linux-kernel - Re: [PATCH v2 4/7] sched/fair: Add SHARED_RUNQ sched feature and skeleton calls

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Date:   Tue, 11 Jul 2023 11:19:26 -0500
From:   David Vernet <void@...ifault.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, 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,
        gautham.shenoy@....com, kprateek.nayak@....com, aaron.lu@...el.com,
        clm@...a.com, tj@...nel.org, roman.gushchin@...ux.dev,
        kernel-team@...a.com
Subject: Re: [PATCH v2 4/7] sched/fair: Add SHARED_RUNQ sched feature and
 skeleton calls

On Tue, Jul 11, 2023 at 11:45:47AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 10, 2023 at 03:03:39PM -0500, David Vernet wrote:
> 
> > @@ -11843,6 +11871,9 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >  	if (!cpu_active(this_cpu))
> >  		return 0;
> >  
> > +	if (sched_feat(SHARED_RUNQ) && shared_runq_pick_next_task(this_rq, rf))
> > +		return -1;
> > +
> 
> Next patch implements shared_runq_pick_next_task() with the same return
> values as newidle_balance(), which would seem to suggest the following
> instead:
> 
> 	if (sched_feat(SHARED_RUNQ)) {
> 		pulled_task = shared_runq_pick_next_task(this_rq, rf);
> 		if (pulled_task)
> 			return pulled_task;
> 	}

Yep, that's cleaner.

> Additionally, I think we wants something like:
> 
> 	sd = rcu_dereference_check_sched_domain(this_rq->sd);
> 	if (sched_feat(SHARED_RUNQ)) {
> 		... /* see above */
> 
> 		sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
> 		sd = sd->parent;
> 	}
> 
> to ensure we skip <=LLC domains, since those have already been covered
> by this shiny new thing, no?

Indeed, another nice optimization. I'll incorporate both of these into
the next version.