Commit 435ccfa8 authored Oct 23, 2020 by Arjun Roy Committed by Jakub Kicinski Oct 23, 2020

tcp: Prevent low rmem stalls with SO_RCVLOWAT.



With SO_RCVLOWAT, under memory pressure,
it is possible to enter a state where:

1. We have not received enough bytes to satisfy SO_RCVLOWAT.
2. We have not entered buffer pressure (see tcp_rmem_pressure()).
3. But, we do not have enough buffer space to accept more packets.

In this case, we advertise 0 rwnd (due to #3) but the application does
not drain the receive queue (no wakeup because of #1 and #2) so the
flow stalls.

Modify the heuristic for SO_RCVLOWAT so that, if we are advertising
rwnd<=rcv_mss, force a wakeup to prevent a stall.

Without this patch, setting tcp_rmem to 6143 and disabling TCP
autotune causes a stalled flow. With this patch, no stall occurs. This
is with RPC-style traffic with large messages.

Fixes: 03f45c88 ("tcp: avoid extra wakeups for SO_RCVLOWAT users")
Signed-off-by: Arjun Roy <arjunroy@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20201023184709.217614-1-arjunroy.kdev@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent dab23422

net/ipv4/tcp.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -485,6 +485,8 @@ static inline bool tcp_stream_is_readable(const struct tcp_sock *tp,
		return true;
		if (tcp_rmem_pressure(sk))
		return true;
		if (tcp_receive_window(tp) <= inet_csk(sk)->icsk_ack.rcv_mss)
		return true;
		}
		if (sk->sk_prot->stream_memory_read)
		return sk->sk_prot->stream_memory_read(sk);

net/ipv4/tcp_input.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -4908,7 +4908,8 @@ void tcp_data_ready(struct sock *sk)
		int avail = tp->rcv_nxt - tp->copied_seq;

		if (avail < sk->sk_rcvlowat && !tcp_rmem_pressure(sk) &&
		!sock_flag(sk, SOCK_DONE))
		!sock_flag(sk, SOCK_DONE) &&
		tcp_receive_window(tp) > inet_csk(sk)->icsk_ack.rcv_mss)
		return;

		sk->sk_data_ready(sk);

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