linux-kernel - Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and scheduling.

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Date:   Fri, 23 Oct 2020 15:54:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and
 scheduling.

On Fri, Oct 23, 2020 at 03:51:29PM +0200, Peter Zijlstra wrote:
> 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.

That is; it should be the exact counterpart of this bit:

> +			/*
> +			 * Optimize the 'normal' case where there aren't any
> +			 * cookies and we don't need to sync up.
> +			 */
> +			if (i == cpu && !need_sync && !p->core_cookie) {
> +				next = p;
> +				goto done;
> +			}

If there is no task found in this class, try the next class, if there
is, we done.