linux-kernel - Re: [PATCH 05/31] sched: Add sched_class->reweight

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Date:   Mon, 12 Dec 2022 10:15:07 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...nel.org, joshdon@...gle.com, brho@...gle.com,
        pjt@...gle.com, derkling@...gle.com, haoluo@...gle.com,
        dvernet@...a.com, dschatzberg@...a.com, dskarlat@...cmu.edu,
        riel@...riel.com, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 05/31] sched: Add sched_class->reweight_task()

On Mon, Dec 12, 2022 at 09:11:36PM +0100, Peter Zijlstra wrote:
> > > Hurmph.. this further propagate the existing problem of thinking that
> > > 'prio' is a useful concept in general (it isn't).
> > 
> > I'm not quite following. Can you please expand on why prio isn't a generally
> > useful concept?
> 
> The whole fixed vs dynamic priority scheduling thing. Specifically
> SCHED_DEADLINE implements a dynamic priority scheme using the sporadic
> task model which just doesn't map well to this single prio value
> (notably every SCHED_DEADLINE task has prio -1, making it impossible to
> order SCHED_DEADLINE tasks based on this).

I see, so the expressive power isn't sufficient to cover some use cases.
Yeah, I mean, it's limited and we can add whatever that's necessary but
given that there are many use cases that make use of priorities, I don't
think SCX can skip supporting it.

Thanks.

-- 
tejun