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Date:   Fri, 23 Oct 2020 15:51:29 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
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Subject: Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and
 scheduling.
Message-ID: <20201023135129.GS2611@hirez.programming.kicks-ass.net>
References: <20201020014336.2076526-1-joel@joelfernandes.org>
 <20201020014336.2076526-7-joel@joelfernandes.org>
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On Mon, Oct 19, 2020 at 09:43:16PM -0400, Joel Fernandes (Google) wrote:
> +			/*
> +			 * If this sibling doesn't yet have a suitable task to
> +			 * run; ask for the most elegible task, given the
> +			 * highest priority task already selected for this
> +			 * core.
> +			 */
> +			p = pick_task(rq_i, class, max);
> +			if (!p) {
> +				/*
> +				 * If there weren't no cookies; we don't need to
> +				 * bother with the other siblings.
> +				 * If the rest of the core is not running a tagged
> +				 * task, i.e.  need_sync == 0, and the current CPU
> +				 * which called into the schedule() loop does not
> +				 * have any tasks for this class, skip selecting for
> +				 * other siblings since there's no point. We don't skip
> +				 * for RT/DL because that could make CFS force-idle RT.
> +				 */
> +				if (i == cpu && !need_sync && class == &fair_sched_class)
> +					goto next_class;
> +
> +				continue;
> +			}

I'm failing to understand the class == &fair_sched_class bit.

IIRC the condition is such that the core doesn't have a cookie (we don't
need to sync the threads) so we'll only do a pick for our local CPU.

That should be invariant of class.