linux-kernel - Re: [PATCH V8 02/13] ntp: add ADJ

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Date:	Wed, 12 Jan 2011 05:32:03 +0900
From:	"Kuwahara,T." <6vvetjsrt26xsrzlh1z0zn4d2grdah@...il.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	netdev@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Lameter <cl@...ux.com>,
	David Miller <davem@...emloft.net>,
	Krzysztof Halasa <khc@...waw.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Rodolfo Giometti <giometti@...ux.it>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V8 02/13] ntp: add ADJ_SETOFFSET mode bit

On Tue, Jan 11, 2011 at 6:06 AM, john stultz <johnstul@...ibm.com> wrote:
> On Tue, 2011-01-11 at 05:45 +0900, Kuwahara,T. wrote:
>> On Tue, Jan 11, 2011 at 1:49 AM, john stultz <johnstul@...ibm.com> wrote:
>> > Leapsecond processing is done via an absolute hrtimer. Thus when the
>> > time offset is set, the hrtimers that should have expired will fire
>> > (just like with settimeofday) and the adjustment will then be made.
>>
>> How do you convert relative time to absolute time?  It's not trivial
>> because TAI offset is also a variable.
>
> I don't believe I understand what you're getting at.
>
> The proposed interface is almost identical in functionality to a
> userland application doing the following:
>
>        offset = my_calculate_offset();
>        clock_gettime(CLOCK_REALTIME, &now);
>        newtime = my_add_ts(now, offset);
>        settimeofday(&newtime, 0);
>
> The only difference is that you avoid the error from the delay between
> the gettime call and the settime call. It just adds the offset directly
> to the CLOCK_REALTIME.

For example, what time was it 3 seconds after 2008-12-31 23:59:59?
You may say, of course it's 2009-01-01 00:00:02.  But it's not true.
You wonder why?  Because a leap second had been added at midnight.

	unix time   UTC     offset
	---------- -------- ---
	1230767997 23:59:57  -2
	1230767998 23:59:58  -1
	1230767999 23:59:59   0
	1230768000 23:59:60   1 (leap second)
	1230768000 00:00:00   2
	1230768001 00:00:01   3
	1230768002 00:00:02   4

That's why I said it's not trivial, and that your patch is broken and
thus useless.  Unfortunately, there's no remedy for this as long as a
nonlinear timescale such as the unix time is being used, since the
leap second insertion/deletion is a non-deterministic event.
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