From d74fc8803f514575f83c6bc19618a9f2d3df3a5a Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <fweisbec@gmail.com>
Date: Mon, 19 Jun 2017 04:12:02 +0200
Subject: [PATCH] sched/fair: Spare idle load balancing on nohz_full CPUs

References: bnc#1101669 optimise numa balancing for fast migrate
Patch-mainline: v4.13-rc1
Git-commit: 387bc8b5536eeb0a92f4b4ab553539eaea2ac0ba

Although idle load balancing obviously only concerns idle CPUs, it can
be a disturbance on a busy nohz_full CPU. Indeed a CPU can only get rid
of an idle load balancing duty once a tick fires while it runs a task
and this can take a while on a nohz_full CPU.

We could fix that and escape the idle load balancing duty from the very
idle exit path but that would bring unecessary overhead. Lets just not
bother and leave that job to housekeeping CPUs (those outside nohz_full
range). The nohz_full CPUs simply don't want any disturbance.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1497838322-10913-4-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/sched/fair.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0ac3f9c7e787..bfefddbcbae4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9118,6 +9118,10 @@ void nohz_balance_enter_idle(int cpu)
 	if (!cpu_active(cpu))
 		return;
 
+	/* Spare idle load balancing on CPUs that don't want to be disturbed: */
+	if (!is_housekeeping_cpu(cpu))
+		return;
+
 	if (test_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu)))
 		return;