linux-kernel - Re: [RFC][PATCH 2/3] sched: Add support to pick functions to take rf

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Message-ID: <20251013110911.GF4068168@noisy.programming.kicks-ass.net>
Date: Mon, 13 Oct 2025 13:09:11 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, juri.lelli@...hat.com, dietmar.eggemann@....com,
	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,
	Joel Fernandes <joelagnelf@...dia.com>
Subject: Re: [RFC][PATCH 2/3] sched: Add support to pick functions to take rf

On Mon, Oct 13, 2025 at 01:04:49PM +0200, Peter Zijlstra wrote:

> Bah; so yeah, this new behaviour is better for indeed always calling
> newidle when it is needed, but you're also right that in case of ext
> this might not be ideal.
> 
> So I have a pile of newidle hacks here:
> 
>   https://lkml.kernel.org/r/20251010170937.GG4067720@noisy.programming.kicks-ass.net
> 
> and while I don't particularly like NI_SPARE (the has_spare_tasks thing
> is fickle); the idea seems to have some merit for this situation --
> where we know we'll not be having fair tasks at all.
> 
> I mean, we can always do something like this to sched_balance_newidle():
> 
> 	if (scx_switched_all())
> 		return 0;
> 
> Not pretty, but should do the job.

Oh, never mind, none of this is needed.

__pick_next_task()

	if (scx_enabled())
	  goto restart;

	...
restart:
	for_each_active_class(class) {
		...
	}


And then we have next_active_class() skip fair_sched_class entirely when
scx_switch_all().

So in the common ext case, we'll not hit pick_next_task_fair() at all.