linux-kernel - Re: [RFC PATCH V3 6/6] sched/fair: Implement starvation monitor

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Message-ID: <20230616115149.GK83892@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Jun 2023 13:51:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        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>
Subject: Re: [RFC PATCH V3 6/6] sched/fair: Implement starvation monitor

On Thu, Jun 08, 2023 at 05:58:18PM +0200, Daniel Bristot de Oliveira wrote:
> From: Juri Lelli <juri.lelli@...hat.com>
> 
> Starting deadline server for lower priority classes right away when
> first task is enqueued might break guarantees, as tasks belonging to
> intermediate priority classes could be uselessly preempted. E.g., a well
> behaving (non hog) FIFO task can be preempted by NORMAL tasks even if
> there are still CPU cycles available for NORMAL tasks to run, as they'll
> be running inside the fair deadline server for some period of time.
> 
> To prevent this issue, implement a starvation monitor mechanism that

Why should this be prevented?  FIFO can be preempted by a higher
priority FIFO/RR, or in this case by DEADLINE, and always by stop_task.

Just accept this.

Anything that's build around FIFO-99 not getting preempted is just plain
broken. Don't try and pander to broken.