docs: scheduler: Start documenting the EEVDF scheduler

Add some documentation regarding the newly introduced scheduler EEVDF.
Signed-off-by: Carlos Bilbao <carlos.bilbao.osdev@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240720002207.444286-2-carlos.bilbao.osdev@gmail.com

Documentation/scheduler/index.rst

@@ -12,6 +12,7 @@ Scheduler
     sched-bwc
     sched-deadline
     sched-design-CFS
+    sched-eevdf
     sched-domains
     sched-capacity
     sched-energy

Documentation/scheduler/sched-design-CFS.rst

@@ -8,10 +8,12 @@ CFS Scheduler
 1.  OVERVIEW
 ============
 
-CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
-scheduler implemented by Ingo Molnar and merged in Linux 2.6.23.  It is the
-replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
-code.
+CFS stands for "Completely Fair Scheduler," and is the "desktop" process
+scheduler implemented by Ingo Molnar and merged in Linux 2.6.23. When
+originally merged, it was the replacement for the previous vanilla
+scheduler's SCHED_OTHER interactivity code. Nowadays, CFS is making room
+for EEVDF, for which documentation can be found in
+Documentation/scheduler/sched-eevdf.rst.
 
 80% of CFS's design can be summed up in a single sentence: CFS basically models
 an "ideal, precise multi-tasking CPU" on real hardware.

Documentation/scheduler/sched-eevdf.rst

+===============
+EEVDF Scheduler
+===============
+
+The "Earliest Eligible Virtual Deadline First" (EEVDF) was first introduced
+in a scientific publication in 1995 [1]. The Linux kernel began
+transitioning to EEVDF in version 6.6 (as a new option in 2024), moving
+away from the earlier Completely Fair Scheduler (CFS) in favor of a version
+of EEVDF proposed by Peter Zijlstra in 2023 [2-4]. More information
+regarding CFS can be found in
+Documentation/scheduler/sched-design-CFS.rst.
+
+Similarly to CFS, EEVDF aims to distribute CPU time equally among all
+runnable tasks with the same priority. To do so, it assigns a virtual run
+time to each task, creating a "lag" value that can be used to determine
+whether a task has received its fair share of CPU time. In this way, a task
+with a positive lag is owed CPU time, while a negative lag means the task
+has exceeded its portion. EEVDF picks tasks with lag greater or equal to
+zero and calculates a virtual deadline (VD) for each, selecting the task
+with the earliest VD to execute next. It's important to note that this
+allows latency-sensitive tasks with shorter time slices to be prioritized,
+which helps with their responsiveness.
+
+There are ongoing discussions on how to manage lag, especially for sleeping
+tasks; but at the time of writing EEVDF uses a "decaying" mechanism based
+on virtual run time (VRT). This prevents tasks from exploiting the system
+by sleeping briefly to reset their negative lag: when a task sleeps, it
+remains on the run queue but marked for "deferred dequeue," allowing its
+lag to decay over VRT. Hence, long-sleeping tasks eventually have their lag
+reset. Finally, tasks can preempt others if their VD is earlier, and tasks
+can request specific time slices using the new sched_setattr() system call,
+which further facilitates the job of latency-sensitive applications.
+
+REFERENCES
+==========
+
+[1] https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=805acf7726282721504c8f00575d91ebfd750564
+
+[2] https://lore.kernel.org/lkml/a79014e6-ea83-b316-1e12-2ae056bda6fa@linux.vnet.ibm.com/
+
+[3] https://lwn.net/Articles/969062/
+
+[4] https://lwn.net/Articles/925371/