linux-kernel - [tip: sched/core] sched/fair: Spread out tasks evenly when not overloaded

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Message-ID: <157164916243.29376.10093977370174875453.tip-bot2@tip-bot2>
Date:   Mon, 21 Oct 2019 09:12:42 -0000
From:   "tip-bot2 for Vincent Guittot" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Mike Galbraith <efault@....de>,
        Morten.Rasmussen@....com, Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>, hdanton@...a.com,
        parth@...ux.ibm.com, pauld@...hat.com, quentin.perret@....com,
        riel@...riel.com, srikar@...ux.vnet.ibm.com,
        valentin.schneider@....com, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/fair: Spread out tasks evenly when not overloaded

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     2ab4092fc82d6001fdd9d51dbba27d04dec967e0
Gitweb:        https://git.kernel.org/tip/2ab4092fc82d6001fdd9d51dbba27d04dec967e0
Author:        Vincent Guittot <vincent.guittot@...aro.org>
AuthorDate:    Fri, 18 Oct 2019 15:26:34 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 21 Oct 2019 09:40:54 +02:00

sched/fair: Spread out tasks evenly when not overloaded

When there is only one CPU per group, using the idle CPUs to evenly spread
tasks doesn't make sense and nr_running is a better metrics.

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Mike Galbraith <efault@....de>
Cc: Morten.Rasmussen@....com
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: hdanton@...a.com
Cc: parth@...ux.ibm.com
Cc: pauld@...hat.com
Cc: quentin.perret@....com
Cc: riel@...riel.com
Cc: srikar@...ux.vnet.ibm.com
Cc: valentin.schneider@....com
Link: https://lkml.kernel.org/r/1571405198-27570-8-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e6a3db0..f489f60 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8591,18 +8591,34 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
 	    busiest->sum_nr_running > local->sum_nr_running + 1)
 		goto force_balance;
 
-	if (busiest->group_type != group_overloaded &&
-	     (env->idle == CPU_NOT_IDLE ||
-	      local->idle_cpus <= (busiest->idle_cpus + 1)))
-		/*
-		 * If the busiest group is not overloaded
-		 * and there is no imbalance between this and busiest group
-		 * wrt. idle CPUs, it is balanced. The imbalance
-		 * becomes significant if the diff is greater than 1 otherwise
-		 * we might end up just moving the imbalance to another
-		 * group.
-		 */
-		goto out_balanced;
+	if (busiest->group_type != group_overloaded) {
+		if (env->idle == CPU_NOT_IDLE)
+			/*
+			 * If the busiest group is not overloaded (and as a
+			 * result the local one too) but this CPU is already
+			 * busy, let another idle CPU try to pull task.
+			 */
+			goto out_balanced;
+
+		if (busiest->group_weight > 1 &&
+		    local->idle_cpus <= (busiest->idle_cpus + 1))
+			/*
+			 * If the busiest group is not overloaded
+			 * and there is no imbalance between this and busiest
+			 * group wrt idle CPUs, it is balanced. The imbalance
+			 * becomes significant if the diff is greater than 1
+			 * otherwise we might end up to just move the imbalance
+			 * on another group. Of course this applies only if
+			 * there is more than 1 CPU per group.
+			 */
+			goto out_balanced;
+
+		if (busiest->sum_h_nr_running == 1)
+			/*
+			 * busiest doesn't have any tasks waiting to run
+			 */
+			goto out_balanced;
+	}
 
 force_balance:
 	/* Looks like there is an imbalance. Compute it */