In my application I need to report audio thread CPU usage with a fair amount of confidence and warn if the CPU is struggeling.
Measuring the proportinal time in an audio callback will only work if there are no thread suspends or -switches. In practice, the audio thread is never interruputed?
Well it really works in the sense that it tells you how long was spent in the audio callback. The usefulness of that statistic in deciding whether or not the audio system is running smoothly is debatable.
I don't think you can assume that the audio thread is never interrupted, but it will certainly have a high priority.