Comment 5 for bug 1763766

I hit a situation when free vcpus on host <= instance flavor used vcpus (CPU pinning requested). The rebuild such instance on the same host (e.g. Guest OS Upgrade with new image) will fail.
Even with 3 other bugs fixes:
    https://bugs.launchpad.net/nova/+bug/1750623
    https://bugs.launchpad.net/nova/+bug/1750618
    https://bugs.launchpad.net/nova/+bug/1772523
NUMAtopology filter's hardware.numa_fit_instance_to_host will will call _numa_fit_instance_cell_with_pinning() which compares host_cell.avail_cpus < required_cpus, then skip the host selection with debug message "Not enough available CPUs to schedule instance. Oversubscription is not possible ...".
With educated guess, memory fit may also fail.

What you pointed out is truly valid.
    "The important thing is that the resource consumption as seen by nova and reported in placement can't
    change. So we need to disallow rebuilds to an image with properties that are incompatible with the
    current resource consumption."