The code is using a variable which can be not defined if sibling_sets (plural) is empty.
# NOTE(sfinucan): If siblings weren't available and we're using PREFER
# (implicitly or explicitly), fall back to linear assignment across
# cores
if (instance_cell.cpu_thread_policy !=
fields.CPUThreadAllocationPolicy.REQUIRE and
not pinning):
pinning = list(zip(sorted(instance_cell.cpuset),
itertools.chain(*sibling_set))) <-----not defined if sibling_sets is empty
So far the only path I could see to be in a situation where sibling_sets is empty at this step would be if two instances get scheduled "in same time" in the same host where we could consider that all the checks to ensure that the host cell provides enough cpus to handle the request have been accepted.
Even if that could happen only in such circumstance which should fix the issue.
[0] https://github.com/openstack/nova/blob/master/nova/virt/hardware.py#L882
Seeing this repeatedly in logs:
Traceback (most recent call last):\n', u' File "/usr/lib/