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SubjectRe: Does smp_reschedule_interrupt really reschedule?
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On Fri, 2005-05-13 at 20:26 +0200, Ingo Molnar wrote:
> 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
>

OK, Forget about vanilla, I'm keeping to your kernel now ;-)

In finish_task_switch, where is need_resched set?


> _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.

No this wasn't from real world applications yet. So the bug may rightly
be with the placement of my timers. I was looking at the code and
didn't know how the reschedule takes place and started testing it. Let
me know where that need_resched is with that finish_task_switch code,
and I'll probably be happy with just that.

Thanks,

-- Steve

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