I would note that the kernel watchdog timeouts here are always at 20 odd
seconds.  They are not increasing so whatever is occuring is progressing
at least as far as the kernel is concerned.  If we assume the systemd
log is still working (and it was shortly before the event when it
reported reaching shutdown state) then we would expect it to be in the
process of attempting to deconstruct the system before calling reboot.
Most of the deconstructors it calls are reported before calling.  There
is one, cg_trim(), which is not announced.  Looking at the
implementation of that it is doing a hierachical remove of the
/sys/fs/cgroup hierachy.  On my system this is some 15000 files in 1300
directories.  If there was a performance issue in there we could easily
spend hours in this call with nothing logged.

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https://bugs.launchpad.net/bugs/1730717

Title:
  Some VMs fail to reboot with "watchdog: BUG: soft lockup - CPU#0 stuck
  for 22s! [systemd:1]"

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