Re: [PATCH] clockevents: return error from tick_broadcast_oneshot_control if !GENERIC_CLOCKEVENTS_BROADCAST

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

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