linux-kernel - Re: [PATCH 04/30] sched: Add sched_class->switching_to() and expose check_class

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Date: Fri, 21 Jun 2024 09:18:46 -1000
From: Tejun Heo <tj@...nel.org>
To: Phil Auld <pauld@...hat.com>
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,
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	joel@...lfernandes.org, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 04/30] sched: Add sched_class->switching_to() and expose
 check_class_changing/changed()

Hello, Phil.

On Fri, Jun 21, 2024 at 12:53:27PM -0400, Phil Auld wrote:
> > A new BPF extensible sched_class will have callbacks that allow the BPF
> > scheduler to keep track of relevant task states (like priority and cpumask).
> > Those callbacks aren't called while a task is on a different sched_class.
> > When a task comes back, we wanna tell the BPF progs the up-to-date state
> 
> "wanna" ?   How about "want to"?
> 
> That makes me wanna stop reading right there... :)

Sorry about that. Have been watching for it recently but this log was
written a while ago, so...

> > +/*
> > + * ->switching_to() is called with the pi_lock and rq_lock held and must not
> > + * mess with locking.
> > + */
> > +void check_class_changing(struct rq *rq, struct task_struct *p,
> > +			  const struct sched_class *prev_class)
> > +{
> > +	if (prev_class != p->sched_class && p->sched_class->switching_to)
> > +		p->sched_class->switching_to(rq, p);
> > +}
> 
> Does this really need wrapper? The compiler may help but it doesn't seem to
> but you're doing a function call and passing in prev_class just to do a
> simple check.  I guess it's not really a fast path. Just seemed like overkill.

This doesn't really matter either way but wouldn't it look weird if it's not
symmetric with check_class_changed()?

Thanks.

-- 
tejun