linux-kernel - Re: [RFC 0/2] ABI for clock_gettime

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Date:	Wed, 14 Dec 2011 19:21:21 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org, Kumar Sundararajan <kumar@...com>,
	Arun Sharma <asharma@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC 0/2] ABI for clock_gettime_ns

On Wed, Dec 14, 2011 at 08:23:52AM -0800, john stultz wrote:
> On Wed, 2011-12-14 at 08:20 +0100, Richard Cochran wrote:
> > Michel Hack wrote an article last year detailing how Linux botches the
> > leap second and suggested a more robust way to handle it.
> 
> Hmm. Do you have a link to the article? 

I don't think it is online. Do you have the magic IEEE access?

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5609776

> I like the idea of having TAI as a kernel clockid. The hard part is
> getting systems to initialize it properly at boot. 
> 
> Also part of the issue with leapseconds is that time functions are such
> a hot path, we can't really add extra conditionals checking for leap
> seconds. Instead the leapsecond occurs on the first tick of the
> leapsecond.

The idea would only involve one conditional and one addition:

- System clock represents TAI
- Table of {threshold; offset} values, read mostly, rarely updated
- Table has index pointing to next event

Get time becomes:

1. read system time
2. test threshold
3. apply correction

> More interestingly to me is Google's recent use of slewed leapseconds.
> However, how that would work on a public network is a bit more fuzzy.
> And being able to support both TAI and slewed leapseconds would require
> quite a bit more logic.

Do you mean smoothing the jump over the entire day (or other
interval)? This is also discussed in Hack's paper.

Thanks,
Richard
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