Merge branch 'UDP-sock_wfree-opts'

Pavel Begunkov says:

====================
UDP sock_wfree optimisations

The series is not UDP specific but that the main beneficiary. 2/3 saves one
atomic in sock_wfree() and on top 3/3 removes an extra barrier.
Tested with UDP over dummy netdev, 2038491 -> 2099071 req/s (or around +3%).

note: in regards to 1/3, there is a "Should agree with poll..." comment
that I don't completely get, and there is no git history to explain it.
Though I can't see how it could rely on having the second check without
racing with tasks woken by wake_up*().

The series was split from a larger patchset, see
https://lore.kernel.org/netdev/cover.1648981570.git.asml.silence@gmail.com/
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -146,6 +146,9 @@
 static DEFINE_MUTEX(proto_list_mutex);
 static LIST_HEAD(proto_list);
 
+static void sock_def_write_space_wfree(struct sock *sk);
+static void sock_def_write_space(struct sock *sk);
+
 /**
  * sk_ns_capable - General socket capability test
  * @sk: Socket to use a capability on or through
@@ -2324,8 +2327,20 @@ void sock_wfree(struct sk_buff *skb)
 {
 	struct sock *sk = skb->sk;
 	unsigned int len = skb->truesize;
+	bool free;
 
 	if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) {
+		if (sock_flag(sk, SOCK_RCU_FREE) &&
+		    sk->sk_write_space == sock_def_write_space) {
+			rcu_read_lock();
+			free = refcount_sub_and_test(len, &sk->sk_wmem_alloc);
+			sock_def_write_space_wfree(sk);
+			rcu_read_unlock();
+			if (unlikely(free))
+				__sk_free(sk);
+			return;
+		}
+
 		/*
 		 * Keep a reference on sk_wmem_alloc, this will be released
 		 * after sk_write_space() call
@@ -3191,20 +3206,42 @@ static void sock_def_write_space(struct sock *sk)
 	/* Do not wake up a writer until he can make "significant"
 	 * progress.  --DaveM
 	 */
-	if ((refcount_read(&sk->sk_wmem_alloc) << 1) <= READ_ONCE(sk->sk_sndbuf)) {
+	if (sock_writeable(sk)) {
 		wq = rcu_dereference(sk->sk_wq);
 		if (skwq_has_sleeper(wq))
 			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
 						EPOLLWRNORM | EPOLLWRBAND);
 
 		/* Should agree with poll, otherwise some programs break */
-		if (sock_writeable(sk))
-			sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
+		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
 	}
 
 	rcu_read_unlock();
 }
 
+/* An optimised version of sock_def_write_space(), should only be called
+ * for SOCK_RCU_FREE sockets under RCU read section and after putting
+ * ->sk_wmem_alloc.
+ */
+static void sock_def_write_space_wfree(struct sock *sk)
+{
+	/* Do not wake up a writer until he can make "significant"
+	 * progress.  --DaveM
+	 */
+	if (sock_writeable(sk)) {
+		struct socket_wq *wq = rcu_dereference(sk->sk_wq);
+
+		/* rely on refcount_sub from sock_wfree() */
+		smp_mb__after_atomic();
+		if (wq && waitqueue_active(&wq->wait))
+			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
+						EPOLLWRNORM | EPOLLWRBAND);
+
+		/* Should agree with poll, otherwise some programs break */
+		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
+	}
+}
+
 static void sock_def_destruct(struct sock *sk)
 {
 }