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.
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.
Regards
Preeti U Murthy
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