linux-kernel - Re: [bisected] Clocksource tsc unstable git

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Message-ID: <20101029083013.GA1553@arch.trippelsdorf.de>
Date:	Fri, 29 Oct 2010 10:30:13 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Borislav Petkov <bp@...en8.de>, john stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...64.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	heiko.carstens@...ibm.com, a.p.zijlstra@...llo.nl, avi@...hat.com,
	mtosatti@...hat.com
Subject: Re: [bisected] Clocksource tsc unstable git

On Fri, Oct 29, 2010 at 10:18:31AM +0200, Borislav Petkov wrote:
> On Wed, Oct 27, 2010 at 08:26:08PM +0200, markus@...ppelsdorf.de wrote:
> > During my search for a reliable testcase I found out that kvm guests also
> > showed random hangs (that didn't happen in v2.6.36) here. These hangs last a
> > few seconds each, during which the mouse is not movable at all.
> > 
> > So I ran git-bisect with this testcase and the result of the bisection is:
> > 
> > 34f971f6f7988be4d014eec3e3526bee6d007ffa is the first bad commit
> > commit 34f971f6f7988be4d014eec3e3526bee6d007ffa
> > Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date:   Wed Sep 22 13:53:15 2010 +0200
> >
> 
> ...
> 
> >
> > Reverting the commit solves the kvm hang issue. (If this issue is
> > related to my original tsc problem is of course open for debate, but I
> > have a strong hunch it is.)
> 
> Well, what do you know, the temporary "slowdown" triggered on my setup
> too. After waiting a minute maybe, the machine recovered and guess what
> appeared in dmesg:
> 
> [44729.650859] Clocksource tsc unstable (delta = -8615263009 ns)
> [44729.656229] Switching to clocksource hpet
> 
> I've reverted the changeset above and will run without it to check
> whether it makes any difference.

After further testing I've found out that the tsc- and the kvm-issue are
unrelated. IOW I get these "slowdowns" even with 34f971f6f7988be4
reverted. 

I guess John was right when he wrote:
> From your dmesg, I'd guess the hpet is the watchdog clocksource, so I
> be on the lookout for hpet related changes.
>
> Maybe does reverting 995bd3bb5c78f3ff71339803c0b8337ed36d64fb hide the
> issue?  

Because with 995bd3bb5c78f reverted I never had a single "slowdown" here.
-- 
Markus
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