Commit b0f71bd3 authored by Francis Yan's avatar Francis Yan Committed by David S. Miller
Browse files

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: default avatarFrancis Yan <francisyyan@gmail.com>
Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 5615f886

net/ipv4/tcp.c

+8 −2
Original line number Diff line number Diff line
@@ -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

+4 −1
Original line number Diff line number Diff line
@@ -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

+12 −0
Original line number Diff line number Diff line
@@ -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);

	}

}

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