linux-kernel - [PATCH 1/2] sched,numa: fix math underflow in task_tick

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Date:	Thu,  5 Nov 2015 15:56:22 -0500
From:	riel@...hat.com
To:	linux-kernel@...r.kernel.org
Cc:	peterz@...radead.org, mingo@...nel.org, jstancek@...hat.com,
	mgorman@...e.de
Subject: [PATCH 1/2] sched,numa: fix math underflow in task_tick_numa

From: Rik van Riel <riel@...hat.com>

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.

Signed-off-by: Rik van Riel <riel@...hat.com>
Reported-and-tested-by: Jan Stancek <jstancek@...hat.com>
---
 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 824aa9f501a3..f04fda8f669c 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;
-- 
2.1.0

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