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

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Message-ID: <20230619120215.GO38236@hirez.programming.kicks-ass.net>
Date:   Mon, 19 Jun 2023 14:02:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        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>,
        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>,
        Vineeth Pillai <vineeth@...byteword.org>,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [RFC PATCH V3 6/6] sched/fair: Implement starvation monitor

On Fri, Jun 16, 2023 at 02:05:07PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 13, 2023 at 03:41:30PM +0200, Daniel Bristot de Oliveira wrote:
> 
> > In an 0-laxity scheduler, the server would run at 0-laxity, jumping in
> > front of DL tasks... that would break EDF. It would be mixing two
> > schedulers in one. It is not required and likely not a good idea either.
> 
> I did consider a hybrid 0-laxity and EDF scheduler for mixed
> criticality, as have others like Ted Baker IIRC. IIRC it can be done
> using an augmented tree, but none of that solves the problems 0-laxity
> has (like over preemption and the general problem of playing chicken by
> doing things at the *VERY* last possible moment).
> 
> I think I did a talk at OSPERT on this at some point many years ago.
> Luckily some bright fellow had this semi-partitioned stuff that would
> make live much simpler :-)

I must clarify; I was thinking Least-Laxity-First, which is ofcourse not
the same as a 0-laxity scheduler.