linux-kernel - Re: [RFC][PATCH 4/4] time: Do leapsecond adjustment in gettime fastpaths

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Date:	Fri, 05 Jun 2015 11:10:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Jan Kara <jack@...e.cz>, Jiri Bohac <jbohac@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Shuah Khan <shuahkh@....samsung.com>
Subject: Re: [RFC][PATCH 4/4] time: Do leapsecond adjustment in gettime
 fastpaths

On Fri, 2015-06-05 at 11:04 +0200, Richard Cochran wrote:
> On Fri, Jun 05, 2015 at 09:29:13AM +0200, Peter Zijlstra wrote:
> > That leaves the question; for who is this exact second edge important?
> 
> Distributed applications using the UTC time scale.
> 
> Many control applications are done with a 1 millisecond period.
> Having the time wrong by a second for 10 or 100 loops is bad news.

Firstly I would strongly suggest such applications not use UTC because
of this, I think TAI was invented for just this reason.

Secondly, how would John's patches help with this? Usespace loops
reading time would be using the VDSO and would still not get the right
time, and timers would be subject to the same IRQ latency that a hrtimer
based leap second insert would, and would still very much not be in-sync
across the cluster.
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