linux-kernel - Re: [PATCH -tip 12/32] sched: Simplify the core pick loop for optimized case

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Date:   Tue, 24 Nov 2020 13:04:38 +0100
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, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 12/32] sched: Simplify the core pick loop for
 optimized case

On Tue, Nov 17, 2020 at 06:19:42PM -0500, Joel Fernandes (Google) wrote:
> +	/*
> +	 * Optimize for common case where this CPU has no cookies
> +	 * and there are no cookied tasks running on siblings.
> +	 */
> +	if (!need_sync) {
> +		for_each_class(class) {
> +			next = class->pick_task(rq);
> +			if (next)
> +				break;
> +		}
> +
> +		if (!next->core_cookie) {
> +			rq->core_pick = NULL;
> +			goto done;
> +		}
>  		need_sync = true;
>  	}

This isn't what I send you here:

  https://lkml.kernel.org/r/20201026093131.GF2628@hirez.programming.kicks-ass.net

Specifically, you've lost the whole cfs-cgroup optimization.

What was wrong/not working with the below?

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5225,8 +5227,6 @@ pick_next_task(struct rq *rq, struct tas
 		return next;
 	}
 
-	put_prev_task_balance(rq, prev, rf);
-
 	smt_mask = cpu_smt_mask(cpu);
 	need_sync = !!rq->core->core_cookie;
 
@@ -5255,17 +5255,14 @@ pick_next_task(struct rq *rq, struct tas
 	 * and there are no cookied tasks running on siblings.
 	 */
 	if (!need_sync) {
-		for_each_class(class) {
-			next = class->pick_task(rq);
-			if (next)
-				break;
-		}
-
+		next = __pick_next_task(rq, prev, rf);
 		if (!next->core_cookie) {
 			rq->core_pick = NULL;
-			goto done;
+			return next;
 		}
-		need_sync = true;
+		put_prev_task(next);
+	} else {
+		put_prev_task_balance(rq, prev, rf);
 	}
 
 	for_each_cpu(i, smt_mask) {