How does VxWorks prioritize interrupt bottom-halves?

Question

Suppose I have two tasks, 'A' and 'B', of differing priority executing on SMP-supported VxWorks. Both 'A' and 'B' issue a command to an I/O device (such as a disk or NIC) and both block waiting for results. That is, both 'A' and 'B' are blocked at the same time. Some time later, the I/O device raises an interrupt and the ISR is invoked. The ISR then dispatches deferred work (aka "bottom-half") to a worker-task. Question: What is the priority of the worker-task?

VxWorks Device Driver Developer's Guide is a bit vague. It appears that the priority of the worker-task is set up a-priori. There are no automatic inheritance mechanisms that will increase the priority of the worker-task based upon the priorities of tasks ('A' and 'B') that are blocked waiting for results. This is similar to how threaded interrupt priorities work in PREEMPT_RT Linux. However, both QNX Neutrino and LynxOS will schedule the worker-task with the maximum priority of the blocked tasks-- Ex. priority(worker) = max_priority(A, B).

Can anyone clarify?

Answer 1

It depends exactly on which mechanism the "ISR dispatched deferred work" uses.

If a semaphore/messageQueue/Event is used, then the recipient task (A or B) will run at the priority specified when the task was created. In this scenario, the interrupt is essentially finished, and the task (A and/or B) are ready to run.

Whichever task is has the highest priority will get to run and perform it's work. Note that the task doesn't have access to any information from the interrupt context. If you use global structures (yuk) or pass data via a message queue, then the task could access those elements.

The network stack task (tNetTask) uses this approach, and a semaphore signals tNetTask when a packet has been received. When tNetTask has processed the packet (packet reassembly, etc...), it is then forwarded to whichever task is waiting on the corresponding socket.

It is possible to defer work from an ISR to tExcTask (via a call to excJobAdd). Note that with this approach, excJobAdd takes the pointer to a function and executes the function in the context of the tExcTask (which is at the highest priority in the system). It does not act as a self-contained task.

Note that some things like file systems, SCSI drivers, USB, etc... are much more than a simple driver with interrupts. They include a number of different components that unfortunately also increases complexity.