Re: [Xenomai-core] [RFC][PATCH 4/4] shorten overrun loops of periodic timers

[Jan Kiszka <jan.kiszka@domain.hid>]

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>  > A simple patch, just like suggested by Gilles, to avoid looping over
>  > periodic xntimer handlers in case of overruns.
>  > 
>  > It saves the current TSC on loop entry and uses this value later when
>  > forwarding the timer. Is is the overhead of re-reading the TSC on all
>  > archs negligible and should we rather go that way?
> 
>  >(...)
>  > -				xntimerh_date(&timer->aplink) +=
>  > -				    nkpod->htimer.interval;
>  > +				while ((xntimerh_date(&timer->aplink) +=
>  > +				        nkpod->htimer.interval) < now);
> 
> I think you are patching the wrong addition, the one you are interested
> in is most probably the one at the bottom of xntimer_do_tick_aperiodic.
> 

Ouch, indeed. Guess I should start reading what I patch. Here comes a
second try.

I'm still in favour of saving the TSC instead of re-reading it.
Otherwise we would have to pave the code with #ifdefs for the case
xnarch_get_cpu_tsc() is slow for a specific setup. Not that nice, is it?

Jan

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---
 ksrc/nucleus/timer.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

Index: xenomai/ksrc/nucleus/timer.c
===================================================================
--- xenomai.orig/ksrc/nucleus/timer.c
+++ xenomai/ksrc/nucleus/timer.c
@@ -200,12 +200,13 @@ static void xntimer_do_tick_aperiodic(vo
 	xntimerq_t *timerq = &sched->timerqueue;
 	xntimerh_t *holder;
 	xntimer_t *timer;
+	xnticks_t now;
 
 	while ((holder = xntimerq_head(timerq)) != NULL) {
 		timer = aplink2timer(holder);
 
-		if (xntimerh_date(&timer->aplink) - nkschedlat >
-		    xnarch_get_cpu_tsc())
+		now = xnarch_get_cpu_tsc();
+		if (xntimerh_date(&timer->aplink) - nkschedlat > now)
 			/* No need to continue in aperiodic mode since timeout
 			   dates are ordered by increasing values. */
 			break;
@@ -247,7 +248,7 @@ static void xntimer_do_tick_aperiodic(vo
 			   translates into precious microsecs on low-end hw. */
 			__setbits(sched->status, XNHTICK);
 
-		xntimerh_date(&timer->aplink) += timer->interval;
+		while ((xntimerh_date(&timer->aplink) += timer->interval) < now);
 		xntimer_enqueue_aperiodic(timer);
 	}
 

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