How does SCHED_FIFO and SCHED_RR interfer with each other?

Question

SCHED_FIFO and SCHED_RR are both meant for real-time uses. I am aware that SCHED_RR can be preempted by time slicing. But say if I have one thread set to SCHED_FIFO, and another set to SCHED_RR, if both threads are ready to run, are they scheduled purely by priority? What if they have same priority?

Answer 1

Conceptually, there is a list of runnable processes associated with each static priority level. These lists can contain both SCHED_FIFO and SCHED_RR processes - the two scheduling policies share the same set of static priorities.

When selecting a process to run, the scheduler takes the process at the head of the non-empty list with the highest static priority, regardless of the scheduling policy of that process.

The scheduling policies affect how the processes move within those lists. For SCHED_FIFO, once a process reaches the head of the list for a given priority it will stay there until it blocks or yields. For SCHED_RR, a runnable process that has exceeded its maximum time quantum will be moved to the end of the list for its static priority.