linux-kernel - Re: [PATCH] clockevents: return error from tick_broadcast_oneshot_control if !GENERIC_CLOCKEVENTS

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Date:	Fri, 26 Jun 2015 16:55:48 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Sudeep Holla <sudeep.holla@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Suzuki Poulose <Suzuki.Poulose@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH] clockevents: return error from tick_broadcast_oneshot_control
 if !GENERIC_CLOCKEVENTS_BROADCAST

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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