Algorithm
The scheduler stores the records about the planned tasks in a red-black tree, using the spent processor time as a key. This allows it to pick efficiently the process that has used the least amount of time (it is stored in the leftmost node of the tree). The entry of the picked process is then removed from the tree, the spent execution time is updated and the entry is then returned to the tree where it normally takes some other location. The new leftmost node is then picked from the tree, repeating the iteration.
If the task spends a lot of its time sleeping, then its spent time value is low and it automatically gets the priority boost when it finally needs it. Hence such tasks do not get less processor time than the tasks that are constantly running.
Read more about this topic: Completely Fair Scheduler