periodic tasks are not independent

Hi,

coming from manila, where the manager inherits from PeriodicTasks https://opendev.org/openstack/manila/src/branch/stable/2023.1/manila/manager.py#L68

the tasks https://opendev.org/openstack/manila/src/branch/stable/2023.1/manila/manager.py#L99-L101 are started by a timer https://opendev.org/openstack/manila/src/branch/stable/2023.1/manila/service.py#L182-L183 with https://opendev.org/openstack/manila/src/branch/stable/2023.1/manila/service.py#L273-L276

For the sake of making an understandable example let's simplify a bit:
I have
- task A and task B decorated by @periodic_task.periodic_task(spacing=10, run_immediately=True) and
- task C decorated by @periodic_task.periodic_task in my manager

My timer interval is 20 seconds, no initial delay.
Task B takes 35 seconds to complete.

Expected behaviour:
- Task A runs every 10 seconds.
- Unsure what to expect for Task B:
-- a) runs every 35 seconds
-- b) runs every 10 seconds, with multiple tasks of the same kind running in parallel
-- c) runs every 40 seconds (when the next timer tick comes around after completion)
- After 60 seconds initial delay task C runs every 60 seconds.

Observed behaviour:
Warning log line 'Function 'manila.service.Service.periodic_tasks' run outlasted interval by 15 sec' not telling me which task is the offender.

- Task A runs every 35 seconds.
- Task B runs every 35 seconds.
- Task C runs after initial delay sometimes after 35 seconds, sometimes after 60+15 seconds

Is is expected that my slow task B is affecting the interval at which my tasks A and C running?

When researching I stumbled upon https://review.opendev.org/c/openstack/neutron/+/147710, I think I'm not alone :)

Should I make different configuration/code in order to get my expected independent periodic tasks or is this behaviour a bug or rather a known limitation?

Thanks,
Maurice