af_unix: Optimise hash table layout.

Commit 6dd4142fb5a9 ("Merge branch 'af_unix-per-netns-socket-hash'") and
commit 51bae889fe11 ("af_unix: Put pathname sockets in the global hash
table.") changed a hash table layout.

Before:
unix_socket_table [0 - 255] : abstract & pathname sockets
[256 - 511] : unnamed sockets

After:
per-netns table [0 - 255] : abstract & pathname sockets
[256 - 511] : unnamed sockets
bsd_socket_table [0 - 255] : pathname sockets (sk_bind_node)

Now, while looking up sockets, we traverse the global table for the
pathname sockets and the first half of each per-netns hash table for
abstract sockets, where pathname sockets are also linked. Thus, the
more pathname sockets we have, the longer we take to look up abstract
sockets. This characteristic has been there before the layout change,
but we can improve it now.

This patch changes the per-netns hash table's layout so that sockets not
requiring lookup reside in the first half and do not impact the lookup of
abstract sockets.

per-netns table [0 - 255] : pathname & unnamed sockets
[256 - 511] : abstract sockets
bsd_socket_table [0 - 255] : pathname sockets (sk_bind_node)

We have run a test that bind()s 100,000 abstract/pathname sockets for
each, bind()s an abstract socket 100,000 times and measures the time
on __unix_find_socket_byname(). The result shows that the patch makes
each lookup faster.

Without this patch:
$ sudo ./funclatency -p 2278 --microseconds __unix_find_socket_byname.isra.44
usec : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 126 | |
16 -> 31 : 1438 |* |
32 -> 63 : 4150 |*** |
64 -> 127 : 9049 |******* |
128 -> 255 : 37704 |******************************* |
256 -> 511 : 47533 |****************************************|

With this patch:
$ sudo ./funclatency -p 3648 --microseconds __unix_find_socket_byname.isra.46
usec : count distribution
0 -> 1 : 109 | |
2 -> 3 : 318 | |
4 -> 7 : 725 | |
8 -> 15 : 2501 |* |
16 -> 31 : 3061 |** |
32 -> 63 : 4028 |*** |
64 -> 127 : 9312 |******* |
128 -> 255 : 51372 |****************************************|
256 -> 511 : 28574 |********************** |

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/unix/af_unix.c

···
 	hash ^= hash >> 8;
 	hash ^= sk->sk_type;
 
-	return UNIX_HASH_MOD + 1 + (hash & UNIX_HASH_MOD);
+	return hash & UNIX_HASH_MOD;
 }
 
 static unsigned int unix_bsd_hash(struct inode *i)
···
 	hash ^= hash >> 8;
 	hash ^= type;
 
-	return hash & UNIX_HASH_MOD;
+	return UNIX_HASH_MOD + 1 + (hash & UNIX_HASH_MOD);
 }
 
 static void unix_table_double_lock(struct net *net,
 				   unsigned int hash1, unsigned int hash2)
 {
-	/* hash1 and hash2 is never the same because
-	 * one is between 0 and UNIX_HASH_MOD, and
-	 * another is between UNIX_HASH_MOD + 1 and UNIX_HASH_SIZE - 1.
-	 */
+	if (hash1 == hash2) {
+		spin_lock(&net->unx.table.locks[hash1]);
+		return;
+	}
+
 	if (hash1 > hash2)
 		swap(hash1, hash2);
 
···
 static void unix_table_double_unlock(struct net *net,
 				     unsigned int hash1, unsigned int hash2)
 {
+	if (hash1 == hash2) {
+		spin_unlock(&net->unx.table.locks[hash1]);
+		return;
+	}
+
 	spin_unlock(&net->unx.table.locks[hash1]);
 	spin_unlock(&net->unx.table.locks[hash2]);
 }