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: Fri, 21 Jun 2024 09:46:56 -1000
From: Tejun Heo <tj@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	peterz@...radead.org, 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,
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	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,

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.

I'll update the patch description to point to the previous message just in
case and apply it to sched_ext/for-6.11.

Thanks.

-- 
tejun