I noticed this while looking at a grenade failure on an unrelated patch:
https://bugs.launchpad.net/nova/+bug/1844929
The details are in the bug but it looks like this showed up around Sept
17 and hits mostly on FortNebula nodes but also OVH nodes. It's
restricted to grenade jobs and while I don't see anything obvious in the
rabbitmq logs (the only errors are about uwsgi [api] heartbeat issues),
it's possible that these are slower infra nodes and we're just not
waiting for something properly during the grenade upgrade. We also don't
seem to have the mysql logs published during the grenade jobs which we
need to fix (and recently did fix for devstack jobs [1] but grenade jobs
are still using devstack-gate so log collection happens there).
I didn't see any changes in nova, grenade or devstack since Sept 16 that
look like they would be related to this so I'm guessing right now it's
just a combination of performance on certain infra nodes (slower?) and
something in grenade/nova not restarting properly or not waiting long
enough for the upgrade to complete.