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

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Date:	Wed, 11 Jul 2012 09:05:53 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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()


>> 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?
> 

Both of those options seem like a lot of work for something that happens once
every 3-4 years, and may not happen ever again[1].  Based on that statement, if
we're going to modify code I would prefer that it be as lightweight as possible.
 So, in terms of the kernel, option 2 is likely the best way to go rather than
introducing new code that will be used once every 3-4 years.

I keep asking the question of why the mechanism of inserting a leap second isn't
moved into userspace ntpd (or some other appropriate daemon).  I suppose there
is a risk of ntpd being starved out on heavily loaded systems...

P.

[1] http://en.wikipedia.org/wiki/Leap_second#Proposal_to_abolish_leap_seconds
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