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

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Message-ID: <alpine.DEB.1.10.0901201251180.2681@gandalf.stny.rr.com>
Date:	Tue, 20 Jan 2009 12:53:53 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Kevin Shanahan <kmshanah@...b.org.au>, 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>,
	Kevin Shanahan <kmshanah@...xo.wumi.org.au>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)


On Tue, 20 Jan 2009, Ingo Molnar wrote:

> 
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Tue, 20 Jan 2009, Ingo Molnar wrote:
> > > Another test would be to build the scheduler latency tracer into your 
> > > kernel:
> > > 
> > >     CONFIG_SCHED_TRACER=y
> > > 
> > > And enable it via:
> > > 
> > >     echo wakeup > /debug/tracing/current_tracer
> > > 
> > > and you should be seeing the worst-case scheduling latency traces in 
> > > /debug/tracing/trace, and the largest observed latency will be in 
> > > /debug/tracing/tracing_max_latency [in microseconds].
> > 
> > Note, the wakeup latency only tests realtime threads, since other 
> > threads can have other issues for wakeup. I could change the wakeup 
> > tracer as wakeup_rt, and make a new "wakeup" that tests all threads, but 
> > it may be difficult to get something accurate.
> 
> hm, that's a significant regression then. The latency tracer used to 
> measure the highest-prio task in the system - be that RT or non-rt.

Well, it is a regression from what was in -rt yes. But not from what ever 
was in mainline.

But I needed to change this to detect the problem that we 
solved with push and pull of rt tasks. The wake up of a non-rt tasks 
always took longer than an -rt task, and by tracing all tasks, I never got 
the wake up latency of an rt task.

As I mentioned earlier, I can make a wakeup-rt to do the rt tracing, and 
make wakeup do all tasks.

-- Steve

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