linux-kernel - Re: [PATCH V7 9/9] sched/rt: Remove default bandwidth control

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Message-ID: <2aqeakf4rbed2hnt46yvdospzntpvp7ndrk4xnv6rrh7ctijes@d3nzwnguheju>
Date: Fri, 29 Nov 2024 21:21:26 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	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>, 
	Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org, 
	Luca Abeni <luca.abeni@...tannapisa.it>, Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>, 
	Thomas Gleixner <tglx@...utronix.de>, Joel Fernandes <joel@...lfernandes.org>, 
	Vineeth Pillai <vineeth@...byteword.org>, Shuah Khan <skhan@...uxfoundation.org>, 
	Phil Auld <pauld@...hat.com>, Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH V7 9/9] sched/rt: Remove default bandwidth control

On Fri, Nov 29, 2024 at 03:44:53PM GMT, Juri Lelli <juri.lelli@...hat.com> wrote:
> You can disable admission control by echoing -1 in sched_rt_runtime_us,
> but still have the dl_server working for SCHED_NORMAL tasks. By
> disabling admission control SCHED_DEADLINE can indeed monopolize CPU
> (over subscription).

| I'm wondering if different values of Δ mean anything or how they can
| be used.

Aha, the knob therefore remains relevant for the DL admission control.

So if I put it together, I will write it down like this:

-- 8< --
Subject: [PATCH] sched/RT: Update paragraphs about RT bandwidth control

This has slightly changed with the introductions 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

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