Clock occasionally jumps backwards on 11.1-RELEASE

[Alan Somers]

Since upgrading my jail server to 11.1-RELEASE, the clock occasionally
jumps backwards by 5-35 minutes for no apparent reason.  Has anybody seen
something like this?

Details
=====

* Happens about once a day on my jail server, and has happened at least
once on a separate bhyve server.

* The jumps almost always happen between 1 and 3 AM, but I've also seen
them happen at 06:30 and 20:15.

* The jumps are always backwards, never forwards.

* Inspecting the logs of both the host and its jails shows nothing
interesting that's correlated with the jumps.  Sometimes I find Amanda
doing a backup, but not always.

* Sometimes the jumps happen immediately after ntpd adds a new server to
its list, but not always.

* I'm using the default ntp.conf file.

* ntpd is running on both, and it should be the only process touching the
clock.   I have a script running "ntpq -c peers" once a minute, which shows
the offset for one server suddenly jump to a large negative number.  Then
the offsets for other servers jump to the same value, then either ntpd
fixes the clock or exits because the offset is too high.

* Said script is sleeping using the monotonic clock, not the realtime
clock.  As expected, successive timestamps differ by about 6.5 minutes when
ntpd corrects a 5.5 minute clock offset.  However, when the clock
presumably jumps backwards I _don't_ see successive timestamps go backwards
too.  They keep marching forward at the expected rate.  This makes me
wonder if the entire machine is hanging.  But it would have to be a pretty
serious hang to stop the clock from ticking.

Any ideas?