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

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Message-ID: <20101029172631.GA23546@arch.trippelsdorf.de>
Date:	Fri, 29 Oct 2010 19:26:31 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Borislav Petkov <bp@...64.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	john stultz <johnstul@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"avi@...hat.com" <avi@...hat.com>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>
Subject: Re: [bisected] Clocksource tsc unstable git

On Fri, Oct 29, 2010 at 07:00:40PM +0200, Borislav Petkov wrote:
> On Fri, Oct 29, 2010 at 08:14:12AM -0400, Thomas Gleixner wrote:
> > On Fri, 29 Oct 2010, Borislav Petkov wrote:
> > > On Fri, Oct 29, 2010 at 04:30:13AM -0400, Markus Trippelsdorf wrote:
> > > That's strange, when we tested this one everything seemed fine so I
> > > guess this is one of those bugs which appear later, just as if out of
> > > nowhere.
> > > 
> > > Thomas, could it be that SMI fires in between the HPET write and
> > > subsequent read:
> > > 
> > > 	hpet_writel(cnt, HPET_Tn_CMP(timer));
> > > 
> > > 	...
> > > 
> > > 	res = (s32)(cnt - hpet_readl(HPET_COUNTER));
> > > 
> > > causing the -ETIME and thus a wait for HPET wraparound? My machine at
> > > least does SMI-initiated C1E so it could very well be the problem.
> > 
> > Well, even if a SMI fires, then the counter will be ahead of cnt and
> > we get -ETIME. The upper layer of clockevents/timers will then
> > reprogram HPET. So that's not an issue.
> 
> Ah, I missed the upper layer, good.
> 
> > The only problem which might hit us is when our assumption of 8 HPET
> > cycles being enough to transfer the new match value into the real
> > match register is wrong.
> 
> So Markus, can you try with 995bd3bb5c78, but by increasing the value
> to, say 16 (I don't know what's a good value here, let's double the old
> one). Simply change the line
> 
> 	return res < 8 ? -ETIME : 0;
> 
> to
> 
> 	return res < 16 ? -ETIME : 0;
> 
> 
> in <arch/x86/kernel/hpet.c:hpet_next_event()>. I'll do that too on the
> machine here when I get around to it.

Yes, I already did this, although I'm running
return res < 12 ? -ETIME : 0; 
at the moment.

-- 
Markus
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