locking/rtmutex: Implement lockless top-waiter wakeup (45ab4eff) · Commits · 戴 / test

Mark the task for later wakeup after the wait_lock has been released. This way, once the next task is awoken, it will have a better chance to of finding the wait_lock free when continuing executing in __rt_mutex_slowlock() when trying to acquire the rtmutex, calling try_to_take_rt_mutex(). Upon contended scenarios, other tasks attempting take the lock may acquire it first, right after the wait_lock is released, but (a) this can also occur with the current code, as it relies on the spinlock fairness, and (b) we are dealing with the top-waiter anyway, so it will always take the lock next. Signed-off-by:

Davidlohr Bueso <dbueso@suse.de> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Mike Galbraith <umgwanakikbuti@gmail.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/r/1432056298-18738-2-git-send-email-dave@stgolabs.net Signed-off-by:

Thomas Gleixner <tglx@linutronix.de>

kernel/locking/rtmutex.c

+10 −11

Original line number	Diff line number	Diff line
		@@ -955,14 +955,13 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
		}

		/*
		* Wake up the next waiter on the lock.
		*
		* Remove the top waiter from the current tasks pi waiter list and
		* wake it up.
		* queue it up.
		*
		* Called with lock->wait_lock held.
		*/
		static void wakeup_next_waiter(struct rt_mutex *lock)
		static void mark_wakeup_next_waiter(struct wake_q_head *wake_q,
		struct rt_mutex *lock)
		{
		struct rt_mutex_waiter *waiter;
		unsigned long flags;
		@@ -991,12 +990,7 @@ static void wakeup_next_waiter(struct rt_mutex *lock)

		raw_spin_unlock_irqrestore(&current->pi_lock, flags);

		/*
		* It's safe to dereference waiter as it cannot go away as
		* long as we hold lock->wait_lock. The waiter task needs to
		* acquire it in order to dequeue the waiter.
		*/
		wake_up_process(waiter->task);
		wake_q_add(wake_q, waiter->task);
		}

		/*
		@@ -1258,6 +1252,8 @@ static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
		static void __sched
		rt_mutex_slowunlock(struct rt_mutex *lock)
		{
		WAKE_Q(wake_q);

		raw_spin_lock(&lock->wait_lock);

		debug_rt_mutex_unlock(lock);
		@@ -1306,10 +1302,13 @@ rt_mutex_slowunlock(struct rt_mutex *lock)
		/*
		* The wakeup next waiter path does not suffer from the above
		* race. See the comments there.
		*
		* Queue the next waiter for wakeup once we release the wait_lock.
		*/
		wakeup_next_waiter(lock);
		mark_wakeup_next_waiter(&wake_q, lock);

		raw_spin_unlock(&lock->wait_lock);
		wake_up_q(&wake_q);

		/* Undo pi boosting if necessary: */
		rt_mutex_adjust_prio(current);

Admin message