linux-kernel - [PATCH] locking/mutex: Allow next waiter lockless wakeup

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Message-ID: <20160125022343.GA3322@linux-uzut.site>
Date:	Sun, 24 Jan 2016 18:23:43 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <waiman.long@....com>
Cc:	Ding Tianhong <dingtianhong@...wei.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Jason Low <jason.low2@...com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: [PATCH] locking/mutex: Allow next waiter lockless wakeup

Make use of wake_q and enable the wakeup to occur after
releasing the wait_lock. This is similar to what we do
with rtmutex top waiter, slightly shortening the critical
region and allow other waiters to acquire the wait_lock
sooner. In low contention cases it can also help the
recently woken waiter to find the wait_lock available
(fastpath) when it continues execution.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
  kernel/locking/mutex.c | 5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 0551c21..e364b42 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -716,6 +716,7 @@ static inline void
  __mutex_unlock_common_slowpath(struct mutex *lock, int nested)
  {
  	unsigned long flags;
+	WAKE_Q(wake_q);
  
  	/*
  	 * As a performance measurement, release the lock before doing other
@@ -743,11 +744,11 @@ __mutex_unlock_common_slowpath(struct mutex *lock, int nested)
  					   struct mutex_waiter, list);
  
  		debug_mutex_wake_waiter(lock, waiter);
-
-		wake_up_process(waiter->task);
+		wake_q_add(&wake_q, waiter->task);
  	}
  
  	spin_unlock_mutex(&lock->wait_lock, flags);
+	wake_up_q(&wake_q);
  }
  
  /*
-- 
2.1.4