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

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Date:	Tue, 2 Jun 2015 07:09:46 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Richard Cochran <richardcochran@...il.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 Tue, Jun 2, 2015 at 2:21 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jun 02, 2015 at 11:01:54AM +0200, Ingo Molnar wrote:
>> > Currently, leapsecond adjustments are done at tick time.
>> >
>> > As a result, the leapsecond was applied at the first timer
>> > tick *after* the leapsecond (~1-10ms late depending on HZ),
>> > rather then exactly on the second edge.
>
> So why not program a hrtimer to expire at the exact right time? Then
> even the VDSO gets its time 'right' and you avoid touching all the
> fast paths.

Well, the hrtimer won't always expire at the exact right time (due to
irq latency, possible clockevent drift/error, etc) , so that would
close the window a bit, but wouldn't eliminate the issue.

thanks
-john
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