linux-kernel - Re: clocksource change of behavior in 2.6.22 compared to 2.6.20 causes massive system clock slowdown

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Message-Id: <20070710002935.f082b7c3.akpm@linux-foundation.org>
Date:	Tue, 10 Jul 2007 00:29:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Alessandro Suardi" <alessandro.suardi@...il.com>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>
Subject: Re: clocksource change of behavior in 2.6.22 compared to 2.6.20
 causes massive system clock slowdown

On Mon, 9 Jul 2007 16:27:59 +0200 "Alessandro Suardi" <alessandro.suardi@...il.com> wrote:

> My oldish AMD K7-800's clock began falling behind after
>  rebooting from 2.6.20 (and 109 days uptime with a spotless
>  clock) into 2.6.22; time lost is about four minutes each hour.
> 
> Turns out that 2.6.22 marks my TSC as unstable and starts
>  using PIT instead. Rebooting 2.6.22 with clocksource=tsc
>  gets the original stable system time back.
> 
> Not sure whether this is supposed to be the default way of
>  fixing the issue on older machines, though; if anyone sees
>  a problem with this, I'm up for providing more information.
> 
> [root@...key ~]# cd /sys/devices/system/clocksource/clocksource0/
> [root@...key clocksource0]# cat available_clocksource
> pit jiffies tsc
> [root@...key clocksource0]# cat current_clocksource
> tsc
> [root@...key clocksource0]# uname -a
> Linux donkey 2.6.22 #1 PREEMPT Mon Jul 9 13:26:38 CEST 2007 i686
> athlon i386 GNU/Linux
> [root@...key clocksource0]# cat /proc/cpuinfo
> processor       : 0
> vendor_id       : AuthenticAMD
> cpu family      : 6
> model           : 4
> model name      : AMD Athlon(tm) Processor
> stepping        : 2
> cpu MHz         : 800.068
> cache size      : 256 KB
> fdiv_bug        : no
> hlt_bug         : no
> f00f_bug        : no
> coma_bug        : no
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 1
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca
> cmov pat pse36 mmx fxsr syscall mmxext 3dnowext 3dnow
> bogomips        : 1603.17
> clflush size    : 32
> 

(cc's added)
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