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 15:32:23 -0400
From: Phil Auld <pauld@...hat.com>
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,
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	dvernet@...a.com, dschatzberg@...a.com, dskarlat@...cmu.edu,
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	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 04/30] sched: Add sched_class->switching_to() and expose
 check_class_changing/changed()

On Fri, Jun 21, 2024 at 09:18:46AM -1000 Tejun Heo wrote:
> 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()?

Fair enough.  It was just a thought.


Cheers,
Phil


> 
> Thanks.
> 
> -- 
> tejun
> 

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