I mean it’s not that hard even on non-rt preemptive schedulers. You can’t avoid false negatives because you could be preempted after func completes but before the time is fetched but if you get a result it will be valid.

    def must_complete_before(func, deadline):
        result = func()
        lower, upper = time.now()
        if upper < deadline:
            return result
        else:
            throw MissedDeadlineException(“Can’t guarantee func completed before deadline”)