tcp: batch calls to sk_flush_backlog()

Starting from commit d41a69f1 ("tcp: make tcp_sendmsg() aware of socket backlog")
loopback flows got hurt, because for each skb sent, the socket receives an
immediate ACK and sk_flush_backlog() causes extra work.

Intent was to not let the backlog grow too much, but we went a bit too far.

We can check the backlog every 16 skbs (about 1MB chunks)
to increase TCP over loopback performance by about 15 %

Note that the call to sk_flush_backlog() handles a single ACK,
thanks to coalescing done on backlog, but cleans the 16 skbs
found in rtx rb-tree.
Reported-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1162,7 +1162,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 	struct sockcm_cookie sockc;
 	int flags, err, copied = 0;
 	int mss_now = 0, size_goal, copied_syn = 0;
-	bool process_backlog = false;
+	int process_backlog = 0;
 	bool zc = false;
 	long timeo;
 
@@ -1254,8 +1254,9 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			if (!sk_stream_memory_free(sk))
 				goto wait_for_sndbuf;
 
-			if (process_backlog && sk_flush_backlog(sk)) {
-				process_backlog = false;
+			if (unlikely(process_backlog >= 16)) {
+				process_backlog = 0;
+				if (sk_flush_backlog(sk))
 					goto restart;
 			}
 			first_skb = tcp_rtx_and_write_queues_empty(sk);
@@ -1264,7 +1265,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			if (!skb)
 				goto wait_for_memory;
 
-			process_backlog = true;
+			process_backlog++;
 			skb->ip_summed = CHECKSUM_PARTIAL;
 
 			skb_entail(sk, skb);