linux-kernel - Re: [PATCH 12/36] sched_ext: Implement BPF extensible scheduler class

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Date: Fri, 22 Mar 2024 22:37:32 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Tejun Heo <tj@...nel.org>
Cc: torvalds@...ux-foundation.org, 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, 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, linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	kernel-team@...a.com, Andrea Righi <andrea.righi@...onical.com>
Subject: Re: [PATCH 12/36] sched_ext: Implement BPF extensible scheduler class

Hello Tejun,

On Fri, Nov 10, 2023 at 04:47:38PM -1000, Tejun Heo wrote:
[..]
> +/*
> + * Omitted operations:
> + *
> + * - wakeup_preempt: NOOP as it isn't useful in the wakeup path because the task
> + *   isn't tied to the CPU at that point.
> + *
> + * - migrate_task_rq: Unncessary as task to cpu mapping is transient.
> + *
> + * - task_fork/dead: We need fork/dead notifications for all tasks regardless of
> + *   their current sched_class. Call them directly from sched core instead.
> + *
> + * - task_woken, switched_from: Unnecessary.
> + */
> +DEFINE_SCHED_CLASS(ext) = {
> +	.enqueue_task		= enqueue_task_scx,
> +	.dequeue_task		= dequeue_task_scx,
> +	.yield_task		= yield_task_scx,
> +	.yield_to_task		= yield_to_task_scx,
> +
> +	.wakeup_preempt		= wakeup_preempt_scx,

I was wondering about the comment above related to 'wakeup_preempt', could
you clarify why it is not useful (NOOP) in the sched-ext class?

wakeup_preempt() may be called via:
sched_ttwu_pending() ->
	ttwu_do_activate() ->
		wakeup_preempt()
			

at which point the enqueue of the task could have already happened via:

sched_ttwu_pending() ->
	ttwu_do_activate() ->
		activate_task() ->
			enqueue_task()

But the comment above says "task isn't tied to the CPU" ?

Apologies in advance if I missed something as I just recently starting
looking into the sched-ext patches.

thanks!

 - Joel