[PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg

From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Yongji Xie <elohimes@gmail.com>,
	mingo@redhat.com, will.deacon@arm.com,
	linux-kernel@vger.kernel.org, xieyongji@baidu.com,
	zhangyu31@baidu.com, liuqi16@baidu.com, yuanlinsi01@baidu.com,
	nixun@baidu.com, lilin24@baidu.com, longman@redhat.com,
	andrea.parri@amarulasolutions.com
Subject: [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg
Date: Sun, 2 Dec 2018 21:31:30 -0800	[thread overview]
Message-ID: <20181203053130.gwkw6kg72azt2npb@linux-r8p5> (raw)
In-Reply-To: <20181129221714.GF11632@hirez.programming.kicks-ass.net>

The cmpxchg will fail when the task is already in the process
of waking up, and as such is an extremely rare occurrence.
Micro-optimize the call and put an unlikely() around it.

To no surprise, when using CONFIG_PROFILE_ANNOTATED_BRANCHES
under a number of workloads the incorrect rate was a mere 1-2%.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 091e089063be..f7747cf6e427 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -408,7 +408,7 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
 	 * This cmpxchg() executes a full barrier, which pairs with the full
 	 * barrier executed by the wakeup in wake_up_q().
 	 */
-	if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
+	if (unlikely(cmpxchg(&node->next, NULL, WAKE_Q_TAIL)))
 		return;
 
 	get_task_struct(task);
-- 
2.16.4

