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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Date: Mon, 24 Jun 2024 11:02:06 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Tejun Heo <tj@...nel.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()

On Thu, Jun 20, 2024 at 03:42:48PM -0700, Linus Torvalds wrote:
> Btw, indirect calls are now expensive enough that when you have only a
> handful of choices, instead of a variable
> 
>         class->some_callback(some_arguments);
> 
> you might literally be better off with a macro that does
> 
>        #define call_sched_fn(class, name, arg...) switch (class) { \
>         case &fair_name_class: fair_name_class.name(arg); break; \
>         ... unroll them all here..
> 
> which then just generates a (very small) tree of if-statements.
> 
> Again, this is entirely too ugly to do unless people *really* care.
> But for situations where you have a small handful of cases known at
> compile-time, it's not out of the question, and it probably does
> generate better code.
> 
> NOTE NOTE NOTE! This is a comp[letely independent aside, and has
> nothing to do with sched_ext except for the very obvious indirect fact
> that sched_ext would be one of the classes in this kind of code.
> 
> And yes, I suspect it is too ugly to actually do this.

Very early on in the retpoline mess I briefly considered doing this, but
I decided against doing the ugly until someone came with numbers bad
enough to warrant them.

We're now many years later and I'm very glad we never really *had* to go
down that route.