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SubjectRe: Does smp_reschedule_interrupt really reschedule?

* Steven Rostedt <rostedt@goodmis.org> wrote:

> As the comment says, do nothing since all the work is automatically
> done at the return from interrupt. But is it? Doesn't the
> need_resched need to be set? Here's what I'm seeing with Ingo's
> kernel. I capture the time in sched.c when the
> smp_send_reschedule_allbutself is called, and also a capture of the
> time when the schedule actually takes place. I'm finding differences
> up to 2 tenths of a second. That's TENTHS! I added the following
> patch:

it's all a bit tricky. The short story is that i think both vanilla and
-RT kernels are fine.

Here is how smp_send_reschedule() is used:

CPU#0 CPU#1

set_tsk_need_resched(rq->curr);
...
smp_send_reschedule()
--- IPI --->
smp_reschedule_interrupt();
...
entry.S's need_resched check

_but_, this is intentionally racy: if CPU#1 happens to reschedule before
the IPI reaches CPU#1 (an IPI can take 10 usecs easily so the window is
not small), then need_resched might be cleared before the IPI hits. In
that case you wont get a reschedule after the IPI hits, because it was
done before!

so the correct thing to measure is what the -RT kernel's wakeup-latency
timing feature does: the time from setting need_resched, to the point
the task starts to run. The feature works on SMP too - and it doesnt
show any large latencies.

are you seeing actual process delays? If not then i think those large
latencies are just the result of the wrong assumptions in your
measurement code.

Ingo
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