linux-kernel - Re: [PATCH 1/6] hrtimer: Provide clock_was_set

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Date:	Wed, 11 Jul 2012 14:45:39 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Prarit Bhargava <prarit@...hat.com>
cc:	John Stultz <johnstul@...ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, stable@...r.kernel.org
Subject: Re: [PATCH 1/6] hrtimer: Provide clock_was_set_delayed()

On Wed, 11 Jul 2012, Prarit Bhargava wrote:
> On 07/10/2012 06:43 PM, John Stultz wrote:
> > clock_was_set() cannot be called from hard interrupt context because
> > it calls on_each_cpu(). For fixing the widely reported leap seconds
> > issue it's necessary to call it from the timer interrupt context.
> > 
> > Provide a new function which denotes it in the hrtimer cpu base
> > structure of the cpu on which it is called and raising the timer
> > softirq.
> > 
> > We then execute the clock_was_set() notificiation in the timer softirq
> > context in hrtimer_run_pending().
> 
> I wish there was a nicer way to do this ... but looking at the code I can't
> figure out a better way.  (no offense John, it's just the way the code is ;) )

Yeah, I had the same discussion with Peter earlier today. There is
only a rather limited set of options.

1) Retrigger the timer interrupt vectors on all CPUs - except the one
   we are running on, but we have no interface for that at the moment

2) Do the nasty __smp_call_function_single() hack

   Preallocate call_single_data for all cpus and do a
   __smp_call_function_single() on all online cpus.

   This can be called from hard interrupt context or irq disabled
   regions.

   That would allow to get rid of the whole delay magic all
   together.

Thoughts?

	tglx

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