[ntp:questions] PPS not working on newer kernel/distros

Miroslav Lichvar mlichvar at redhat.com
Mon Oct 15 12:01:42 UTC 2018

On Mon, Oct 15, 2018 at 07:31:02AM -0400, Brandon Applegate wrote:
> > On Oct 15, 2018, at 1:51 AM, Vitezslav Samel <vitezslav at samel.cz> wrote:
> > 
> > ntpq -c kerninfo
> Hmm, thanks for the tip.  I guess even though I have flag3 set to 1 I’m 'falling back' to userland PPS ?

It's not a userland PPS. The PPS timestamps are still from the kernel.
The difference is in the clock discipline. Instead of kernel using the
PPS samples directly to control the clock, they are processed by ntpd
and fed back to the kernel PLL/FLL loop at the normal update interval.

> root at ice:~# ntpq -c kerninfo | grep ^kernel
> kernel status:         pll nano
> So would this be a kernel issue causing this ?

The kernel PPS discipline is not supported in the NOHZ "tickless"
configuration, which I think is enabled in all major Linux

AFAIK the only thing that has changed in the last few years is how
ntpd handles the error when the PPS discipline cannot be enabled. It
used to be ignored.

Miroslav Lichvar

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