linux-kernel - [PATCH] workqueue: remove the unneeded cpu_relax() in __queue

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Message-ID: <1400748257-14165-1-git-send-email-laijs@cn.fujitsu.com>
Date:	Thu, 22 May 2014 16:44:16 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	<linux-kernel@...r.kernel.org>
CC:	Tejun Heo <tj@...nel.org>, Lai Jiangshan <laijs@...fujitsu.com>
Subject: [PATCH] workqueue: remove the unneeded cpu_relax() in __queue_work()

When pwq->refcnt == 0, the retrying is guaranteed to make forward-progress.
The comment above the code explains it well:

	/*
	 * pwq is determined and locked.  For unbound pools, we could have
	 * raced with pwq release and it could already be dead.  If its
	 * refcnt is zero, repeat pwq selection.  Note that pwqs never die
	 * without another pwq replacing it in the numa_pwq_tbl or while
	 * work items are executing on it, so the retrying is guaranteed to
	 * make forward-progress.
	 */

It means the cpu_relax() here is useless and sometimes misleading,
it should retry directly and make some progress rather than waste time.

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---
 kernel/workqueue.c |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 23f9a2b..98b38b5 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1368,7 +1368,6 @@ retry:
 	if (unlikely(!pwq->refcnt)) {
 		if (wq->flags & WQ_UNBOUND) {
 			spin_unlock(&pwq->pool->lock);
-			cpu_relax();
 			goto retry;
 		}
 		/* oops */
-- 
1.7.4.4

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