From d7bf7e976d8d74627507fb225c1cbc26da07c335 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Tue, 3 Dec 2019 17:01:05 +0100
Subject: [PATCH] sched: Spare resched IPI when prio changes on a single fair
 task

References: bnc#1155798 (CPU scheduler functional and performance backports)
Patch-mainline: v5.6-rc1
Git-commit: 7c2e8bbd87db661122e92d71a394dd7bb3ada4d3

The runqueue of a fair task being remotely reniced is going to get a
resched IPI in order to reassess which task should be the current
running on the CPU. However that evaluation is useless if the fair task
is running alone, in which case we can spare that IPI, preventing
nohz_full CPUs from being disturbed.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20191203160106.18806-2-frederic@kernel.org
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 13bbf7ee50e2..fba6de07db5e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10369,6 +10369,9 @@ prio_changed_fair(struct rq *rq, struct task_struct *p, int oldprio)
 	if (!task_on_rq_queued(p))
 		return;
 
+	if (rq->cfs.nr_running == 1)
+		return;
+
 	/*
 	 * Reschedule if we are currently running on this runqueue and
 	 * our priority decreased, or if we are not currently running on