linux-kernel - Re: [PATCH] sched: Document that RT task priorities are 1…99

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Date:   Wed, 03 Apr 2019 23:17:52 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de
Subject: Re: [PATCH] sched: Document that RT task priorities are
 1…99

On 2019-04-03, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> John identified three files which claim that RT task priorities start at
> zero. As far as I understand, 0 is used for DL and has nothing to do
> wihich RT priorities as identified by the RT policy.
>
> Correct the comment, valid RT priorities are in the range from 1 to 99.
>
> Reported-by: John Ogness <john.ogness@...utronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  Documentation/scheduler/sched-rt-group.txt | 2 +-
>  include/linux/sched/prio.h                 | 2 +-
>  kernel/sched/cpupri.h                      | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/scheduler/sched-rt-group.txt b/Documentation/scheduler/sched-rt-group.txt
> index d8fce3e784574..23f8f8465a775 100644
> --- a/Documentation/scheduler/sched-rt-group.txt
> +++ b/Documentation/scheduler/sched-rt-group.txt
> @@ -175,7 +175,7 @@ get their allocated time.
>  
>  Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
>  the biggest challenge as the current linux PI infrastructure is geared towards
> -the limited static priority levels 0-99. With deadline scheduling you need to
> +the limited static priority levels 1-99. With deadline scheduling you need to
>  do deadline inheritance (since priority is inversely proportional to the
>  deadline delta (deadline - now)).
>  
> diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
> index 7d64feafc408e..6986c32356842 100644
> --- a/include/linux/sched/prio.h
> +++ b/include/linux/sched/prio.h
> @@ -8,7 +8,7 @@
>  
>  /*
>   * Priority of a process goes from 0..MAX_PRIO-1, valid RT
> - * priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH
> + * priority is 1..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH

Actually, valid RT priority is 0..MAX_RT_PRIO-2 (0-98). This comment is
talking about the kernel representation, not the userspace one.

>   * tasks are in the range MAX_RT_PRIO..MAX_PRIO-1. Priority
>   * values are inverted: lower p->prio value means higher priority.
>   *
> diff --git a/kernel/sched/cpupri.h b/kernel/sched/cpupri.h
> index 7dc20a3232e72..40257a97fb8f2 100644
> --- a/kernel/sched/cpupri.h
> +++ b/kernel/sched/cpupri.h
> @@ -5,7 +5,7 @@
>  #define CPUPRI_INVALID		-1
>  #define CPUPRI_IDLE		 0
>  #define CPUPRI_NORMAL		 1
> -/* values 2-101 are RT priorities 0-99 */
> +/* values 2-101 are RT priorities 1-99 */

I suppose this also should be 0-98.

>  
>  struct cpupri_vec {
>  	atomic_t		count;

IMHO it is a bit crazy that userspace RT prio 99 maps to kernel prio
0. This leaves a hole at kernel prio 99. Wouldn't it be better just to
map userspace RT prio 1-99 to kernel prio 99-1?

John Ogness