Scheduler for all "threads"

In operating system, not only the traditional process and thread are schedule-able entities, but also the IRQ handler, software IRQ, tasklet, timer operations and work queue should be the schedule-able entities too, conceptually.

Some threads can have higher priority than some tasklets, so, it can preempt these tasklets. It can even have higher priority than some IRQ handler, so that the IRQ is not serviced during thread executing. I think this is a useful concept in real-time system. I think the thread-style IRQ handing in FreeBSD maybe follow the same concept.