linux-kernel - [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q

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Message-ID: <20181203053130.gwkw6kg72azt2npb@linux-r8p5>
Date:   Sun, 2 Dec 2018 21:31:30 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Yongji Xie <elohimes@...il.com>, mingo@...hat.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        xieyongji@...du.com, zhangyu31@...du.com, liuqi16@...du.com,
        yuanlinsi01@...du.com, nixun@...du.com, lilin24@...du.com,
        longman@...hat.com, andrea.parri@...rulasolutions.com
Subject: [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg

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@...e.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