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

Steven Rostedt
2009-01-22T01:48:00Z

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