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docs: scheduler: Start documenting the EEVDF scheduler

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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
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Carlos Bilbao 2024-07-19 19:22:06 -05:00 committed by Jonathan Corbet
parent e9c7acd723
commit 602bce7e5e
3 changed files with 50 additions and 4 deletions
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Documentation/scheduler/index.rst
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@ -12,6 +12,7 @@ Scheduler
sched-bwc
sched-deadline
sched-design-CFS
sched-eevdf
sched-domains
sched-capacity
sched-energy

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Documentation/scheduler/sched-design-CFS.rst
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@ -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.

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Documentation/scheduler/sched-eevdf.rst Normal file
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===============
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/