[PATCH v3 8/8] Documentation/sched: Add fairserver to sched-deadline

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

The fair_server ensures the fair-class gets CPU run-time and throttles
even the rt-class if needed. Add this bit of wisdom and the recently
added sysfs knobs for configutiation.

Assisted-by: LLM
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 Documentation/scheduler/sched-deadline.rst | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/Documentation/scheduler/sched-deadline.rst b/Documentation/scheduler/sched-deadline.rst
index 23ea88b2ed7dc..f0d667e09d10c 100644
--- a/Documentation/scheduler/sched-deadline.rst
+++ b/Documentation/scheduler/sched-deadline.rst
@@ -577,7 +577,8 @@ Deadline Task Scheduling
  level.
 
  A main difference between deadline bandwidth management and RT-throttling
- is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
+ is that -deadline tasks have bandwidth on their own (while -rt have only
+ throttling via the fair_server),
  and thus we don't need a higher level throttling mechanism to enforce the
  desired bandwidth. In other words, this means that interface parameters are
  only used at admission control time (i.e., when the user calls
@@ -604,6 +605,24 @@ Deadline Task Scheduling
  be thus free of oversubscribing the system up to any arbitrary level.
  This is done by writing -1 in /sys/kernel/sched/dl_runtime_us.
 
+4.1.1 Fair server settings
+--------------------------
+
+ The fair server ensures that fair-class tasks (SCHED_OTHER) are not starved
+ and can be scheduled even when tasks in the RT scheduling class
+ (SCHED_FIFO and SCHED_RR) are consuming all CPU time. It replaces the
+ legacy RT throttling mechanism, which was configured through the procfs
+ interface.
+
+ The fair server executes fair-class tasks through a deadline-scheduling
+ server with a runtime budget. It guarantees that the runtime configured by
+ fair_server_runtime is provided within the interval defined by
+ fair_server_period.
+
+ Both values are specified in nanoseconds and can be configured on a
+ per-CPU basis through /sys/kernel/sched/cpuX. Setting
+ fair_server_runtime to 0 disables the fair server on the corresponding
+ CPU.
 
 4.2 Task interface
 ------------------

-- 
2.55.0