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 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
>