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From: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Subject: Re: upstream regression (IO-APIC?)
Date: Sun, 2 Nov 2008 21:24:24 +0100
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Cc: linux-kernel@vger.kernel.org, Alok N Kataria <akataria@vmware.com>,
       Robert Hancock <hancockr@shaw.ca>,
       Arjan van de Ven <arjan@infradead.org>, Pavel Machek <pavel@suse.cz>
References: <fa.mewpISr5S4wcdUNw1DVfOnQfl8s@ifi.uio.no> <4909011F.1050102@shaw.ca> <200811021537.24771.bzolnier@gmail.com>
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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)...

[ The commit works for HZ == 250 because it does tsc_khz * 1000 / HZ,
  tsc_khz * 4 => lpj assumption holds true and there is no frequency
  scaling at boot. ]

The quick fix would be to replace 1000 / HZ by the magic number "4"
but the major question is whether can we reliably depend on the tsc_khz
for lpj?

Thanks,
Bart
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