So far we've talked about scheduling policies and thread states, but we haven't said much yet about why and when things are rescheduled.
There's a common misconception that rescheduling just “occurs,” without any real causes. Actually, this is a useful abstraction during design! But it's important to understand the conditions that cause rescheduling. Recall the diagram “Scheduling roadmap” (in “The rules” in the section on the kernel's role).
Rescheduling occurs only because of: