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

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Message-ID: <alpine.DEB.1.10.0901211011170.14817@gandalf.stny.rr.com>
Date:	Wed, 21 Jan 2009 10:13:59 -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 should have replied to this email :-)

Yeah, I'm working on making wakeup latency tracer work with non rt tasks.

The "wakeup" tracer will now trace all tasks where as a new "wakeup_rt" 
tracer will only trace rt tasks. I did it for rt tasks only because it 
only records the highest latency wake ups and the non rt tasks were always 
bigger than the rt tasks which made what I was tracing useless (the rt 
scheduling).

But by not having an option for all tasks, it makes the wakeup tracer 
useless for everyone else ;-)

-- Steve

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