Comment 14 for bug 2039346

>> Any reason this could NOT be backported to Zed as well?
>The change which we suspect introduced this problem has never been backported to zed.

OK, maybe the cause is something else then as we are regularly observe issues across multiple daemons:
(This on Yoga, but are just about to upgrade to Zed, thus my question about backporting any fixed to Zed)

nova-conductor:

```
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: Traceback (most recent call last):
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: File "/usr/lib/python3/dist-packages/eventlet/hubs/hub.py", line 476, in fire_timers
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: timer()
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: File "/usr/lib/python3/dist-packages/eventlet/hubs/timer.py", line 59, in __call__
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: cb(*args, **kw)
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: File "/usr/lib/python3/dist-packages/eventlet/semaphore.py", line 152, in _do_acquire
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: waiter.switch()
Mar 06 13:59:29 ctrl-03 nova-conductor[6503]: greenlet.error: cannot switch to a different thread

```

nova-compute:

```
Mar 26 14:42:29 comp-23 nova-compute[3154276]: Traceback (most recent call last):
Mar 26 14:42:29 comp-23 nova-compute[3154276]: File "/usr/lib/python3/dist-packages/eventlet/hubs/hub.py", line 476, in fire_timers
Mar 26 14:42:29 comp-23 nova-compute[3154276]: timer()
Mar 26 14:42:29 comp-23 nova-compute[3154276]: File "/usr/lib/python3/dist-packages/eventlet/hubs/timer.py", line 59, in __call__
Mar 26 14:42:29 comp-23 nova-compute[3154276]: cb(*args, **kw)
Mar 26 14:42:29 comp-23 nova-compute[3154276]: File "/usr/lib/python3/dist-packages/eventlet/semaphore.py", line 152, in _do_acquire
Mar 26 14:42:29 comp-23 nova-compute[3154276]: waiter.switch()
Mar 26 14:42:29 comp-23 nova-compute[3154276]: greenlet.error: cannot switch to a different thread
```

neutron-server:
```
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: Traceback (most recent call last):
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: File "/usr/lib/python3/dist-packages/eventlet/hubs/hub.py", line 476, in fire_timers
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: timer()
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: File "/usr/lib/python3/dist-packages/eventlet/hubs/timer.py", line 59, in __call__
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: cb(*args, **kw)
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: File "/usr/lib/python3/dist-packages/eventlet/semaphore.py", line 152, in _do_acquire
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: waiter.switch()
Mar 26 14:42:32 ctrl-01 neutron-server[7192]: greenlet.error: cannot switch to a different thread
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: Traceback (most recent call last):
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: File "/usr/lib/python3/dist-packages/eventlet/hubs/hub.py", line 476, in fire_timers
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: timer()
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: File "/usr/lib/python3/dist-packages/eventlet/hubs/timer.py", line 59, in __call__
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: cb(*args, **kw)
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: File "/usr/lib/python3/dist-packages/eventlet/semaphore.py", line 152, in _do_acquire
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: waiter.switch()
Mar 26 14:56:04 ctrl-01 neutron-server[7191]: greenlet.error: cannot switch to a different thread

```

Unfortunately

a) daemons don't crash, but turn into non working zombies
b) there is no indication / trace to what causes the issue with(in) eventlet

I also discussed this in https://github.com/eventlet/eventlet/issues/432#issuecomment-1983373808

Can I provide anything to find the cause of the greenlet issues I observe?