Merge branch 'tcp-mem-pressure-vs-SO_RCVLOWAT'

Eric Dumazet says:

====================
tcp: mem pressure vs SO_RCVLOWAT

First patch fixes an issue for applications using SO_RCVLOWAT
to reduce context switches.

Second patch is a cleanup.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1431,12 +1431,29 @@ void tcp_cleanup_rbuf(struct sock *sk, int copied);
  */
 static inline bool tcp_rmem_pressure(const struct sock *sk)
 {
-	int rcvbuf = READ_ONCE(sk->sk_rcvbuf);
-	int threshold = rcvbuf - (rcvbuf >> 3);
+	int rcvbuf, threshold;
+
+	if (tcp_under_memory_pressure(sk))
+		return true;
+
+	rcvbuf = READ_ONCE(sk->sk_rcvbuf);
+	threshold = rcvbuf - (rcvbuf >> 3);
 
 	return atomic_read(&sk->sk_rmem_alloc) > threshold;
 }
 
+static inline bool tcp_epollin_ready(const struct sock *sk, int target)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	int avail = READ_ONCE(tp->rcv_nxt) - READ_ONCE(tp->copied_seq);
+
+	if (avail <= 0)
+		return false;
+
+	return (avail >= target) || tcp_rmem_pressure(sk) ||
+	       (tcp_receive_window(tp) <= inet_csk(sk)->icsk_ack.rcv_mss);
+}
+
 extern void tcp_openreq_init_rwin(struct request_sock *req,
 				  const struct sock *sk_listener,
 				  const struct dst_entry *dst);

net/ipv4/tcp.c

@@ -481,19 +481,11 @@ static void tcp_tx_timestamp(struct sock *sk, u16 tsflags)
 	}
 }
 
-static inline bool tcp_stream_is_readable(const struct tcp_sock *tp,
-					  int target, struct sock *sk)
+static bool tcp_stream_is_readable(struct sock *sk, int target)
 {
-	int avail = READ_ONCE(tp->rcv_nxt) - READ_ONCE(tp->copied_seq);
-
-	if (avail > 0) {
-		if (avail >= target)
-			return true;
-		if (tcp_rmem_pressure(sk))
-			return true;
-		if (tcp_receive_window(tp) <= inet_csk(sk)->icsk_ack.rcv_mss)
+	if (tcp_epollin_ready(sk, target))
 		return true;
-	}
+
 	if (sk->sk_prot->stream_memory_read)
 		return sk->sk_prot->stream_memory_read(sk);
 	return false;
@@ -568,7 +560,7 @@ __poll_t tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
 		    tp->urg_data)
 			target++;
 
-		if (tcp_stream_is_readable(tp, target, sk))
+		if (tcp_stream_is_readable(sk, target))
 			mask |= EPOLLIN | EPOLLRDNORM;
 
 		if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {

net/ipv4/tcp_input.c

@@ -4924,14 +4924,7 @@ int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
 
 void tcp_data_ready(struct sock *sk)
 {
-	const struct tcp_sock *tp = tcp_sk(sk);
-	int avail = tp->rcv_nxt - tp->copied_seq;
-
-	if (avail < sk->sk_rcvlowat && !tcp_rmem_pressure(sk) &&
-	    !sock_flag(sk, SOCK_DONE) &&
-	    tcp_receive_window(tp) > inet_csk(sk)->icsk_ack.rcv_mss)
-		return;
-
+	if (tcp_epollin_ready(sk, sk->sk_rcvlowat))
 		sk->sk_data_ready(sk);
 }