linux-kernel - Re: constant_tsc and TSC unstable

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Message-ID: <474F0D47.7020006@zytor.com>
Date:	Thu, 29 Nov 2007 11:04:39 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Paul Rolland (ポール・ロラン)" 
	<rol@...be.net>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: constant_tsc and TSC unstable

Paul Rolland (ポール・ロラン) wrote:
> Hello,
> 
> I've a machine with a Core2Duo CPU. /proc/cpuinfo reports the flag
> constant_tsc, but at boot time, I have the log :
> 
> ...
> Total of 2 processors activated (6919.15 BogoMIPS).
> ENABLING IO-APIC IRQs
> ..TIMER: vector=0x31 apic1=0 pin1=2 apic2=-1 pin2=-1
> checking TSC synchronization [CPU#0 -> CPU#1]:
> Measured 3978592228 cycles TSC warp between CPUs, turning off TSC clock.
> Marking TSC unstable due to: check_tsc_sync_source failed.
> Brought up 2 CPUs
> ...
> 
> This machine is running 2.6.23.1-21.fc7. I know I should report to Fedora,
> but I was wondering if this is a bug or a feature ;)
> 

constant_tsc means the TSC doesn't change with CPU core frequency.

The problem you're having is that the TSCs of your two cores are 
completely different, over a second apart.  This is a bug, unrelated to 
constant_tsc.

	-hpa



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