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

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Message-ID: <20101029170040.GJ21125@aftab>
Date:	Fri, 29 Oct 2010 19:00:40 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	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 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.

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
Einsteinring 24, 85609 Dornach
General Managers: Alberto Bozzo, Andrew Bowd
Registration: Dornach, Gemeinde Aschheim, Landkreis Muenchen
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