linux-kernel - Re: [PATCH 03/12] sched: Fold sched_class::switch{ing,ed}

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Message-ID: <20251009135408.GD4067720@noisy.programming.kicks-ass.net>
Date: Thu, 9 Oct 2025 15:54:08 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: tj@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, longman@...hat.com, hannes@...xchg.org,
	mkoutny@...e.com, void@...ifault.com, arighi@...dia.com,
	changwoo@...lia.com, cgroups@...r.kernel.org,
	sched-ext@...ts.linux.dev, liuwenfang@...or.com, tglx@...utronix.de
Subject: Re: [PATCH 03/12] sched: Fold
 sched_class::switch{ing,ed}_{to,from}() into the change pattern

On Thu, Oct 09, 2025 at 03:30:02PM +0200, Dietmar Eggemann wrote:
> On 06.10.25 12:44, Peter Zijlstra wrote:
> > Add {DE,EN}QUEUE_CLASS and fold the sched_class::switch* methods into
> > the change pattern. This completes and makes the pattern more
> > symmetric.
> > 
> > This changes the order of callbacks slightly:
> > 
> > 				|
> > 				|  switching_from()
> >   dequeue_task();		|  dequeue_task()
> >   put_prev_task();		|  put_prev_task()
> > 				|  switched_from()
> > 				|
> >   ... change task ...		|  ... change task ...
> > 				|
> >   switching_to();		|  switching_to()
> >   enqueue_task();		|  enqueue_task()
> >   set_next_task();		|  set_next_task()
> >   prev_class->switched_from()	|
> >   switched_to()			|  switched_to()
> > 				|
> > 
> > Notably, it moves the switched_from() callback right after the
> > dequeue/put. Existing implementations don't appear to be affected by
> > this change in location -- specifically the task isn't enqueued on the
> > class in question in either location.
> > 
> > Make (CLASS)^(SAVE|MOVE), because there is nothing to save-restore
> > when changing scheduling classes.
> 
> This one causes a DL bw related warning when I run a simple 1 DL task
> rt-app workload:

> Not sure yet how this is related to switched_from_dl() being now called earlier?

Ooh, I might see a problem. task_non_contending() uses dl_task(), which
uses p->prio. The move above means it is now called using the 'old'
prio, whereas it used to run with the 'new' prio.

I suppose it does this to distinguish 'real' DL tasks from PI boosted DL
tasks.

Let me see if I can figure out something for this.