[LEAPSECS] Improvements since 2012

[Martin Burnicki]

Folks, originally I wrote this email about 9 hours ago, but sent it 
using a different email address which is not subscribed to the list. So 
apparingly the first email was blocked, and I'm sending it once more.

Ask Bjørn Hansen wrote:
>> Steve Allen <sla at ucolick.org> wrote:
>
> [...]
>> NTP and the NTP pool are more robust too, so that will be better.
>
> What's better about NTP and the NTP Pool since 2012?
>
> I try not to mean that as a retorical question; I'm interested -- it
> would be good news. I don't know what's better about them and in the
> case of the pool I should probably know. :-)

The main problem with the last leap seconds was due to implementation 
faults in the Linux kernel, when the leap second was handled.

At the leap second in June 2012 this could cause a load spike (and thus 
increased power consumption). See:
https://lkml.org/lkml/2012/7/1/27

At the leap second at the end of 2008 it was even worse and could cause 
a deadlock of the Linux kernel. See:
https://lkml.org/lkml/2009/1/2/373

I don't know exactly from memory in which versions these bugs have been 
fixed, but I'll have a closer look at this the next days.

If I remember correctly the Linux kernel maintainers have very carefully 
checked the leap second handling code, and provided some testing 
routines for it.

So unless a new bug has been introduced I'd expect this works now in 
current kernel versions.


In ntpd there has been a bug which could cause a "leap second loop" when 
a leap second warning was propagated in a loop and could cause another 
leap second warning for the end of the next month after a real leap second.

If I remember correctly then this happened only under certain 
conditions, i.e. with certain configurations, and as far as I know there 
has been a patch in 4.2.8 which avoids this.

Anyway, if the pool's NTP server monitoring software checks the leap 
second warning bit and detects a leap second warning is still lit after 
the leap second has already occurred you have 1 month time to fix this, 
which means you have just to restart ntpd on the affected machine.

As always, this depends on the versions of the kernel and ntpd you are 
running.

Martin