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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Date:	Wed, 11 Jul 2007 14:06:19 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Alessandro Suardi <alessandro.suardi@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: clocksource change of behavior in 2.6.22 compared to 2.6.20
	causes massive system clock slowdown

On Wed, 2007-07-11 at 01:31 +0200, Alessandro Suardi wrote:
> On 7/10/07, john stultz <johnstul@...ibm.com> wrote:
> > On Tue, 2007-07-10 at 00:29 -0700, Andrew Morton wrote:
> > > 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.
> >
> > Alessandro,
> > Can you send me dmesg output for 2.6.20 and 2.6.22 (without
> > clocksource=tsc)?
> 
> Actually, I lied a little bit - it was 2.6.22 with clock=tsc (which
>  warns on boot about clock= being deprecated in favor of
>  clocksource= ). I assume behavior is identical for now.
> 
> Please find attached the dmesg ring (incomplete, as the
>  kernel ring size I have seems too small to hold the full
>  buffer, but it seems to have all the interesting stuff) of
>  2.6.20, 2.6.22, 2.6.22 with clock=tsc.
> 
> If you need more info, just ask. I'll be out of the country
>  from July 12 to the morning of July 16, and again from
>  July 17 to July 20, so if you'd rather get at this later on,
>  it's okay for me ;)

You're dmesg output got chopped at the top. Please increase the kernel
log buffer size.

Sounds like your PIT frequency is out of whack, and I'm guessing the
generic clocksource watchdog blames the TSC and disqualifies it.

Few things to check:
1) Make sure you're running the latest BIOS.
2) See if booting w/ noapic changes anything


-john

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