Philippe Gerum wrote: > On Sun, 2006-07-30 at 21:33 +0200, Jan Kiszka wrote: >> Philippe Gerum wrote: >>> On Sat, 2006-07-29 at 16:20 +0200, Jan Kiszka wrote: >>>>>> :|func 6 xnintr_clock_handler (__ipipe_dispatch_wired) >>>>>> :|func 6 xnintr_irq_handler (xnintr_clock_handler) >>>>>> :|func 7 xnpod_announce_tick (xnintr_irq_handler) >>>>>> :|func 8+ xntimer_do_tick_aperiodic (xnpod_announce_tick) >>>>>> :|func 9 xnthread_periodic_handler (xntimer_do_tick_aperiodic) >>>>>> :|func 10 xnpod_resume_thread (xnthread_periodic_handler) >>>>>> :|[21559] 11+ xnpod_resume_thread (xnthread_periodic_handler) >>>>>> :|func 13+ xnthread_periodic_handler (xntimer_do_tick_aperiodic) >>>> ... >>>> >>>>>> :|func 363+ xnthread_periodic_handler (xntimer_do_tick_aperiodic) >>>> That are a lot of overruns. Haven't counted, but it should be one >>>> xnthread_periodic_handler per missed 100 us period (20000 / 100 = 200!). >>>> [BTW, I think we should handle even this failure scenario without >>>> looping. >>> We need to loop in the aperiodic handler in order to catch timers that >>> could have elapsed while processing the current tick. However, >> No, that was not what I meant. I know that we need the timer loop. But I >> was thinking of something like this for the tick handler's error path: >> >> if (unlikely((timer.date += timer.interval) < now)) >> timer.date = now + timer.interval - >> (now - timer.date) % timer.interval; >> >>> xnpod_wait_thread_period() - over which rt_task_wait_period() is based - >>> does not loop, but rather computes the actual count of overruns by >>> substracting the current time from the deadline. >> ...but by looping for some scenarios instead of dividing for all. Why >> optimising the slow path here? > > Division is utterly expensive and having a jitter that would not fit > in 32bit is seldom (and the definitive sign of serious brokenness anyway), > so this is actually the fast error path which gets optimized. > >>> Which brings us an interesting question: why does the aperiodic handler >>> loop frenetically in the first place? I would be pretty interested in >>> checking the TSC values returned by xnarch_get_cpu_tsc() while spinning >>> inside this deadly loop... >> You can already read those TSCs: each trace point got recorded with the >> current TSC value, fresh from the hardware. >> > > I'd like to explain why we don't we see any other routines than > xnthread_aperiodic_handler called from xntimer_do_tick_aperiodic in the > call frame? Even in case of massive jittery (e.g. > 300 us late) in one > shot, we should not spin in this code, due to the resync done in > xnpod_wait_thread_timeout - assuming we only have a single outstanding > timer (+ the host tick, but this should not be an issue).
xnpod_wait_thread_timeout? Do you mean xnpod_wait_thread_period? How should it help us as long as we are in the tick handler? > >> I rather think, also when looking at Julien's second trace, that we have >> some issue with X in user-space here, probably in combination with weird >> VIA hardware stalling IRQ delivery for a "few" microseconds. Let's see >> if the irqbench gives similar results. >> > > The problem is that I can reproduce X-related jittery (> 2 ms in a row) > on one of my test boxen when dragging windows over the screen, without > triggering the NMI watchdog set to 100 us (and guess what, the chipset > in question is from VIA). Does NMI management happen in the CPU or has the chipset any influence as well? If yes, I could imagine what VIA does here... Have you already checked what irqbench records? Jan
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