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

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Date:   Mon, 12 Dec 2022 21:11:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.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 07:34:27AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Mon, Dec 12, 2022 at 12:22:43PM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 29, 2022 at 10:22:47PM -1000, Tejun Heo wrote:
> > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > index a4a20046e586..08799b2a566e 100644
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -2193,6 +2193,8 @@ struct sched_class {
> > >  	 */
> > >  	void (*switched_from)(struct rq *this_rq, struct task_struct *task);
> > >  	void (*switched_to)  (struct rq *this_rq, struct task_struct *task);
> > > +	void (*reweight_task)(struct rq *this_rq, struct task_struct *task,
> > > +			      int newprio);
> > >  	void (*prio_changed) (struct rq *this_rq, struct task_struct *task,
> > >  			      int oldprio);
> > 
> > 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).