With the above patch, instances are still not launching, returning this traceback [1] so this cgroup or namespace change is also affecting nova_libvirtd presumably.

[1]
~~~
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [req-b526d04d-1bfe-49e3-8a9d-19f6e93225e4 aa9659a889c3439485a7a307256e4250 4a886606e5a44e9e838582c33347166d - default default] [instance: 347d7250-b14a-4bfb-8aab-7a3580847865] Instance failed to spawn: libvirt.libvirtError: unable to open '/sys/fs/cgroup/machine/qemu-1-instance-00000002.libvirt-qemu/': No such file or directory
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865] Traceback (most recent call last):
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/compute/manager.py", line 2640, in _build_resources
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     yield resources
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/compute/manager.py", line 2413, in _build_and_run_instance
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     accel_info=accel_info)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/driver.py", line 4193, in spawn
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     cleanup_instance_disks=created_disks)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/driver.py", line 7250, in _create_guest_with_network
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     cleanup_instance_disks=cleanup_instance_disks)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/oslo_utils/excutils.py", line 227, in __exit__
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     self.force_reraise()
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/oslo_utils/excutils.py", line 200, in force_reraise
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     raise self.value
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/driver.py", line 7218, in _create_guest_with_network
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     post_xml_callback=post_xml_callback)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/driver.py", line 7155, in _create_guest
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     guest.launch(pause=pause)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/guest.py", line 163, in launch
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     self._encoded_xml, errors='ignore')
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/oslo_utils/excutils.py", line 227, in __exit__
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     self.force_reraise()
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/oslo_utils/excutils.py", line 200, in force_reraise
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     raise self.value
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/nova/virt/libvirt/guest.py", line 159, in launch
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     return self._domain.createWithFlags(flags)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 190, in doit
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     result = proxy_call(self._autowrap, f, *args, **kwargs)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 148, in proxy_call
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     rv = execute(f, *args, **kwargs)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 129, in execute
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     six.reraise(c, e, tb)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/six.py", line 703, in reraise
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     raise value
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib/python3.6/site-packages/eventlet/tpool.py", line 83, in tworker
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     rv = meth(*args, **kwargs)
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]   File "/usr/lib64/python3.6/site-packages/libvirt.py", line 1385, in createWithFlags
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865]     raise libvirtError('virDomainCreateWithFlags() failed')
nova-compute.log:2021-07-14 00:15:23.206 2 ERROR nova.compute.manager [instance: 347d7250-b14a-4bfb-8aab-7a3580847865] libvirt.libvirtError: unable to open '/sys/fs/cgroup/machine/qemu-1-instance-00000002.libvirt-qemu/': No such file or directory
~~~