Re: x86: tsc: v2 make TSC calibration more immune to interrupts

[john stultz]

On Wed, 2011-04-20 at 15:39 -0700, Josh Triplett wrote:
> On Wed, Apr 20, 2011 at 11:22:19PM +0200, Kasper Pedersen wrote:
> > When a SMI or plain interrupt occurs during the delayed part
> > of TSC calibration, and the SMI/irq handler is good and fast
> > so that is does not exceed SMI_TRESHOLD, tsc_khz can be a bit
> > off (10-30ppm).
> > 
> > We should not depend on interrupts being longer than 50000
> > clocks, so, in the refined calibration, always do the 5
> > tries, and use the best sample we get.
> > 
> > This should work always for any four periodic or rate-limited
> > interrupt sources. If we get 5 interrupts with 500ns gaps in
> > a row, behaviour should be as without this patch.
> > 
> > It is safe to use the first value that passes SMI_TRESHOLD
> > for the initial calibration: As long as tsc_khz is above
> > 100MHz, SMI_TRESHOLD represents less than 1% of error.
> > 
> > The 8 additional samples costs us 28 microseconds in startup
> > time.
> > 
> > measurements:
> > On a 700MHz P3 I see t2-t1=~22000, and 31ppm error.
> > A Core2 is similar: http://n1.taur.dk/tscdeviat.png
> > (while mostly t2-t1=~1000, in about 1 of 3000 tests
> > I see t2-t1=~20000 for both machines.)
> > vmware ESX4 has t2-t1=~8000 and up.
> > 
> > v2: John Stulz suggested limiting best uncertainty to
> > where it is needed, saving ~170usec startup time.
> 
> Have you considered disabling interrupts while calibrating?  That would
> ensure that you only have to care about SMIs, not arbitrary interrupts.

This calibration is actually timer based (and runs for 1 second,
allowing the system to continue booting in the meantime), so disabling
irqs wouldn't work. You could just disable irqs during the tsc_getref,
but that still has the possibility to get hit by SMIs, which are the
real issue.

> Also, on more recent x86 systems you could look at MSR_SMI_COUNT (MSR
> 0x34) to detect if any SMIs have occurred during the sample period.
> rdmsr, start sample period, stop sample period, rdmsr, if delta of 0
> then no SMIs occurred.  Exists on Nehalem and newer, at least.

That's interesting... but probably still too machine specific to be
generally useful.

thanks
-john

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