linux-kernel - Re: [Bug #15672] KVM bug, git bisected

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Date:	Wed, 21 Apr 2010 10:45:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, Rik van Riel <riel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [Bug #15672] KVM bug, git bisected

On Wed, 2010-04-21 at 08:20 +0200, Borislav Petkov wrote:
> From: "Rafael J. Wysocki" <rjw@...k.pl>
> Date: Wed, Apr 21, 2010 at 07:02:02AM +0200
> 
> > On Wednesday 21 April 2010, Rik van Riel wrote:
> > > On 04/20/2010 05:11 PM, Rik van Riel wrote:
> > > > On 04/19/2010 11:19 PM, Rafael J. Wysocki wrote:
> > > >> This message has been generated automatically as a part of a summary
> > > >> report
> > > >> of recent regressions.
> > > >>
> > > >> The following bug entry is on the current list of known regressions
> > > >> from 2.6.33. Please verify if it still should be listed and let the
> > > >> tracking team
> > > >> know (either way).
> > > >>
> > > >>
> > > >> Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=15672
> > > >> Subject : KVM bug, git bisected
> > > >> Submitter : Kent Overstreet<kent.overstreet@...il.com>
> > > >> Date : 2010-03-27 12:43 (24 days old)
> > > >> First-Bad-Commit:
> > > >> http://kernel.org/git/linus/5beb49305251e5669852ed541e8e2f2f7696c53e
> > > >> Message-ID :<4BADFD74.8060904@...il.com>
> > > >> References : http://marc.info/?l=linux-kernel&m=126969385121711&w=2
> > > >
> > > > Should be fixed by commit ea90002b0fa7bdee86ec22eba1d951f30bf043a6
> > > 
> > > Never mind me - this is a harmless (but loud) overflow
> > > of PREEMPT_BITS in the preempt count.
> > 
> > OK, what am I supposed to do with this entry, then?  Close?
> 
> FWIW, I hit that warning too when chasing the anon_vma regression. It
> seems on certain workloads (for me it was several kvm guests) we're
> close to max preemption depth.
> 
> Anyway, adding some more people to Cc.

Right, so my proposed solution to this is to make those locks
preemptible, but that's a large and unfinished patch-set.

As it is, its only a warning, nothing really serious should happen, but
the situation does suck.

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