From: Wanpeng Li <wanpeng.li@hotmail.com>
Date: Wed, 4 May 2016 06:45:34 +0000 (+0800)
Subject: sched/nohz: Fix affine unpinned timers mess
X-Git-Tag: firefly_0821_release~176^2~4^2~31^2~243
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=15abaa07a2f0dabb66dfa637162fdaa66b839141;p=firefly-linux-kernel-4.4.55.git

sched/nohz: Fix affine unpinned timers mess

commit 444969223c81c7d0a95136b7b4cfdcfbc96ac5bd upstream.

The following commit:

  9642d18eee2c ("nohz: Affine unpinned timers to housekeepers")'

intended to affine unpinned timers to housekeepers:

  unpinned timers(full dynaticks, idle)   =>   nearest busy housekeepers(otherwise, fallback to any housekeepers)
  unpinned timers(full dynaticks, busy)   =>   nearest busy housekeepers(otherwise, fallback to any housekeepers)
  unpinned timers(houserkeepers, idle)    =>   nearest busy housekeepers(otherwise, fallback to itself)

However, the !idle_cpu(i) && is_housekeeping_cpu(cpu) check modified the
intention to:

  unpinned timers(full dynaticks, idle)   =>   any housekeepers(no mattter cpu topology)
  unpinned timers(full dynaticks, busy)   =>   any housekeepers(no mattter cpu topology)
  unpinned timers(housekeepers, idle)     =>   any busy cpus(otherwise, fallback to any housekeepers)

This patch fixes it by checking if there are busy housekeepers nearby,
otherwise falls to any housekeepers/itself. After the patch:

  unpinned timers(full dynaticks, idle)   =>   nearest busy housekeepers(otherwise, fallback to any housekeepers)
  unpinned timers(full dynaticks, busy)   =>   nearest busy housekeepers(otherwise, fallback to any housekeepers)
  unpinned timers(housekeepers, idle)     =>   nearest busy housekeepers(otherwise, fallback to itself)

Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[ Fixed the changelog. ]
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Fixes: 'commit 9642d18eee2c ("nohz: Affine unpinned timers to housekeepers")'
Link: http://lkml.kernel.org/r/1462344334-8303-1-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 67d1e1597d9c..ea863bc22caf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -627,7 +627,10 @@ int get_nohz_timer_target(void)
 	rcu_read_lock();
 	for_each_domain(cpu, sd) {
 		for_each_cpu(i, sched_domain_span(sd)) {
-			if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) {
+			if (cpu == i)
+				continue;
+
+			if (!idle_cpu(i) && is_housekeeping_cpu(i)) {
 				cpu = i;
 				goto unlock;
 			}