RE: [PATCH] Split futex global spinlock futex_lock

[Hu, Boris]

Sound good. Here is the update of the patch. :)

--- bk-linux-2.6/kernel/futex.c	2003-09-10 13:16:54.000000000 +0800
+++ bk-linux-2.6.dev/kernel/futex.c	2003-09-11 14:57:08.000000000
+0800
@@ -79,9 +79,17 @@
 	struct file *filp;
 };
 
+/*
+ * Split the global futex_lock into every hash list lock.
+ */
+struct futex_hash_bucket {
+       spinlock_t              lock;
+       struct list_head       chain;
+};
+
 /* The key for the hash is the address + index + offset within page */
-static struct list_head futex_queues[1<<FUTEX_HASHBITS];
-static spinlock_t futex_lock = SPIN_LOCK_UNLOCKED;
+static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS] \
+	__cacheline_aligned_in_smp;
 
 /* Futex-fs vfsmount entry: */
 static struct vfsmount *futex_mnt;
@@ -89,7 +97,7 @@
 /*
  * We hash on the keys returned from get_futex_key (see below).
  */
-static inline struct list_head *hash_futex(union futex_key *key)
+static inline struct futex_hash_bucket *hash_futex(union futex_key
*key)
 {
 	return &futex_queues[hash_long(key->both.word
 				       + (unsigned long) key->both.ptr
@@ -214,6 +222,7 @@
 static int futex_wake(unsigned long uaddr, int num)
 {
 	struct list_head *i, *next, *head;
+	struct futex_hash_bucket *bh;
 	union futex_key key;
 	int ret;
 
@@ -223,9 +232,10 @@
 	if (unlikely(ret != 0))
 		goto out;
 
-	head = hash_futex(&key);
+	bh = hash_futex(&key);
+	spin_lock(&bh->lock);
+	head = &bh->chain;
 
-	spin_lock(&futex_lock);
 	list_for_each_safe(i, next, head) {
 		struct futex_q *this = list_entry(i, struct futex_q,
list);
 
@@ -239,7 +249,7 @@
 				break;
 		}
 	}
-	spin_unlock(&futex_lock);
+	spin_unlock(&bh->lock);
 
 out:
 	up_read(&current->mm->mmap_sem);
@@ -254,6 +264,7 @@
 				int nr_wake, int nr_requeue)
 {
 	struct list_head *i, *next, *head1, *head2;
+	struct futex_hash_bucket *bh1, *bh2;
 	union futex_key key1, key2;
 	int ret;
 
@@ -266,10 +277,19 @@
 	if (unlikely(ret != 0))
 		goto out;
 
-	head1 = hash_futex(&key1);
-	head2 = hash_futex(&key2);
+	bh1 = hash_futex(&key1);
+	bh2 = hash_futex(&key2);
+	if (bh1 < bh2) {
+		spin_lock(&bh1->lock);
+		spin_lock(&bh2->lock);
+	} else {
+		spin_lock(&bh2->lock);
+		if (bh1 > bh2)
+			spin_lock(&bh1->lock);
+	}
+	head1 = &bh1->chain;
+	head2 = &bh2->chain;
 
-	spin_lock(&futex_lock);
 	list_for_each_safe(i, next, head1) {
 		struct futex_q *this = list_entry(i, struct futex_q,
list);
 
@@ -291,8 +311,14 @@
 			}
 		}
 	}
-	spin_unlock(&futex_lock);
-
+	if (bh1 < bh2) {
+		spin_unlock(&bh2->lock);
+		spin_unlock(&bh1->lock);
+	} else {
+		if (bh1 > bh2)
+			spin_unlock(&bh1->lock);
+		spin_unlock(&bh2->lock);
+	}
 out:
 	up_read(&current->mm->mmap_sem);
 	return ret;
@@ -301,28 +327,30 @@
 static inline void queue_me(struct futex_q *q, union futex_key *key,
 			    int fd, struct file *filp)
 {
-	struct list_head *head = hash_futex(key);
+	struct futex_hash_bucket *bh = hash_futex(key);
+	struct list_head *head = &bh->chain;
 
 	q->key = *key;
 	q->fd = fd;
 	q->filp = filp;
 
-	spin_lock(&futex_lock);
+	spin_lock(&bh->lock);
 	list_add_tail(&q->list, head);
-	spin_unlock(&futex_lock);
+	spin_unlock(&bh->lock);
 }
 
