[patch 62/63] locking/rtmutex: Add adaptive spinwait mechanism

[Thomas Gleixner <tglx@linutronix.de>]

From: Steven Rostedt <rostedt@goodmis.org>

Going to sleep when a spinlock or rwlock is contended can be quite
inefficient when the contention time is short and the lock owner is running
on a different CPU. The MCS mechanism is not applicable to rtmutex based
locks, so provide a simple adaptive spinwait mechanism for the RT specific
spin/rwlock implementations.

[ tglx: Provide a contemporary changelog ]

Originally-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/locking/rtmutex.c |   50 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 49 insertions(+), 1 deletion(-)
---
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -8,6 +8,11 @@
  *  Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
  *  Copyright (C) 2005 Kihon Technologies Inc., Steven Rostedt
  *  Copyright (C) 2006 Esben Nielsen
+ * Adaptive Spinlocks:
+ *  Copyright (C) 2008 Novell, Inc., Gregory Haskins, Sven Dietrich,
+ *				     and Peter Morreale,
+ * Adaptive Spinlocks simplification:
+ *  Copyright (C) 2008 Red Hat, Inc., Steven Rostedt <srostedt@redhat.com>
  *
  *  See Documentation/locking/rt-mutex-design.rst for details.
  */
@@ -1529,6 +1534,43 @@ static __always_inline int __rt_mutex_lo
  * Functions required for spin/rw_lock substitution on RT kernels
  */
 
+#ifdef CONFIG_SMP
+/*
+ * Note that owner is a speculative pointer and dereferencing relies
+ * on rcu_read_lock() and the check against the lock owner.
+ */
+static bool rtlock_adaptive_spinwait(struct rt_mutex_base *lock,
+				     struct task_struct *owner)
+{
+	bool res = true;
+
+	rcu_read_lock();
+	for (;;) {
+		/* Owner changed. Trylock again */
+		if (owner != rt_mutex_owner(lock))
+			break;
+		/*
+		 * Ensure that owner->on_cpu is dereferenced _after_
+		 * checking the above to be valid.
+		 */
+		barrier();
+		if (!owner->on_cpu) {
+			res = false;
+			break;
+		}
+		cpu_relax();
+	}
+	rcu_read_unlock();
+	return res;
+}
+#else
+static bool rtlock_adaptive_spinwait(struct rt_mutex_base *lock,
+				     struct task_struct *owner)
+{
+	return false;
+}
+#endif
+
 /**
  * rtlock_slowlock_locked - Slow path lock acquisition for RT locks
  * @lock:	The underlying rt mutex
@@ -1536,6 +1578,7 @@ static __always_inline int __rt_mutex_lo
 static void __sched rtlock_slowlock_locked(struct rt_mutex_base *lock)
 {
 	struct rt_mutex_waiter waiter;
+	struct task_struct *owner;
 
 	lockdep_assert_held(&lock->wait_lock);
 
@@ -1554,9 +1597,14 @@ static void __sched rtlock_slowlock_lock
 		if (try_to_take_rt_mutex(lock, current, &waiter))
 			break;
 
+		if (&waiter == rt_mutex_top_waiter(lock))
+			owner = rt_mutex_owner(lock);
+		else
+			owner = NULL;
 		raw_spin_unlock_irq(&lock->wait_lock);
 
-		schedule_rtlock();
+		if (!owner || !rtlock_adaptive_spinwait(lock, owner))
+			schedule_rtlock();
 
 		raw_spin_lock_irq(&lock->wait_lock);
 		set_current_state(TASK_RTLOCK_WAIT);