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From:   John Ogness <john.ogness@linutronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, tglx@linutronix.de
Subject: Re: [PATCH] sched: Document that RT task priorities are
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References: <20190403210821.10916-1-bigeasy@linutronix.de>
        <87sguypz3z.fsf@linutronix.de>
Date:   Thu, 04 Apr 2019 09:45:53 +0200
In-Reply-To: <87sguypz3z.fsf@linutronix.de> (John Ogness's message of "Wed, 03
        Apr 2019 23:17:52 +0200")
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On 2019-04-03, John Ogness <john.ogness@linutronix.de> wrote:
> 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?

FWIW, the current mapping:

    userspace         kernel
    ---------         ------
    rt 1...99         98...0
    nice -20...+19    100...139

has been in place since the introduction of Ingo's scalable scheduler:

Author: linus1 <torvalds@athlon.transmeta.com>
Date:   Sun Dec 16 12:00:00 2001 -0800

    v2.5.1.9 -> v2.5.1.10
    
    - Kai Germaschewski: ISDN updates
    - Al Viro: start moving buffer cache indexing to "struct
    - block_device *"
    - Greg KH: USB update
    - Russell King: fix up some ARM merge issues
    - Ingo Molnar: scalable scheduler

With that commit, the then separate nice and rt_priority task_struct
fields were combined to the new prio field, and thus the mapping was
born.

John Ogness