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Subject: Re: [PATCH][GIT PULL] tracing: Fix compile issue for
 trace_sched_wakeup.c
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Jason Baron <jbaron@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
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Date: Fri, 22 Oct 2010 16:23:02 +0200
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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?

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

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.. 

Hopefully that disables ftrace before it starts dumping otherwise the
dump is useless -- Steven?