linux-kernel - [PATCH 1/2] sched: Spare resched IPI when prio changes on a single fair task

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Message-Id: <20191203160106.18806-2-frederic@kernel.org>
Date:   Tue,  3 Dec 2019 17:01:05 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH 1/2] 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@...radead.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/sched/fair.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 08a233e97a01..6d2560cb24f0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10322,6 +10322,8 @@ 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
-- 
2.23.0