linux-kernel - [PATCH 5/5] sched/fair: evenly spread tasks when not overloaded

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Date:   Fri, 19 Jul 2019 09:58:25 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org
Cc:     quentin.perret@....com, dietmar.eggemann@....com,
        Morten.Rasmussen@....com, pauld@...hat.com,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 5/5] sched/fair: evenly spread tasks when not overloaded

When there is only 1 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>
---
 kernel/sched/fair.c | 42 +++++++++++++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c221713..a60ddef 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8353,7 +8353,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 		env->src_grp_type = migrate_task;
 		imbalance = max_t(long, 0, (local->idle_cpus - busiest->idle_cpus) >> 1);
 
-		if (sds->prefer_sibling)
+		if (sds->prefer_sibling || busiest->group_weight == 1)
 			/*
 			 * When prefer sibling, evenly spread running tasks on
 			 * groups.
@@ -8531,18 +8531,34 @@ 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 to just move the imbalance on 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_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 */
-- 
2.7.4