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

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Date:	Mon, 16 Mar 2015 15:56:10 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tick/broadcast-hrtimer : Fix suspicious RCU usage in
 idle loop

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.

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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