linux-kernel - Re: [PATCH][GIT PULL] tracing: Fix compile issue for trace_sched

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Message-ID: <20101022143656.GB6498@redhat.com>
Date:	Fri, 22 Oct 2010 10:36:57 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>, tj@...nel.org
Subject: Re: [PATCH][GIT PULL] tracing: Fix compile issue for
	trace_sched_wakeup.c

On Fri, Oct 22, 2010 at 04:23:02PM +0200, Peter Zijlstra wrote:
> On Fri, 2010-10-22 at 10:13 -0400, Jason Baron wrote:
> > On Fri, Oct 22, 2010 at 10:14:31AM +0200, Peter Zijlstra wrote:
> > > On Thu, 2010-10-21 at 21:44 -0400, Jason Baron wrote:
> > > 
> > > > I finally found that we actually continue to run after the above
> > > > apparent 'hang'. That is, we continue to make progress updating the jump
> > > > labels. And doing a dump of all the system tasks at the time of the hang
> > > > showed the processes in various places besides the stop machine threads.
> > > > Thus, I thought that perhaps, for some reason the stop machine threads
> > > > weren't being scheduled.
> > > > 
> > > > Thus, I tried commenting out the special scheduling that is set up for
> > > > stop machine threads, and that fixed the hang. I haven't yet looked into
> > > > what might be going wrong with that scheduling...but maybe somebody else
> > > > knows...
> > > 
> > > Hrmm, so are you saying rq->stop was runnable but not running?
> > 
> > yes, that's what it seems like.
> > 
> > > 
> > > That would imply broken wakeup-preemption, does something like the below
> > > cure that?
> > > 
> > > 
> > 
> > no still seeing the same hang with the below patch...also, as a data point I
> > backed out the patch that adds the stop_sched_class and that resolved the hang
> > as well - just as a data point.
> 
> Weird... and there's no hotplugging happening, right?
> 

don't think so.

> Can this be reproduced in qemu? I don't have a i386-smp machine around
> to use.
> 

its actually an amd 64-bit system with a 32-bit install of f13...

> Could you trace the thing with all sched (except the sched_stat)
> tracepoints enabled? I think there's a sysrq key to dump the trace once
> the machine's stuck.. 
> 

the current test is triguring this on boot testing enabling/disabling the
tracepoints...so it would be interesting to disable the boot test, and see if I
can re-produce it at run-time...which will make getting the trace much easier :)

-Jason
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