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Date:	Mon, 03 Nov 2008 10:04:04 -0800
From:	Alok Kataria <akataria@...are.com>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Robert Hancock <hancockr@...w.ca>,
	Arjan van de Ven <arjan@...radead.org>,
	Pavel Machek <pavel@...e.cz>
Subject: Re: upstream regression (IO-APIC?)

On Mon, 2008-11-03 at 09:28 -0800, Bartlomiej Zolnierkiewicz wrote:
> On Sunday 02 November 2008, Bartlomiej Zolnierkiewicz wrote:
> > On Sunday 02 November 2008, Bartlomiej Zolnierkiewicz wrote:
> > > On Thursday 30 October 2008, Robert Hancock wrote:
> > > > Bartlomiej Zolnierkiewicz wrote:
> > > > > The current Linus tree as of commit e946217e4fdaa67681bbabfa8e6b18641921f750
> > > > > is broken for me.  I get either the following panic (see log from qemu below)
> > > > > or lost IRQs on ATA init...  Is this a known issue?
> > > > > 
> > > > > PS The tree that I used before and was supposedly good (sorry, I'm too tired
> > > > > to verify it now) had commit 57f8f7b60db6f1ed2c6918ab9230c4623a9dbe37 at head.
> > > 
> > > Unfortunately 57f8f7b60db6f1ed2c6918ab9230c4623a9dbe37 (v2.6.28-rc1)
> > > is also bad.  Bisecting it further was a real pain (i.e. I hit broken
> > > build with x86 irqbalance changes, broken build with netfilter nat
> > > changes and jbd journal problem).  In the end it turned out that 2.6.27
> > > is bad too!  However with 2.6.27 the panic occurs only once per several
> > > attempts and if there is no panic kernel boots normally (no lost IRQs).
> > > 
> > > [...]
> > > 
> > > I finally managed to narrow it down to change making x86 use tsc_khz
> > > for loops_per_jiffy -- commit 3da757daf86e498872855f0b5e101f763ba79499
> > > ("x86: use cpu_khz for loops_per_jiffy calculation").  This approach
> > > seems too simplistic (as I see now Arjan & Pavel expressed concerns
> > > about it back when the patch was posted initially [1][2]).  Also it
> > > would probably be preferred to re-use existing preset_lpj variable
> > > (just like KVM does it for similar purpose [3]) instead of adding a
> > > lpj_tsc one and increasing complexity.
> > 
> > It turned out that I can boot a kernel with different config with
> > HZ == 250 just fine and switching to HZ == 1000 makes it fail.
> > 
> > 
> > Looking into it some more:
> > 
> > HZ == 250 kernel (good):
> > 
> > Calibrating delay loop (skipped), value calculated using timer frequency.. 2986.79 BogoMIPS (lpj=5973580)
> > 
> > HZ == 1000 kernel (bad):
> > 
> > Calibrating delay loop (skipped), using tsc calculated value.. 2990.35 BogoMIPS (lpj=1495176)
> > 
> > HZ == 1000 kernel with hackyfix (good):
> > 
> > Calibrating delay using timer specific routine.. 3016.68 BogoMIPS (lpj=6033376)
> > 
> > 
> > Argggh... lpj is used for udelay() & friends so this bug is quite
> > dangerous (since udelay() & friends are used for hardware delays)...
> 
> It may be not as severe as I initially thought,
> (obviously) the real hardware works fine:
> 
> calibrate_delay_direct(): lpj=1495884
> Calibrating delay loop (skipped), value calculated using timer frequency.. 2990.36 BogoMIPS (lpj=1495183)
> 
> So the issue only affects qemu ATM.
Oh so its on a emulator, something wrong in the timer emulation logic in
qemu ?


> 
> Thanks,
> Bart

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