ipv6: factorize sk_wmem_alloc updates done by __ip6_append_data()

While testing my inet defrag changes, I found that the senders
could spend ~20% of cpu cycles in skb_set_owner_w() updating
sk->sk_wmem_alloc for every fragment they cook, competing
with TX completion of prior skbs possibly happening on another cpus.

The solution to this problem is to use alloc_skb() instead
of sock_wmalloc() and manually perform a single sk_wmem_alloc change.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -1259,6 +1259,7 @@ static int __ip6_append_data(struct sock *sk,
 	struct ipv6_txoptions *opt = v6_cork->opt;
 	int csummode = CHECKSUM_NONE;
 	unsigned int maxnonfragsize, headersize;
+	unsigned int wmem_alloc_delta = 0;
 
 	skb = skb_peek_tail(queue);
 	if (!skb) {
@@ -1411,10 +1412,9 @@ static int __ip6_append_data(struct sock *sk,
 						(flags & MSG_DONTWAIT), &err);
 			} else {
 				skb = NULL;
-				if (refcount_read(&sk->sk_wmem_alloc) <=
+				if (refcount_read(&sk->sk_wmem_alloc) + wmem_alloc_delta <=
 				    2 * sk->sk_sndbuf)
-					skb = sock_wmalloc(sk,
-							   alloclen + hh_len, 1,
+					skb = alloc_skb(alloclen + hh_len,
 							sk->sk_allocation);
 				if (unlikely(!skb))
 					err = -ENOBUFS;
@@ -1474,6 +1474,11 @@ static int __ip6_append_data(struct sock *sk,
 			/*
 			 * Put the packet on the pending queue
 			 */
+			if (!skb->destructor) {
+				skb->destructor = sock_wfree;
+				skb->sk = sk;
+				wmem_alloc_delta += skb->truesize;
+			}
 			__skb_queue_tail(queue, skb);
 			continue;
 		}
@@ -1520,12 +1525,13 @@ static int __ip6_append_data(struct sock *sk,
 			skb->len += copy;
 			skb->data_len += copy;
 			skb->truesize += copy;
-			refcount_add(copy, &sk->sk_wmem_alloc);
+			wmem_alloc_delta += copy;
 		}
 		offset += copy;
 		length -= copy;
 	}
 
+	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
 	return 0;
 
 error_efault:
@@ -1533,6 +1539,7 @@ static int __ip6_append_data(struct sock *sk,
 error:
 	cork->length -= length;
 	IP6_INC_STATS(sock_net(sk), rt->rt6i_idev, IPSTATS_MIB_OUTDISCARDS);
+	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
 	return err;
 }