net: avoid extra access to sk->sk_wmem_alloc in sock_wfree()

UDP TX packets destructor is sock_wfree().

It suffers from a cache line bouncing in sock_def_write_space_wfree().

Instead of reading sk->sk_wmem_alloc after we just did an atomic RMW
on it, use __refcount_sub_and_test() to get the old value for free,
and pass the new value to sock_def_write_space_wfree().

Add __sock_writeable() helper.

Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/net/sock.h

···
 
 bool sk_page_frag_refill(struct sock *sk, struct page_frag *pfrag);
 
+static inline bool __sock_writeable(const struct sock *sk, int wmem_alloc)
+{
+	return wmem_alloc < (READ_ONCE(sk->sk_sndbuf) >> 1);
+}
 /*
  *	Default write policy as shown to user space via poll/select/SIGIO
  */
 static inline bool sock_writeable(const struct sock *sk)
 {
-	return refcount_read(&sk->sk_wmem_alloc) < (READ_ONCE(sk->sk_sndbuf) >> 1);
+	return __sock_writeable(sk, refcount_read(&sk->sk_wmem_alloc));
 }
 
 static inline gfp_t gfp_any(void)

net/core/sock.c

···
 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_wfree(struct sock *sk, int wmem_alloc);
 static void sock_def_write_space(struct sock *sk);
 
 /**
···
  */
 void sock_wfree(struct sk_buff *skb)
 {
-	struct sock *sk = skb->sk;
 	unsigned int len = skb->truesize;
+	struct sock *sk = skb->sk;
 	bool free;
+	int old;
 
 	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);
+			free = __refcount_sub_and_test(len, &sk->sk_wmem_alloc,
+						       &old);
+			sock_def_write_space_wfree(sk, old - len);
 			rcu_read_unlock();
 			if (unlikely(free))
 				__sk_free(sk);
···
  * 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)
+static void sock_def_write_space_wfree(struct sock *sk, int wmem_alloc)
 {
 	/* Do not wake up a writer until he can make "significant"
 	 * progress.  --DaveM
 	 */
-	if (sock_writeable(sk)) {
+	if (__sock_writeable(sk, wmem_alloc)) {
 		struct socket_wq *wq = rcu_dereference(sk->sk_wq);
 
 		/* rely on refcount_sub from sock_wfree() */