linux-kernel - [patch] BFS 421: remove mem barrier when setting task cpu

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Date:	Thu, 14 Jun 2012 20:25:03 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>,
	Hillf Danton <dhillf@...il.com>
Subject: [patch] BFS 421: remove mem barrier when setting task cpu

With runqueue lock held, update tasks cpu without mem barrier.

--- a/kernel/sched/bfs.c	Thu Jun 14 20:05:42 2012
+++ b/kernel/sched/bfs.c	Thu Jun 14 20:06:22 2012
@@ -1034,13 +1034,8 @@ void set_task_cpu(struct task_struct *p,
 	if (task_cpu(p) != cpu)
 		perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, NULL, 0);

-	/*
-	 * After ->cpu is set up to a new value, task_grq_lock(p, ...) can be
-	 * successfully executed on another CPU. We must ensure that updates of
-	 * per-task data have been completed by this moment.
-	 */
-	smp_wmb();
-	task_thread_info(p)->cpu = cpu;
+	if (task_thread_info(p)->cpu != cpu)
+		task_thread_info(p)->cpu = cpu;
 }

 static inline void clear_sticky(struct task_struct *p)
--
--
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