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sched/rt: Update limit of sched_rt sysctl in documentation

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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@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/20250306052954.452005-3-sshegde@linux.ibm.com
This commit is contained in:
Shrikanth Hegde 2025-03-06 10:59:54 +05:30 • committed by Ingo Molnar
parent 14672f059d
commit b1536481c8
1 changed files with 3 additions and 0 deletions
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3
Documentation/scheduler/sched-rt-group.rst
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@ -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