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

* Miklos Szeredi <> wrote:

> 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.


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.

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)

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?

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