[PATCH v2] mutex: do not unnecessarily deal with waiters

[Davidlohr Bueso]

From: Davidlohr Bueso <davidlohr.bueso@xxxxxx>

Upon entering the slowpath, we immediately attempt to acquire the lock
by checking if it is already unlocked. If we are lucky enough that this
is the case, then we don't need to deal with any waiter related logic.

Furthermore any checks for an empty wait_list are unnecessary as we
already know that count is non-negative and hence no one is waiting for
the lock.

Move the count check and xchg calls to be done before any waiters are
setup - including waiter debugging. Upon failure to acquire the lock,
the xchg sets the counter to 0, instead of -1 as it was originally.
This can be done here since we set it back to -1 right at the beginning
of the loop so other waiters are woken up when the lock is released.

When tested on a 8-socket (80 core) system against a vanilla 3.10-rc1
kernel, this patch provides some small performance benefits (+2-6%).
While it could be considered in the noise level, the average percentages
were stable across multiple runs and no performance regressions were seen.
Two big winners, for small amounts of users (10-100), were the short and
compute workloads had a +19.36% and +%15.76% in jobs per minute.

Also change some break statements to 'goto slowpath', which IMO makes a
little more intuitive to read.

Signed-off-by: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
---
v1->v2: Rebase on -tip, dealing with the new W/W mutexes.

 kernel/mutex.c | 41 ++++++++++++++++++-----------------------
 1 file changed, 18 insertions(+), 23 deletions(-)

diff --git a/kernel/mutex.c b/kernel/mutex.c
index e581ada..61cce1f 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -460,7 +460,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 			 * performed the optimistic spinning cannot be done.
 			 */
 			if (ACCESS_ONCE(ww->ctx))
-				break;
+				goto slowpath;
 		}
 
 		/*
@@ -471,7 +471,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		owner = ACCESS_ONCE(lock->owner);
 		if (owner && !mutex_spin_on_owner(lock, owner)) {
 			mspin_unlock(MLOCK(lock), &node);
-			break;
+			goto slowpath;
 		}
 
 		if ((atomic_read(&lock->count) == 1) &&
@@ -486,8 +486,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
 			mutex_set_owner(lock);
 			mspin_unlock(MLOCK(lock), &node);
-			preempt_enable();
-			return 0;
+			goto done;
 		}
 		mspin_unlock(MLOCK(lock), &node);
 
@@ -498,7 +497,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		 * the owner complete.
 		 */
 		if (!owner && (need_resched() || rt_task(task)))
-			break;
+			goto slowpath;
 
 		/*
 		 * The cpu_relax() call is a compiler barrier which forces
@@ -512,6 +511,10 @@ slowpath:
 #endif
 	spin_lock_mutex(&lock->wait_lock, flags);
 
+	/* once more, can we acquire the lock? */
+	if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1))
+		goto skip_wait;
+
 	debug_mutex_lock_common(lock, &waiter);
 	debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
 
@@ -519,9 +522,6 @@ slowpath:
 	list_add_tail(&waiter.list, &lock->wait_list);
 	waiter.task = task;
 
-	if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
-		goto done;
-
 	lock_contended(&lock->dep_map, ip);
 
 	for (;;) {
@@ -535,7 +535,7 @@ slowpath:
 		 * other waiters:
 		 */
 		if (MUTEX_SHOW_NO_WAITER(lock) &&
-		   (atomic_xchg(&lock->count, -1) == 1))
+		    (atomic_xchg(&lock->count, -1) == 1))
 			break;
 
 		/*
@@ -560,24 +560,25 @@ slowpath:
 		schedule_preempt_disabled();
 		spin_lock_mutex(&lock->wait_lock, flags);
 	}
+	mutex_remove_waiter(lock, &waiter, current_thread_info());
+	/* set it to 0 if there are no waiters left: */
+	if (likely(list_empty(&lock->wait_list)))
+		atomic_set(&lock->count, 0);
+	debug_mutex_free_waiter(&waiter);
 
-done:
+skip_wait:
+	/* got the lock - cleanup and rejoice! */
 	lock_acquired(&lock->dep_map, ip);
-	/* got the lock - rejoice! */
-	mutex_remove_waiter(lock, &waiter, current_thread_info());
 	mutex_set_owner(lock);
 
 	if (!__builtin_constant_p(ww_ctx == NULL)) {
-		struct ww_mutex *ww = container_of(lock,
-						      struct ww_mutex,
-						      base);
+		struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
 		struct mutex_waiter *cur;
 
 		/*
 		 * This branch gets optimized out for the common case,
 		 * and is only important for ww_mutex_lock.
 		 */
-
 		ww_mutex_lock_acquired(ww, ww_ctx);
 		ww->ctx = ww_ctx;
 
@@ -591,15 +592,9 @@ done:
 		}
 	}
 
-	/* set it to 0 if there are no waiters left: */
-	if (likely(list_empty(&lock->wait_list)))
-		atomic_set(&lock->count, 0);
-
 	spin_unlock_mutex(&lock->wait_lock, flags);
-
-	debug_mutex_free_waiter(&waiter);
+done:
 	preempt_enable();
-
 	return 0;
 
 err:
-- 
1.7.11.7



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