> > 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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