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

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Date:	Tue, 20 Jan 2009 15:25:15 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kevin Shanahan <kmshanah@...b.org.au>
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)


* Kevin Shanahan <kmshanah@...b.org.au> wrote:

> 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?

Correct.

> 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.

Yeah, that would be lovely. You dont even have to trim it much - just give 
us a timestamp to look at for the delay incident. You might also want to 
start the kvm session while the script is already running - that way we'll 
get fresh statistics and see the whole thing.

	Ingo
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