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

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Date:	Wed, 21 Jan 2009 20:48:00 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Avi Kivity <avi@...hat.com>
cc:	Kevin Shanahan <kmshanah@...b.org.au>, Ingo Molnar <mingo@...e.hu>,
	"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>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	bugme-daemon@...zilla.kernel.org
Subject: Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)




On Wed, 21 Jan 2009, Avi Kivity wrote:

> Kevin Shanahan wrote:
> > > > --- hermes-old.wumi.org.au ping statistics ---
> > > > 900 packets transmitted, 900 received, 0% packet loss, time 899326ms
> > > > rtt min/avg/max/mdev = 0.093/0.157/3.611/0.117 ms
> > > > 
> > > > So, a _huge_ difference. But what does it mean?
> > > >       
> > > It means, a scheduling problem.  Can you run the latency tracer (which
> > > only works with realtime priority), so we can tell if it is (a) kvm
> > > failing to wake up the vcpu properly or (b) the scheduler delaying the
> > > vcpu from running.
> > >     
> > 
> > Sorry, but are you sure that's going to be useful?
> > 
> > If it only works on realtime threads and I'm not seeing the problem when
> > running kvm with realtime priority, is this going to tell you what you
> > want to know?
> > 
> > Not trying to be difficult, but that just didn't make sense to me.
> >   
> 
> You're right, wasn't thinking properly.
> 
> This is a tough one.  I'll see if I can think of something.  Ingo, any ideas?

I fixed up the wakeup latency tracer to work with all tasks (as well as 
other fixes). You can checkout the following:

git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git

  branch: tip/devel

compile with CONFIG_FUNCTION_TRACER and CONFIG_SCHED_TRACER and just

echo 0 > /debug/tracing/tracing_enabled
echo wakeup > /debug/tracing/current_tracer

echo 1 > /debug/tracing/tracing_enabled
run your test
echo 0 > /debug/tracing/tracing_enabled

and then look at /debug/tracing/latency_trace

-- Steve

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