[tip: sched/core] sched: Spare resched IPI when prio changes on a single fair task

["tip-bot2 for Frederic Weisbecker" <tip-bot2@linutronix.de>]

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

Commit-ID:     7c2e8bbd87db661122e92d71a394dd7bb3ada4d3
Gitweb:        https://git.kernel.org/tip/7c2e8bbd87db661122e92d71a394dd7bb3ada4d3
Author:        Frederic Weisbecker <frederic@kernel.org>
AuthorDate:    Tue, 03 Dec 2019 17:01:05 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 17 Dec 2019 13:32:50 +01:00

sched: Spare resched IPI when prio changes on a single fair task

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
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 08a233e..846f50b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10322,6 +10322,9 @@ 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