[PATCH v3] lib/dlock-list: Scale dlock_lists_empty()

[Davidlohr Bueso]

Instead of the current O(N) implementation, at the cost
of adding an atomic counter, we can convert the call to
an atomic_read(). The counter only serves for accounting
empty to non-empty transitions, and vice versa; therefore
only modified twice for each of the lists during the
lifetime of the dlock (while used).

In addition, to be able to unaccount a list_del(), we
add a dlist pointer to each head, thus minimizing the
overall memory footprint.

Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---

Changes from v2: Removed some bogus lines and an overoptimistic
               changelog.

include/linux/dlock-list.h |  2 ++
lib/dlock-list.c           | 52 +++++++++++++++++++++++++++++++++++++---------
2 files changed, 44 insertions(+), 10 deletions(-)

diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
index c00c7f92ada4..d176a2d00cd1 100644
--- a/include/linux/dlock-list.h
+++ b/include/linux/dlock-list.h
@@ -32,10 +32,12 @@
struct dlock_list_head {
	struct list_head list;
	spinlock_t lock;
+	struct dlock_list_heads *dlist;
} ____cacheline_aligned_in_smp;

struct dlock_list_heads {
	struct dlock_list_head *heads;
+	atomic_t waiters;
};

/*
diff --git a/lib/dlock-list.c b/lib/dlock-list.c
index a4ddecc01b12..5814e42c5b81 100644
--- a/lib/dlock-list.c
+++ b/lib/dlock-list.c
@@ -122,8 +122,11 @@ int __alloc_dlock_list_heads(struct dlock_list_heads *dlist,

		INIT_LIST_HEAD(&head->list);
		head->lock = __SPIN_LOCK_UNLOCKED(&head->lock);
+		head->dlist = dlist;
		lockdep_set_class(&head->lock, key);
	}
+
+	atomic_set(&dlist->waiters, 0);
	return 0;
}
EXPORT_SYMBOL(__alloc_dlock_list_heads);
@@ -139,29 +142,36 @@ void free_dlock_list_heads(struct dlock_list_heads *dlist)
{
	kfree(dlist->heads);
	dlist->heads = NULL;
+	atomic_set(&dlist->waiters, 0);
}
EXPORT_SYMBOL(free_dlock_list_heads);

/**
 * dlock_lists_empty - Check if all the dlock lists are empty
 * @dlist: Pointer to the dlock_list_heads structure
- * Return: true if list is empty, false otherwise.
 *
- * This can be a pretty expensive function call. If this function is required
- * in a performance critical path, we may have to maintain a global count
- * of the list entries in the global dlock_list_heads structure instead.
+ * Return: true if all dlock lists are empty, false otherwise.
 */
bool dlock_lists_empty(struct dlock_list_heads *dlist)
{
-	int idx;
-
	/* Shouldn't be called before nr_dlock_lists is initialized */
	WARN_ON_ONCE(!nr_dlock_lists);

-	for (idx = 0; idx < nr_dlock_lists; idx++)
-		if (!list_empty(&dlist->heads[idx].list))
-			return false;
-	return true;
+	/*
+	 * Serialize dlist->waiters such that a 0->1 transition is not missed
+	 * by another thread checking if any of the dlock lists are used.
+	 *
+	 * CPU0				    CPU1
+	 * dlock_list_add()                 dlock_lists_empty()
+	 *   [S] atomic_inc(waiters);
+	 *       smp_mb__after_atomic();
+	 *					  smp_mb__before_atomic();
+	 *				      [L] atomic_read(waiters)
+	 *       list_add()
+	 *
+	 */
+	smp_mb__before_atomic();
+	return !atomic_read(&dlist->waiters);
}
EXPORT_SYMBOL(dlock_lists_empty);

@@ -179,6 +189,16 @@ void dlock_lists_add(struct dlock_list_node *node,
	struct dlock_list_head *head = &dlist->heads[this_cpu_read(cpu2idx)];

	/*
+	 * Bump the waiters counter _before_ taking the head->lock
+	 * such that we don't miss a thread adding itself to a list
+	 * while spinning for the lock.
+	 */
+	if (list_empty_careful(&head->list)) {
+		atomic_inc(&dlist->waiters);
+		smp_mb__after_atomic();
+	}
+
+	/*
	 * There is no need to disable preemption
	 */
	spin_lock(&head->lock);
@@ -212,6 +232,18 @@ void dlock_lists_del(struct dlock_list_node *node)
		spin_lock(&head->lock);
		if (likely(head == node->head)) {
			list_del_init(&node->list);
+			/*
+			 * We still hold the head->lock, a normal list_empty()
+			 * check will do.
+			 */
+			if (list_empty(&head->list)) {
+				struct dlock_list_heads *dlist;
+				dlist = node->head->dlist;
+
+				atomic_dec(&dlist->waiters);
+				smp_mb__after_atomic();
+			}
+
			node->head = NULL;
			retry = false;
		} else {
--
2.13.6