vxworks message queue "lost" a task blocked on it. What can be a reason?

Question

We have a quite large multitasking communication system implemented on Vxworks 5.5 and PPC8260. The system should handle a lot of Ethernet traffic and also handle some cyclic peripheral control activities via RS-232, memory mapped I/O etc. What happens is that in some moment few message queues we are using for inter task communication become overflowed (I see it by log inspection). When I check the status of tasks responsible for serving this Message queues (that is doing receive on them) they appear to be READY.When I inspect msgQShow for the queues themselves they are full but no tasks appear to be blocked on them. But looking at task stack trace shows that a task actually pending inside msgQReceive call.Specifically in the qJobGet kernel call or something alike.

Answer 1

It is unlikely in the extreme that a message queue "lost" a task that was blocked on it.

From your description we can assume:

1. A message Queue has overflowed. Presumably you have detected this by checking the return value from msgQSend, which has been invoked either with a timeout value, or NO_WAIT.
2. msgQShow confirms the Q is full
3. Tasks that should be reading the queue are in the READY state.

The READY state is the state that tasks are in when they are available to run. Tasks are held in a queue (strictly, one queue per priority level), and when the reach the head of the queue they will be scheduled.

If tasks are persistently showing as READY, that suggests that they are not getting CPU time. The fact that the msgQ does not appear to empty supports that.

You should use tools such as system viewer to diagnose. You may need to raise the priority of the reader tasks. If your msgQSend is using NO_WAIT, you may need to use a timeout value