> and we'd re-scale the timer irq and other calibrated values 
> accordingly.
> 
> Alternatively, a much simpler method: why doesnt ntpd save its own 
> notion of cpu_khz once it has reached stability, and reads cpu_khz 
> (from /proc/cpuinfo) during bootup and re-scales its initial offset 
> and phase shift accordingly, compensating for that noise? (if it's 
> within 1MHz)

Yea, I'm with George on this. It would be a very linux-specific cludge
(really even further, linux-x86 specific). Further it would require NTP
to figure out which clocksource is being used (as ACPI PM and HPET don't
have this calibration error) before applying. 

Seems poor to push the problem to NTP when the in-kernel calibration
code is really the cause of the issue.

thanks
-john


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