Re: Does smp_reschedule_interrupt really reschedule?

[Steven Rostedt <rostedt@goodmis.org>]

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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