 /* Return 1 if we were still queued (ie. 0 means we were woken) */
 static inline int unqueue_me(struct futex_q *q)
 {
+	struct futex_hash_bucket *bh = hash_futex(&q->key);
 	int ret = 0;
 
-	spin_lock(&futex_lock);
+	spin_lock(&bh->lock);
 	if (!list_empty(&q->list)) {
 		list_del(&q->list);
 		ret = 1;
 	}
-	spin_unlock(&futex_lock);
+	spin_unlock(&bh->lock);
 	return ret;
 }
 
@@ -332,6 +360,7 @@
 	int ret, curval;
 	union futex_key key;
 	struct futex_q q;
+	struct futex_hash_bucket *bh = NULL;
 
  try_again:
 	init_waitqueue_head(&q.waiters);
@@ -373,19 +402,20 @@
 	 * the waiter from the list.
 	 */
 	add_wait_queue(&q.waiters, &wait);
-	spin_lock(&futex_lock);
+	bh = hash_futex(&key);
+	spin_lock(&bh->lock);
 	set_current_state(TASK_INTERRUPTIBLE);
 
 	if (unlikely(list_empty(&q.list))) {
 		/*
 		 * We were woken already.
 		 */
-		spin_unlock(&futex_lock);
+		spin_unlock(&bh->lock);
 		set_current_state(TASK_RUNNING);
 		return 0;
 	}
 
-	spin_unlock(&futex_lock);
+	spin_unlock(&bh->lock);
 	time = schedule_timeout(time);
 	set_current_state(TASK_RUNNING);
 
@@ -435,13 +465,14 @@
 			       struct poll_table_struct *wait)
 {
 	struct futex_q *q = filp->private_data;
+	struct futex_hash_bucket *bh = hash_futex(&q->key);
 	int ret = 0;
 
 	poll_wait(filp, &q->waiters, wait);
-	spin_lock(&futex_lock);
+	spin_lock(&bh->lock);
 	if (list_empty(&q->list))
 		ret = POLLIN | POLLRDNORM;
-	spin_unlock(&futex_lock);
+	spin_unlock(&bh->lock);
 
 	return ret;
 }
@@ -587,8 +618,10 @@
 	register_filesystem(&futex_fs_type);
 	futex_mnt = kern_mount(&futex_fs_type);
 
-	for (i = 0; i < ARRAY_SIZE(futex_queues); i++)
-		INIT_LIST_HEAD(&futex_queues[i]);
+	for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
+		INIT_LIST_HEAD(&futex_queues[i].chain);
+		futex_queues[i].lock = SPIN_LOCK_UNLOCKED;
+	}
 	return 0;
 }
 __initcall(init);



> Hu, Boris wrote:
> > Split futex global spinlock futex_lock into hash bucket spinlocks.
> 
> > +/*
> > + * Split the global futex_lock into every hash list lock.
> > + */
> > +struct futex_hash_bucket {
> > +       struct list_head        chain;
> > +       spinlock_t              lock;
> > +};
> 
> Put "lock" first: it is always the first field accessed.  That will
> save a few clock cycles on some systems.
> 
> I was going to suggest something about cache alignment, but of course
> that doesn't make sense.  If the structure is made any larger,
> it might as well contain more hash buckets.
> 
> Thinking a little deeper, it occurs to me that for scalable SMP
> performance, you want:
> 
>   (1 << FUTEX_HASHBITS) > some factor * SMP_CACHE_BYTES * NR_CPUS /
sizeof
> (bucket)
> 
> To put it into perspective, consider a hypothetical 16-way P4.
> SMP_CACHE_BYTES is 128 on a P4.  That's 24 cache lines in the whole
hash
> table.
> 
> If there are only a few futexes in the table at any time, the dominant
> time for each operation is going to be the spinlock and associated
> cache line transfers, not traversing a bucket's list.
> 
> So that hypothetical box would have an effective hash table of only 24
> buckets.
> 
> What I'm saying is:
> 
> 	If you're able to benchmark changes to the code on a big box,
> 	and you want to tune it's performance, try changing
> 	FUTEX_HASHBITS.  Also try adding a dummy word into
> 	futex_hash_bucket, and use __cache_aligned_in_smp on the array
so
> that
> 	no buckets straddle two cache lines.
> 
> Thought for the day...
> -- Jamie
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