linux-kernel - [PATCH v3 2/3] docs: scheduler-rt: Clarify & fix sched_rt

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Date:   Mon,  2 Oct 2023 13:55:52 +0200
From:   Cyril Hrubis <chrubis@...e.cz>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org
Cc:     ltp@...ts.linux.it, Cyril Hrubis <chrubis@...e.cz>
Subject: [PATCH v3 2/3] docs: scheduler-rt: Clarify & fix sched_rt_* sysctl docs

- Describe explicitly that sched_rt_runtime_us is allocated from
  sched_rt_period_us and hence always less or equal to that value.

- The limit for sched_rt_runtime_us is not INT_MAX-1, but rather it's
  limited by the value of sched_rt_period_us. If sched_rt_period_us is
  INT_MAX then sched_rt_runtime_us can be set to INT_MAX as well.

Signed-off-by: Cyril Hrubis <chrubis@...e.cz>
---
 Documentation/scheduler/sched-rt-group.rst | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/Documentation/scheduler/sched-rt-group.rst b/Documentation/scheduler/sched-rt-group.rst
index 655a096ec8fb..a16bee8f74c2 100644
--- a/Documentation/scheduler/sched-rt-group.rst
+++ b/Documentation/scheduler/sched-rt-group.rst
@@ -87,18 +87,20 @@ lack an EDF scheduler to make non-uniform periods usable.
 The system wide settings are configured under the /proc virtual file system:
 
 /proc/sys/kernel/sched_rt_period_us:
-  The scheduling period that is equivalent to 100% CPU bandwidth
+  The scheduling period that is equivalent to 100% CPU bandwidth.
 
 /proc/sys/kernel/sched_rt_runtime_us:
-  A global limit on how much time realtime scheduling may use.  Even without
-  CONFIG_RT_GROUP_SCHED enabled, this will limit time reserved to realtime
-  processes. With CONFIG_RT_GROUP_SCHED it signifies the total bandwidth
-  available to all realtime groups.
+  A global limit on how much time realtime 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 realtime tasks. Even without CONFIG_RT_GROUP_SCHED enabled,
+  this will limit time reserved to realtime processes. With
+  CONFIG_RT_GROUP_SCHED=y it signifies the total bandwidth available to all
+  realtime groups.
 
   * Time is specified in us because the interface is s32. This gives an
     operating range from 1us to about 35 minutes.
   * sched_rt_period_us takes values from 1 to INT_MAX.
-  * sched_rt_runtime_us takes values from -1 to (INT_MAX - 1).
+  * sched_rt_runtime_us takes values from -1 to sched_rt_period_us.
   * A run time of -1 specifies runtime == period, ie. no limit.
 
 
-- 
2.41.0