locking/mutex: Allow next waiter lockless wakeup

Davidlohr Bueso · ingomolnar · commit 1329ce6fbbe4 · 2016-02-29T10:02:42.000+01:00
Make use of wake-queues 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. Reviewed-by: Waiman Long <Waiman.Long@hpe.com> Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Ding Tianhong <dingtianhong@huawei.com> Cc: Jason Low <jason.low2@hp.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul E. McKenney <paulmck@us.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Waiman Long <waiman.long@hpe.com> Cc: Will Deacon <Will.Deacon@arm.com> Link: http://lkml.kernel.org/r/20160125022343.GA3322@linux-uzut.site Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git 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);
 }
 
 /*

Original file line number	Diff line number	Diff line change
`@@ -716,6 +716,7 @@ static inline void`
`716`	`716`	`__mutex_unlock_common_slowpath(struct mutex *lock, int nested)`
`717`	`717`	`{`
`718`	`718`	`unsigned long flags;`
	`719`	`+ WAKE_Q(wake_q);`
`719`	`720`
`720`	`721`	`/*`
`721`	`722`	`* As a performance measurement, release the lock before doing other`
`@@ -743,11 +744,11 @@ __mutex_unlock_common_slowpath(struct mutex *lock, int nested)`
`743`	`744`	`struct mutex_waiter, list);`
`744`	`745`
`745`	`746`	`debug_mutex_wake_waiter(lock, waiter);`
`746`		`-`
`747`		`- wake_up_process(waiter->task);`
	`747`	`+ wake_q_add(&wake_q, waiter->task);`
`748`	`748`	`}`
`749`	`749`
`750`	`750`	`spin_unlock_mutex(&lock->wait_lock, flags);`
	`751`	`+ wake_up_q(&wake_q);`
`751`	`752`	`}`
`752`	`753`
`753`	`754`	`/*`