Questions About Priority Implementations

[ZhennanWu]

Currently the guarantee we have is "A higher priority will cause the task to be run sooner". It would be helpful to document more details about this priority mechanism. The most pressing issue in my mind is that:

1. How strict is this priority followed? It could be a heuristic, or a guaranteed order without preempting existing running tasks, or a guaranteed order that would preempt another low-priority running task if the executor is busy, or to preempt every other low-priority tasks no matter what (tho this would be unlikely).
2. If it is not a heuristic, does it cause priority inversion problem? If N_cpu high-priority spin-lock-style tasks are waiting for the result of a low-priority task, will the system progress?

The priority inversion problem in my view is an important one, since it has a dramatic impact in system design choices.