linux-kernel - Re: scheduling anomaly on uml (was: -rt doesn't compile for UML)

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Message-Id: <E1Ixhgf-0001CF-MC@dorka.pomaz.szeredi.hu>
Date:	Thu, 29 Nov 2007 12:36:09 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	mingo@...e.hu
CC:	jdike@...toit.com, user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: scheduling anomaly on uml (was: -rt doesn't compile for UML)

> > I can't say I'm understading these traces very well, but here's a 
> > snippet that looks a bit strange.  I'm running 'while true; do date; 
> > done' in parallel with the dd.
> > 
> > For some time it is doing 100% CPU as expected, then it goes into a 
> > second or so of mosty idle (afaics), and then returns to the normal 
> > pattern again.
> 
> try:
> 
>   echo 1 > /proc/sys/kernel/stackframe_tracing
> 
> to get symbolic stack backdumps for the wakeup points, and add 
> trace_special_sym() calls to generate extra stackdump entries at 
> arbitrary places. schedule() does not have it right now - it might make 
> sense to add it.

Umm, trace_special_sym() is ifdefed out, because UML doesn't have
save_stack_trace().

> also, enabling mcount:
> 
>  echo 1 > /proc/sys/kernel/mcount_enabled
> 
> will give you a _lot_ more verbose trace. Likewise:
> 
>  echo 1 > /proc/sys/kernel/syscall_tracing
> 
> (but for that you'd have to add the sys_call()/sys_ret() instrumentation 
> that x86 has in entry_32.S)

I'll have a look.

> but even this highlevel trace shows something weird:
> > events/0-4     0.... 16044512us+: schedule <<idle>-0> (20 -5)
> >   <idle>-0     0.... 16044564us!: schedule <events/0-4> (-5 20)
> >   <idle>-0     0.Nh. 16076072us+: __trace_start_sched_wakeup <date-7133> (120 -1)
> >   <idle>-0     0.Nh. 16076075us+: __trace_start_sched_wakeup <dd-6444> (120 -1)
> >   <idle>-0     0.Nh. 16076078us+: __trace_start_sched_wakeup <kswapd0-33> (115 -1)
> >       dd-6444  0.... 16076104us+: schedule <<idle>-0> (20 0)
> 
> how come UML idled for 30 msecs here, while the workload was supposed to 
> be CPU-bound? It's not IO bound anywhere, right? No SMP artifacts 
> either, right?

Yes.  The UML kernel is UP, and I don't think 'date' or 'bash' want to
do any disk I/O.

Could disk I/O be blocking the tty?  I think UML uses separate threads
for these, but I don't know the details.

Miklos
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