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From:   Aubrey Li <aubrey.li@intel.com>
To:     vincent.guittot@linaro.org, mingo@redhat.com, peterz@infradead.org,
        juri.lelli@redhat.com, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        linux-kernel@vger.kernel.org
Cc:     tim.c.chen@linux.intel.com, vpillai@digitalocean.com,
        joel@joelfernandes.org, Aubrey Li <aubrey.li@intel.com>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Phil Auld <pauld@redhat.com>
Subject: [PATCH] sched/fair: Fix negative imbalance in imbalance calculation
Date:   Thu, 26 Mar 2020 13:42:29 +0800
Message-Id: <1585201349-70192-1-git-send-email-aubrey.li@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

A negative imbalance value was observed after imbalance calculation,
this happens when the local sched group type is group_fully_busy,
and the average load of local group is greater than the selected
busiest group. Fix this problem by comparing the average load of the
local and busiest group before imbalance calculation formula.

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
Cc: Phil Auld <pauld@redhat.com>
---
 kernel/sched/fair.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c1217bf..4a2ba3f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8761,6 +8761,14 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 
 		sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
 				sds->total_capacity;
+		/*
+		 * If the local group is more loaded than the selected
+		 * busiest group don't try to pull any tasks.
+		 */
+		if (local->avg_load >= busiest->avg_load) {
+			env->imbalance = 0;
+			return;
+		}
 	}
 
 	/*
-- 
2.7.4