linux-kernel - Re: [PATCH sched_ext/for-6.11] sched, sched_ext: Replace scx_next_task_picked() with sched_class->switch

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Message-ID: <ZnnelpsfuVPK7rE2@slm.duckdns.org>
Date: Mon, 24 Jun 2024 11:01:10 -1000
From: Tejun Heo <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>, 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, changwoo@...lia.com, himadrics@...ia.fr,
	memxor@...il.com, andrea.righi@...onical.com,
	joel@...lfernandes.org, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH sched_ext/for-6.11] sched, sched_ext: Replace
 scx_next_task_picked() with sched_class->switch_class()

Hello, Peter.

On Mon, Jun 24, 2024 at 10:59:27AM +0200, Peter Zijlstra wrote:
> > @@ -5907,7 +5907,10 @@ restart:
> >  	for_each_active_class(class) {
> >  		p = class->pick_next_task(rq);
> >  		if (p) {
> > -			scx_next_task_picked(rq, p, class);
> > +			const struct sched_class *prev_class = prev->sched_class;
> > +
> > +			if (class != prev_class && prev_class->switch_class)
> > +				prev_class->switch_class(rq, p);
> 
> I would much rather see sched_class::pick_next_task() get an extra
> argument so that the BPF thing can do what it needs in there and we can
> avoid this extra code here.

Hmm... but here, the previous class's ->pick_next_task() might not be called
at all, so I'm not sure how that'd work. For context, sched_ext is using
this to tell the BPF scheduler that it lost a CPU to a higher priority class
(be that RT or CFS) os that the BPF scheduler can respond if necessary (e.g.
punting tasks that were queued on that CPU somewhere else and so on).

Imagine a case where a sched_ext task was running but then a RT task wakes
up on the CPU. We'd enter the scheduling path, RT's pick_next_task() would
return the new RT task to run. We now need to tell the BPF scheduler that we
lost the CPU to the RT task but haven't called its pick_next_task() yet.

Thanks.

-- 
tejun