linux-kernel - [PATCH 4.5 018/238] sched/fair: Avoid using decay_load

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Message-Id: <20160410183457.301824235@linuxfoundation.org>
Date:	Sun, 10 Apr 2016 11:33:15 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Byungchul Park <byungchul.park@....com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Christoph Lameter <cl@...ux.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Mike Galbraith <efault@....de>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, perterz@...radead.org,
	Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 4.5 018/238] sched/fair: Avoid using decay_load_missed() with a negative value

4.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Byungchul Park <byungchul.park@....com>

commit 7400d3bbaa229eb8e7631d28fb34afd7cd2c96ff upstream.

decay_load_missed() cannot handle nagative values, so we need to prevent
using the function with a negative value.

Reported-by: Dietmar Eggemann <dietmar.eggemann@....com>
Signed-off-by: Byungchul Park <byungchul.park@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Chris Metcalf <cmetcalf@...hip.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Luiz Capitulino <lcapitulino@...hat.com>
Cc: Mike Galbraith <efault@....de>
Cc: Paul E . McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: perterz@...radead.org
Fixes: 59543275488d ("sched/fair: Prepare __update_cpu_load() to handle active tickless")
Link: http://lkml.kernel.org/r/20160115070749.GA1914@X58A-UD3R
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/sched/fair.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4459,9 +4459,17 @@ static void __update_cpu_load(struct rq

 		/* scale is effectively 1 << i now, and >> i divides by scale */

-		old_load = this_rq->cpu_load[i] - tickless_load;
+		old_load = this_rq->cpu_load[i];
 		old_load = decay_load_missed(old_load, pending_updates - 1, i);
-		old_load += tickless_load;
+		if (tickless_load) {
+			old_load -= decay_load_missed(tickless_load, pending_updates - 1, i);
+			/*
+			 * old_load can never be a negative value because a
+			 * decayed tickless_load cannot be greater than the
+			 * original tickless_load.
+			 */
+			old_load += tickless_load;
+		}
 		new_load = this_load;
 		/*
 		 * Round up the averaging division if load is increasing. This