linux-kernel - Re: [PATCH 16/23] sched,watchdog: Convert to sched_set

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Date:   Wed, 22 Apr 2020 15:24:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        qais.yousef@....com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, wim@...ux-watchdog.org,
        Christophe Leroy <christophe.leroy@....fr>
Subject: Re: [PATCH 16/23] sched,watchdog: Convert to sched_set_fifo()

On Wed, Apr 22, 2020 at 08:51:55AM -0400, Steven Rostedt wrote:
> On Wed, 22 Apr 2020 13:27:35 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Because SCHED_FIFO is a broken scheduler model (see previous patches)
> > take away the priority field, the kernel can't possibly make an
> > informed decision.
> > 
> > Effectively changes prio from 99 to 50.
> 
> Hmm, this being a watchdog, and looking at commit 38a1222ae4f364d
> ("watchdog: core: make sure the watchdog worker always works")
> 
> I wonder if we should add a sched_set_high(), or have some other kind of
> watchdog handler that is guaranteed to trigger.

It's FIFO, it'll never win from either a deadline or a stop-task. After
that it doesn't matter.

fifo_high() is most definitely a bad idea, because then we're back into
the whole 'fifo priority' has meaning -- it does not. At least, it
doesn't until you've got system design information.

Maybe we should rename fifo_low to get away from that. I just drew a
blank on a better name there.