linux-kernel - System clock frequency offset changes drastically across reboots

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Date:	Mon, 03 Sep 2007 12:21:37 +0200
From:	John Sigler <linux.kernel@...e.fr>
To:	linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>
CC:	johnstul@...ibm.com, tglx@...utronix.de
Subject: System clock frequency offset changes drastically across reboots

[ Re-sending... Please feel free to comment, even if you don't have
"The Solution". I'd just like to get some feedback. ]

Hello everyone,

I'm using 2.6.20.7-rt8 on a P3.

I've noticed that the frequency offset of my system clock (computed
either by ntpd, or by hand) changes drastically across reboots.

By drastically, I mean +/- 60 ppm every time I reboot.

Apparently this is caused by some imprecision in the frequency
estimation done in calculate_cpu_khz() (arch/i386/kernel/tsc.c)

My brutally crude work-around was to skip the computation altogether and
hard-code the "correct" value for cpu_khz.

http://linux.kernel.free.fr/cpu_khz.adlink

Has this issue been investigated more thoroughly by other users?

Regards.

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