4.19-stable review patch. If anyone has any objections, please let me know.
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From: Neal Cardwell ncardwell@google.com
[ Upstream commit 4720852ed9afb1c5ab84e96135cb5b73d5afde6f ]
This commit fixes poor delayed ACK behavior that can cause poor TCP latency in a particular boundary condition: when an application makes a TCP socket write that is an exact multiple of the MSS size.
The problem is that there is painful boundary discontinuity in the current delayed ACK behavior. With the current delayed ACK behavior, we have:
(1) If an app reads data when > 1*MSS is unacknowledged, then tcp_cleanup_rbuf() ACKs immediately because of:
tp->rcv_nxt - tp->rcv_wup > icsk->icsk_ack.rcv_mss ||
(2) If an app reads all received data, and the packets were < 1*MSS, and either (a) the app is not ping-pong or (b) we received two packets < 1*MSS, then tcp_cleanup_rbuf() ACKs immediately beecause of:
((icsk->icsk_ack.pending & ICSK_ACK_PUSHED2) || ((icsk->icsk_ack.pending & ICSK_ACK_PUSHED) && !inet_csk_in_pingpong_mode(sk))) &&
(3) *However*: if an app reads exactly 1*MSS of data, tcp_cleanup_rbuf() does not send an immediate ACK. This is true even if the app is not ping-pong and the 1*MSS of data had the PSH bit set, suggesting the sending application completed an application write.
Thus if the app is not ping-pong, we have this painful case where
1*MSS gets an immediate ACK, and <1*MSS gets an immediate ACK, but a
write whose last skb is an exact multiple of 1*MSS can get a 40ms delayed ACK. This means that any app that transfers data in one direction and takes care to align write size or packet size with MSS can suffer this problem. With receive zero copy making 4KB MSS values more common, it is becoming more common to have application writes naturally align with MSS, and more applications are likely to encounter this delayed ACK problem.
The fix in this commit is to refine the delayed ACK heuristics with a simple check: immediately ACK a received 1*MSS skb with PSH bit set if the app reads all data. Why? If an skb has a len of exactly 1*MSS and has the PSH bit set then it is likely the end of an application write. So more data may not be arriving soon, and yet the data sender may be waiting for an ACK if cwnd-bound or using TX zero copy. Thus we set ICSK_ACK_PUSHED in this case so that tcp_cleanup_rbuf() will send an ACK immediately if the app reads all of the data and is not ping-pong. Note that this logic is also executed for the case where len > MSS, but in that case this logic does not matter (and does not hurt) because tcp_cleanup_rbuf() will always ACK immediately if the app reads data and there is more than an MSS of unACKed data.
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Neal Cardwell ncardwell@google.com Reviewed-by: Yuchung Cheng ycheng@google.com Reviewed-by: Eric Dumazet edumazet@google.com Cc: Xin Guo guoxin0309@gmail.com Link: https://lore.kernel.org/r/20231001151239.1866845-2-ncardwell.sw@gmail.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_input.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 9e1ec69fe5b46..0052a6194cc1a 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -172,6 +172,19 @@ static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb) if (unlikely(len > icsk->icsk_ack.rcv_mss + MAX_TCP_OPTION_SPACE)) tcp_gro_dev_warn(sk, skb, len); + /* If the skb has a len of exactly 1*MSS and has the PSH bit + * set then it is likely the end of an application write. So + * more data may not be arriving soon, and yet the data sender + * may be waiting for an ACK if cwnd-bound or using TX zero + * copy. So we set ICSK_ACK_PUSHED here so that + * tcp_cleanup_rbuf() will send an ACK immediately if the app + * reads all of the data and is not ping-pong. If len > MSS + * then this logic does not matter (and does not hurt) because + * tcp_cleanup_rbuf() will always ACK immediately if the app + * reads data and there is more than an MSS of unACKed data. + */ + if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_PSH) + icsk->icsk_ack.pending |= ICSK_ACK_PUSHED; } else { /* Otherwise, we make more careful check taking into account, * that SACKs block is variable.