linux-kernel - Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

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Message-Id: <1232461380.4895.33.camel@kulgan.wumi.org.au>
Date:	Wed, 21 Jan 2009 00:53:00 +1030
From:	Kevin Shanahan <kmshanah@...b.org.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Avi Kivity <avi@...hat.com>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

On Tue, 2009-01-20 at 13:56 +0100, Ingo Molnar wrote:
> * Kevin Shanahan <kmshanah@...b.org.au> wrote:
> > > This suggests some sort of KVM-specific problem. Scheduler latencies 
> > > in the seconds that occur under normal load situations are noticed and 
> > > reported quickly - and there are no such open regressions currently.
> > 
> > It at least suggests a problem with interaction between the scheduler 
> > and kvm, otherwise reverting that scheduler patch wouldn't have made the 
> > regression go away.
> 
> the scheduler affects almost everything, so almost by definition a 
> scheduler change can tickle a race or other timing bug in just about any 
> code - and reverting that change in the scheduler can make the bug go 
> away. But yes, it could also be a genuine scheduler bug - that is always a 
> possibility.

Okay, I understand.

> Could you please run a cfs-debug-info.sh session on a CONFIG_SCHED_DEBUG=y 
> and CONFIG_SCHEDSTATS=y kernel, while you are experiencing those 
> latencies:
> 
>   http://people.redhat.com/mingo/cfs-scheduler/tools/cfs-debug-info.sh
> 
> and post that (relatively large) somewhere, or send it as a reply after 
> bzip2 -9 compressing it? It will include a lot of information about the 
> delays your tasks are experiencing.

Running it while the problem is occuring will be tricky, as it only
lasts for a few seconds at a time. Is it going to be useful at all to
just see those statistics if the system is running normally?

I might need to modify the script a little. Am I right that everything
above "gathering statistics..." is pretty much static information?

I could run top, vmstat and cat /proc/sched_debug in a loop until the
problem occurs and then trim it. Something like:

while true; do
  date                                >> $FILE
  echo "-- top: --"                   >> $FILE
  top -H -c -b -d 1 -n 0.5            >> $FILE 2>/dev/null
  echo "-- vmstat: --"                >> $FILE
  vmstat                              >> $FILE 2>/dev/null
  echo "-- sched_debug #$i: --"       >> $FILE
  cat /proc/sched_debug               >> $FILE 2>/dev/null
done

That should take a snapshot every half second or so.

Regards,
Kevin.

P.S. Please keep kmshanah@...xo.wumi.org.au out of the CC list (it won't
     route properly anyway). I don't know how it got added - the only
     place it would have appeared was in the "revert" commit message
     when I was testing 2.6.28 with the commit I bisected down to
     removed.


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