linux-kernel - [tip:sched/urgent] sched/numa: Fix math underflow in task_tick

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Message-ID: <tip-25b3e5a3344e1f700c1efec5b6f0199f04707fb1@git.kernel.org>
Date:	Mon, 9 Nov 2015 22:40:25 -0800
From:	tip-bot for Rik van Riel <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	riel@...hat.com, jstancek@...hat.com, mingo@...nel.org,
	linux-kernel@...r.kernel.org, hpa@...or.com, peterz@...radead.org,
	tglx@...utronix.de, torvalds@...ux-foundation.org
Subject: [tip:sched/urgent] sched/numa:
  Fix math underflow in task_tick_numa()

Commit-ID:  25b3e5a3344e1f700c1efec5b6f0199f04707fb1
Gitweb:     http://git.kernel.org/tip/25b3e5a3344e1f700c1efec5b6f0199f04707fb1
Author:     Rik van Riel <riel@...hat.com>
AuthorDate: Thu, 5 Nov 2015 15:56:22 -0500
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 9 Nov 2015 16:13:27 +0100

sched/numa: Fix math underflow in task_tick_numa()

The NUMA balancing code implements delays in scanning by
advancing curr->node_stamp beyond curr->se.sum_exec_runtime.

With unsigned math, that creates an underflow, which results
in task_numa_work being queued all the time, even when we
don't want to.

Avoiding the math underflow makes it possible to reduce CPU
overhead in the NUMA balancing code.

Reported-and-tested-by: Jan Stancek <jstancek@...hat.com>
Signed-off-by: Rik van Riel <riel@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: mgorman@...e.de
Link: http://lkml.kernel.org/r/1446756983-28173-2-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 824aa9f..f04fda8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2302,7 +2302,7 @@ void task_tick_numa(struct rq *rq, struct task_struct *curr)
 	now = curr->se.sum_exec_runtime;
 	period = (u64)curr->numa_scan_period * NSEC_PER_MSEC;
 
-	if (now - curr->node_stamp > period) {
+	if (now > curr->node_stamp + period) {
 		if (!curr->node_stamp)
 			curr->numa_scan_period = task_scan_min(curr);
 		curr->node_stamp += period;
--
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