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Date:   Tue, 11 Jul 2023 11:45:47 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Vernet <void@manifault.com>
Cc:     linux-kernel@vger.kernel.org, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        gautham.shenoy@amd.com, kprateek.nayak@amd.com, aaron.lu@intel.com,
        clm@meta.com, tj@kernel.org, roman.gushchin@linux.dev,
        kernel-team@meta.com
Subject: Re: [PATCH v2 4/7] sched/fair: Add SHARED_RUNQ sched feature and
 skeleton calls
Message-ID: <20230711094547.GE3062772@hirez.programming.kicks-ass.net>
References: <20230710200342.358255-1-void@manifault.com>
 <20230710200342.358255-5-void@manifault.com>
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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;
	}

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?