On 06/26/2015 05:20 PM, Thomas Gleixner wrote: > On Fri, 26 Jun 2015, Preeti U Murthy wrote: >> On 06/26/2015 01:17 PM, Thomas Gleixner wrote: >>> On Fri, 26 Jun 2015, Preeti U Murthy wrote: >>>> What about the case where GENERIC_CLOCKEVENTS_BROADCAST=y and >>>> TICK_ONESHOT=n (HZ_PERIODIC=y) ? Have you tested this ? >>>> >>>> This will hang the kernel at boot if you are using the hrtimer mode of >>>> broadcast. This is because the local timers of all cpus are shutdown >>>> when the cpuidle driver registers itself, on finding out that there are >>>> idle states where local tick devices stop. The broadcast tick device is >>>> then in charge of waking up the cpus at every period. In hrtimer mode of >>>> broadcast, there is no such real device and we hang. >>> >>> Hmm, no. tick-broadcast-hrtimer.o depends on TICK_ONESHOT=y. So this >>> is covered by the check for the broadcast device, which is NULL. >>> >>> But there is another variant: >>> >>> GENERIC_CLOCKEVENTS_BROADCAST=y TICK_ONESHOT=y and 'highres=off >>> nohz=off' on the kernel command line. >> >> Can this happen at all? It is during tick_init_highres() or >> tick_nohz_switch_to_nohz() that we switch to oneshot mode, not otherwise >> AFAICT. > > And how does that matter? If 'highres=off nohz=off' is on the kernel > command line none of the switchovers happens. So system stays in > periodic mode and the broadcast hrtimer thing is registered, right?
Yes we are good here. I overlooked the fact that we could disable high resolution/nohz just before boot. > >> I was actually talking of the following scenario. In periodic mode, >> where GENERIC_CLOCKEVENTS_BROADCAST=y, the arch can execute >> tick_setup_hrtimer_broadcast(), which will return nothing as you point >> out above. So there is no broadcast clockevent device. >> >> When the cpuidle driver registers with the cpuidle core however, >> cpuidle_setup_broadcast_timer() on every cpu is executed if it finds >> that there is an idle state where ticks stop. >> >> cpuidle_setup_broadcast_timer() >> tick_broadcast_enable() >> tick_broadcast_control(BROADCAST_ON) >> bc = tick_broadcast_device.evtdev which is NULL in this case >> >> TICK_BROADCAST_ON: >> checks for periodic mode of the broadcast device - succeeds >> although we haven't registered a broadcast device because >> value of TICKDEV_PERIODIC is 0, the default value of td.mode. >> >> clockevents_shutdown(dev) >> >> At this point all cpus stop. > > Right. That's a different one, if tick_broadcast_device.evtdev == NULL. > > We should not stop any cpu local timer in that case. Combined with the > patch I sent, we prevent the idle stuff from going into a state where > the cpu local timers stop. Right. We need a check above too. Regards Preeti U Murthy > > Thanks, > > tglx > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
