tcp: instrument how long TCP is limited by insufficient send buffer

This patch measures the amount of time when TCP runs out of new data
to send to the network due to insufficient send buffer, while TCP
is still busy delivering (i.e. write queue is not empty). The goal
is to indicate either the send buffer autotuning or user SO_SNDBUF
setting has resulted network under-utilization.

The measurement starts conservatively by checking various conditions
to minimize false claims (i.e. under-estimation is more likely).
The measurement stops when the SOCK_NOSPACE flag is cleared. But it
does not account the time elapsed till the next application write.
Also the measurement only starts if the sender is still busy sending
data, s.t. the limit accounted is part of the total busy time.

Signed-off-by: Francis Yan <francisyyan@gmail.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -996,8 +996,11 @@ do_error:
 		goto out;
 out_err:
 	/* make sure we wake any epoll edge trigger waiter */
-	if (unlikely(skb_queue_len(&sk->sk_write_queue) == 0 && err == -EAGAIN))
+	if (unlikely(skb_queue_len(&sk->sk_write_queue) == 0 &&
+		     err == -EAGAIN)) {
 		sk->sk_write_space(sk);
+		tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
+	}
 	return sk_stream_error(sk, flags, err);
 }
 
@@ -1331,8 +1334,11 @@ do_error:
 out_err:
 	err = sk_stream_error(sk, flags, err);
 	/* make sure we wake any epoll edge trigger waiter */
-	if (unlikely(skb_queue_len(&sk->sk_write_queue) == 0 && err == -EAGAIN))
+	if (unlikely(skb_queue_len(&sk->sk_write_queue) == 0 &&
+		     err == -EAGAIN)) {
 		sk->sk_write_space(sk);
+		tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
+	}
 	release_sock(sk);
 	return err;
 }

net/ipv4/tcp_input.c

@@ -5059,8 +5059,11 @@ static void tcp_check_space(struct sock *sk)
 		/* pairs with tcp_poll() */
 		smp_mb__after_atomic();
 		if (sk->sk_socket &&
-		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
+		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
 			tcp_new_space(sk);
+			if (!test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
+				tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
+		}
 	}
 }
 

net/ipv4/tcp_output.c

@@ -1514,6 +1514,18 @@ static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
 		if (sysctl_tcp_slow_start_after_idle &&
 		    (s32)(tcp_time_stamp - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto)
 			tcp_cwnd_application_limited(sk);
+
+		/* The following conditions together indicate the starvation
+		 * is caused by insufficient sender buffer:
+		 * 1) just sent some data (see tcp_write_xmit)
+		 * 2) not cwnd limited (this else condition)
+		 * 3) no more data to send (null tcp_send_head )
+		 * 4) application is hitting buffer limit (SOCK_NOSPACE)
+		 */
+		if (!tcp_send_head(sk) && sk->sk_socket &&
+		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags) &&
+		    (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))
+			tcp_chrono_start(sk, TCP_CHRONO_SNDBUF_LIMITED);
 	}
 }