linux-kernel - Re: Leap Second, nohz, and ABSOLUTE timers problem

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Message-ID: <CALAqxLUdHGLVxic89CVtX-NcG3kX1LXyDzgDjsyLMfjnQk5SAQ@mail.gmail.com>
Date:	Wed, 27 May 2015 16:09:36 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Frank Hirtz <fhirtz@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Leap Second, nohz, and ABSOLUTE timers problem

On Wed, May 27, 2015 at 7:55 AM, Prarit Bhargava <prarit@...hat.com> wrote:
> John,
>
> We've identified an issue with ABSOLUTE timers and the leap second that
> effects, AFAICT, all kernels.
>
> This is the scenario.  Suppose you have a userspace program that has an
> ABSOLUTE timer that will expire @ midnight (00:00:00) UTC.  The timer expiry
> should occur at the end of the leap second event but it does not always occur
> at the end.  In fact it may occur during the leap second event and effectively
> one second too early.
>
> In the NOHZ kernel, timers are not necessarily aligned to the ticks the
> following does happen in userspace:

Hrm.. Thanks for the report! Looks like this could happen on !NOHZ as
well, and is an artifact of the fact the leapsecond is being applied
by a timer.

I'm working on a patch and will send it out shortly.

thanks
-john
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