linux-kernel - Re: [PATCH] tick/broadcast-hrtimer : Fix suspicious RCU usage in idle loop

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Date:	Tue, 17 Mar 2015 09:49:45 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] tick/broadcast-hrtimer : Fix suspicious RCU usage in
 idle loop

On 03/16/2015 08:26 PM, Peter Zijlstra wrote:
> On Thu, Mar 05, 2015 at 10:06:30AM +0530, Preeti U Murthy wrote:
>>
>> On 03/02/2015 08:23 PM, Peter Zijlstra wrote:
>>> On Thu, Feb 26, 2015 at 08:52:02AM +0530, Preeti U Murthy wrote:
>>>> The hrtimer mode of broadcast queues hrtimers in the idle entry
>>>> path so as to wakeup cpus in deep idle states. 
>>>
>>> Callgraph please...
>>
>> cpuidle_idle_call()
>> |____ clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, ....))
>>      |_____tick_broadcast_set_event()
>>            |____clockevents_program_event()
>>                 |____bc_set_next()
>>>
>>>> hrtimer_{start/cancel}
>>>> functions call into tracing which uses RCU. But it is not legal to call
>>>> into RCU in cpuidle because it is one of the quiescent states. Hence
>>>> protect this region with RCU_NONIDLE which informs RCU that the cpu
>>>> is momentarily non-idle.
>>>
>>> It it not clear to me that every user of bc_set_next() is from IDLE.
>>> From what I can tell it ends up being clockevents_program_event() and
>>> that is called quite a lot.
>>
>> bc_set_next() is called from at places:
>> 1. Idle entry : It is called when a cpu in its idle entry path finds the
>> need to reset the broadcast hrtimer.
>> 2. CPU offline operations : When the cpu on which the broadcast hrtimer
>> is being queued goes offline.
>>
>> So you see that almost all the time, it is called in idle entry path.
> 
> How about:
> 
> 	hrtimer_reprogram()
> 	  tick_program_event()
> 	    clockevents_program_event()
> 	      ->set_next_ktime()
> 
> That is called from !idle loads of times. I guess I'm not seeing what
> avoids &ce_broadcast_hrtimer from being the 'normal' clock event.

Ok I see your point now. Sorry about having misinterpreted it
previously. ce_broadcast_hrtimer is not the per-cpu clock device. It is
not a real clock device. It is a pseudo clock device, which is called
only from the guts of the broadcast framework.
When it is programmed, it queues a hrtimer and programs the per-cpu
clock device. in the fashion mentioned above.

No hrtimer programming/starting/canceling will get routed through
bc_set_next(). The broadcast framework makes use of a separate broadcast
clock device, which is never the per-cpu clock device to wake cpus from
idle. This device is programmed explicitly when required and not
indirectly via timer queueing. *Only* when this broadcast clock device
needs to reprogrammed, bc_set_next() gets called on those archs which
*do not have a real broadcast clock device*. And the whole thing kicks
in when cpus go idle only, not just for PowerPC but for ARM as well.

Regards
Preeti U Murthy
> 
> Sure; it might be that for power you only end up with that broadcast
> crap enabled on idle/hotplug, but is this always so?
> 

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