linux-kernel - Re: constant_tsc and TSC unstable

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Message-ID: <474F30D5.8070404@trash.net>
Date:	Thu, 29 Nov 2007 22:36:21 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	"Paul Rolland (ポール・ロラン)" 
	<rol@...be.net>, Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: constant_tsc and TSC unstable

H. Peter Anvin wrote:
> 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.

On my core2duo I noticed that the TSC of the second core doesn't
start counting until the CPU is activated. Since they're not
synchronized manually anymore this makes the TSC check fail.


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