linux-kernel - [PATCH 2/2] sched/rt: update limit of sched

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Message-ID: <20250306052954.452005-3-sshegde@linux.ibm.com>
Date: Thu,  6 Mar 2025 10:59:54 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: juri.lelli@...hat.com, linux-kernel@...r.kernel.org
Cc: sshegde@...ux.ibm.com, mingo@...nel.org, peterz@...radead.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, hbathini@...ux.ibm.com, joelagnelf@...dia.com
Subject: [PATCH 2/2] sched/rt: update limit of sched_rt sysctl in documentation

By default fair_server dl_server allocates 5% of the bandwidth to the root
domain. Due to this writing any value less than 5% fails due to -EBUSY.

cat /proc/sys/kernel/sched_rt_period_us
1000000
$ echo 49999 > /proc/sys/kernel/sched_rt_runtime_us
-bash: echo: write error: Device or resource busy
$ echo 50000 > /proc/sys/kernel/sched_rt_runtime_us
$

Since the sched_rt_runtime_us allows -1 as the minimum, put this
restriction in the documentation.

One should check average of runtime/period in 
/sys/kernel/debug/sched/fair_server/cpuX/* for exact value.

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 Documentation/scheduler/sched-rt-group.rst | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Documentation/scheduler/sched-rt-group.rst b/Documentation/scheduler/sched-rt-group.rst
index 80b05a3009ea..ab464335d320 100644
--- a/Documentation/scheduler/sched-rt-group.rst
+++ b/Documentation/scheduler/sched-rt-group.rst
@@ -102,6 +102,9 @@ The system wide settings are configured under the /proc virtual file system:
   * sched_rt_period_us takes values from 1 to INT_MAX.
   * sched_rt_runtime_us takes values from -1 to sched_rt_period_us.
   * A run time of -1 specifies runtime == period, ie. no limit.
+  * sched_rt_runtime_us/sched_rt_period_us > 0.05 inorder to preserve
+    bandwidth for fair dl_server. For accurate value check average of
+    runtime/period in /sys/kernel/debug/sched/fair_server/cpuX/
 
 
 2.2 Default behaviour
-- 
2.39.3