Re: [PATCH] locking/mutex: Allow next waiter lockless wakeup

From: Waiman Long
Date: Mon Jan 25 2016 - 18:02:39 EST

On 01/24/2016 09:23 PM, Davidlohr Bueso wrote:
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@xxxxxxx>
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);


This patch looks good to me.

Reviewed-by: Waiman Long <Waiman.Long@xxxxxxx>