linux-kernel - [tip:sched/core] locking/rtmutex: Implement lockless top-waiter wakeup

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Date:	Thu, 18 Jun 2015 13:30:35 -0700
From:	tip-bot for Davidlohr Bueso <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	umgwanakikbuti@...il.com, rostedt@...dmis.org, hpa@...or.com,
	tglx@...utronix.de, bigeasy@...utronix.de,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, dave@...olabs.net, dbueso@...e.de,
	mingo@...nel.org
Subject: [tip:sched/core] locking/rtmutex:
  Implement lockless top-waiter wakeup

Commit-ID:  45ab4effc3bee6f8a5cb05652b7bb895ec5b6a7a
Gitweb:     http://git.kernel.org/tip/45ab4effc3bee6f8a5cb05652b7bb895ec5b6a7a
Author:     Davidlohr Bueso <dave@...olabs.net>
AuthorDate: Tue, 19 May 2015 10:24:55 -0700
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 18 Jun 2015 22:27:46 +0200

locking/rtmutex: Implement lockless top-waiter wakeup

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@...e.de>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/1432056298-18738-2-git-send-email-dave@stgolabs.net
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/locking/rtmutex.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index b025295..44ee8f8 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -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);
--
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