I’d like to use xQueueSendToBackFromISR(). That function can’t block, but just fails if the queue is full. So for correct operation, I need to set the queue size so it will never be full under load. Is there some way to measure a queue’s “high water mark”, ie the maximum number of items that have been in the queue at once, a bit like what can be done for a task stack? — Craig McQueen

No, but that sounds like a good enhancement that would enable people to tune their systems. You can query how many spaces their are in a queue, but not the minimum there have ever been.