Messages in this thread | | | Date | Fri, 26 Jun 2015 13:50:40 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] clockevents: return error from tick_broadcast_oneshot_control if !GENERIC_CLOCKEVENTS_BROADCAST |
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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?
> 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.
Thanks,
tglx
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