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

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Date:	Mon, 25 Jan 2016 18:02:29 -0500
From:	Waiman Long <waiman.long@....com>
To:	Davidlohr Bueso <dave@...olabs.net>
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: Re: [PATCH] locking/mutex: Allow next waiter lockless wakeup

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@...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);
>  }
>
>  /*

This patch looks good to me.

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