linux-kernel - [PATCH] Documentation: sched/RT: Update paragraphs about RT bandwidth control

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Message-ID: <20241211170052.2449581-1-mkoutny@suse.com>
Date: Wed, 11 Dec 2024 18:00:52 +0100
From: Michal Koutný <mkoutny@...e.com>
To: linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Jonathan Corbet <corbet@....net>,
	Juri Lelli <juri.lelli@...hat.com>,
	Michal Koutný <mkoutny@...e.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Christian Loehle <christian.loehle@....com>,
	Frederic Weisbecker <fweisbecker@...e.com>
Subject: [PATCH] Documentation: sched/RT: Update paragraphs about RT bandwidth control

This has slightly changed with the introduction of fair_server.
Update the most relevant parts.

Link: https://lore.kernel.org/r/Z0c8S8i3qt7SEU14@jlelli-thinkpadt14gen4.remote.csb/
Signed-off-by: Michal Koutný <mkoutny@...e.com>
---
 Documentation/scheduler/sched-deadline.rst | 13 +++++++------
 Documentation/scheduler/sched-rt-group.rst |  8 ++++----
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/Documentation/scheduler/sched-deadline.rst b/Documentation/scheduler/sched-deadline.rst
index 22838ed8e13aa..a727827b8dd52 100644
--- a/Documentation/scheduler/sched-deadline.rst
+++ b/Documentation/scheduler/sched-deadline.rst
@@ -591,12 +591,13 @@ Deadline Task Scheduling
 
  The system wide settings are configured under the /proc virtual file system.
 
- For now the -rt knobs are used for -deadline admission control and the
- -deadline runtime is accounted against the -rt runtime. We realize that this
- isn't entirely desirable; however, it is better to have a small interface for
- now, and be able to change it easily later. The ideal situation (see 5.) is to
- run -rt tasks from a -deadline server; in which case the -rt bandwidth is a
- direct subset of dl_bw.
+ For now the -rt knobs are used for -deadline admission control and with
+ CONFIG_RT_GROUP_SCHED the -deadline runtime is accounted against the (root)
+ -rt runtime. With !CONFIG_RT_GROUP_SCHED the knob only serves for the -dl
+ admission control. We realize that this isn't entirely desirable; however, it
+ is better to have a small interface for now, and be able to change it easily
+ later. The ideal situation (see 5.) is to run -rt tasks from a -deadline
+ server; in which case the -rt bandwidth is a direct subset of dl_bw.
 
  This means that, for a root_domain comprising M CPUs, -deadline tasks
  can be created while the sum of their bandwidths stays below:
diff --git a/Documentation/scheduler/sched-rt-group.rst b/Documentation/scheduler/sched-rt-group.rst
index d685609ed3d7a..80b05a3009ea2 100644
--- a/Documentation/scheduler/sched-rt-group.rst
+++ b/Documentation/scheduler/sched-rt-group.rst
@@ -92,10 +92,10 @@ The system wide settings are configured under the /proc virtual file system:
 /proc/sys/kernel/sched_rt_runtime_us:
   A global limit on how much time real-time scheduling may use. This is always
   less or equal to the period_us, as it denotes the time allocated from the
-  period_us for the real-time tasks. Even without CONFIG_RT_GROUP_SCHED enabled,
-  this will limit time reserved to real-time processes. With
-  CONFIG_RT_GROUP_SCHED=y it signifies the total bandwidth available to all
-  real-time groups.
+  period_us for the real-time tasks. Without CONFIG_RT_GROUP_SCHED enabled,
+  this only serves for admission control of deadline tasks. With
+  CONFIG_RT_GROUP_SCHED=y it also signifies the total bandwidth available to
+  all real-time groups.
 
   * Time is specified in us because the interface is s32. This gives an
     operating range from 1us to about 35 minutes.
-- 
2.47.0