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

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Message-ID: <20251013102347.GY3419281@noisy.programming.kicks-ass.net>
Date: Mon, 13 Oct 2025 12:23:47 +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 06:50:55PM +0200, Dietmar Eggemann wrote:

> Yes, but there seems to be more ... missing DEQUEUE_SAVE (a.k.a.
> ENQUEUE_RESTORE) in
> 
>   enqueue_dl_entity()
> 
>     if (flags & (ENQUEUE_RESTORE|ENQUEUE_MIGRATING))
>                  ^^^^^^^^^^^^^^^
>       ...
>       add_running_bw(dl_se, dl_rq)
> 
> and
> 
>   __sched_setscheduler()
> 
>     ...
>     if (prev_class != next_class)
>       queue_flags |= DEQUEUE_CLASS;
>       queue_flags &= ~(DEQUEUE_SAVE | DEQUEUE_MOVE);
>                        ^^^^^^^^^^^^
> 
> as well as
> 
>   sched_change_begin()
> 
>     ...
>     if (flags & DEQUEUE_CLASS) {
>       if (WARN_ON_ONCE(flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)))
>         flags &= ~(DEQUEUE_SAVE | DEQUEUE_MOVE);
>                    ^^^^^^^^^^^^
> 

Urgh.. SAVE/RESTORE while changing CLASS is so weird.

But yeah, let me take that bit out for now -- I'll make a note in a
comment that we should look at perhaps cleaning that up instead.