linux-kernel - [PATCH v3 3/6] workqueue: change value of lcpu in __queue_delayed_work

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Date:	Wed, 15 Aug 2012 23:25:38 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Joonsoo Kim <js1304@...il.com>
Subject: [PATCH v3 3/6] workqueue: change value of lcpu in __queue_delayed_work_on()

We assign cpu id into work struct's data field in __queue_delayed_work_on().
In current implementation, when work is come in first time,
current running cpu id is assigned.
If we do __queue_delayed_work_on() with CPU A on CPU B,
__queue_work() invoked in delayed_work_timer_fn() go into
the following sub-optimal path in case of WQ_NON_REENTRANT.

	gcwq = get_gcwq(cpu);
	if (wq->flags & WQ_NON_REENTRANT &&
		(last_gcwq = get_work_gcwq(work)) && last_gcwq != gcwq) {

Change lcpu to @cpu and rechange lcpu to local cpu if lcpu is WORK_CPU_UNBOUND.
It is sufficient to prevent to go into sub-optimal path.

Signed-off-by: Joonsoo Kim <js1304@...il.com>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c29f2dc..32c4f79 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1356,9 +1356,16 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 	if (!(wq->flags & WQ_UNBOUND)) {
 		struct global_cwq *gcwq = get_work_gcwq(work);
 
-		if (gcwq && gcwq->cpu != WORK_CPU_UNBOUND)
+		/*
+		 * If we cannot get gcwq from work directly, we should
+		 * deliberately select last cpu not to go into sub-optimal
+		 * path of reentrance detection for delayed work. In this case,
+		 * we assign requested cpu to lcpu except WORK_CPU_UNBOUND
+		 */
+		lcpu = cpu;
+		if (gcwq)
 			lcpu = gcwq->cpu;
-		else
+		if (lcpu == WORK_CPU_UNBOUND)
 			lcpu = raw_smp_processor_id();
 	} else {
 		lcpu = WORK_CPU_UNBOUND;
-- 
1.7.9.5

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