linux-kernel - Possible error in tsc frequency calibration

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Date:	Tue, 13 May 2008 16:05:21 -0700
From:	Alok Kataria <akataria@...are.com>
To:	tglx@...utronix.de, johnstul@...ibm.com, mingo@...hat.com,
	andi@...stfloor.org, linux-kernel@...r.kernel.org
Cc:	Dan Hecht <dhecht@...are.com>, Tim Mann <mann@...are.com>,
	Zachary Amsden <zach@...are.com>
Subject: Possible error in tsc frequency calibration

I was looking at the tsc calibration code (tsc_calibrate) in the recent
kernels (2.6.26-rc1). With the newer code of calibrating against the
pm_timer and with the added checks for SMI's the tsc_calibration code
looks pretty robust, at least for x86_64 systems.

However, from the code, (tsc_calibrate) SMI_THRESHOLD is 50000, so in
the worst case, values returned by tsc_read_refs, i.e. tsc1 & tsc2,
could be off by 50000 ticks. On a 2Ghz processor this would mean an
error of 25us. The tsc frequency is calibrated over a period of 50ms;
hence the worst case error can be around 500ppm. 

In addition to that, the pmtimer hardware itself has some drift,
typically in the range of 20 to 100ppm.  If this drift happens to be in
the same direction as the error in measuring the tsc against the
pmtimer, we could have a total error of more than 500ppm in the tsc
frequency calibration code.  The maximum drift that NTP is willing to
correct is 500ppm.  So we can have cases when we may end up
mis-calibrating tsc frequency beyond the NTP threshold.

One thing that we can do to reduce this possible error is to
double the calibration period from the current 50msec to 100msec.
This halves the maximum error to 250ppm. 

Comments?

Now looking at the 32bit code, the frequency calibration code looks
really broken in the sense that we are not checking for SMI's at all.  
The maximum error that can go undetected on these systems is very high.
The best way to solve this would be to unify the calibration code for 32
bit and 64bit. Is anybody already working on this?

Thanks,
Alok

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