From b0c05d0e99d98d7f0cd41efc1eeec94efdc3325d Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Thu, 12 Jul 2018 06:04:52 -0700 Subject: [PATCH 1/2] tcp: fix dctcp delayed ACK schedule Previously, when a data segment was sent an ACK was piggybacked on the data segment without generating a CA_EVENT_NON_DELAYED_ACK event to notify congestion control modules. So the DCTCP ca->delayed_ack_reserved flag could incorrectly stay set when in fact there were no delayed ACKs being reserved. This could result in sending a special ECN notification ACK that carries an older ACK sequence, when in fact there was no need for such an ACK. DCTCP keeps track of the delayed ACK status with its own separate state ca->delayed_ack_reserved. Previously it may accidentally cancel the delayed ACK without updating this field upon sending a special ACK that carries a older ACK sequence. This inconsistency would lead to DCTCP receiver never acknowledging the latest data until the sender times out and retry in some cases. Packetdrill script (provided by Larry Brakmo) 0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 0.000 setsockopt(3, SOL_TCP, TCP_CONGESTION, "dctcp", 5) = 0 0.000 bind(3, ..., ...) = 0 0.000 listen(3, 1) = 0 0.100 < [ect0] SEW 0:0(0) win 32792 0.100 > SE. 0:0(0) ack 1 0.110 < [ect0] . 1:1(0) ack 1 win 257 0.200 accept(3, ..., ...) = 4 0.200 < [ect0] . 1:1001(1000) ack 1 win 257 0.200 > [ect01] . 1:1(0) ack 1001 0.200 write(4, ..., 1) = 1 0.200 > [ect01] P. 1:2(1) ack 1001 0.200 < [ect0] . 1001:2001(1000) ack 2 win 257 0.200 write(4, ..., 1) = 1 0.200 > [ect01] P. 2:3(1) ack 2001 0.200 < [ect0] . 2001:3001(1000) ack 3 win 257 0.200 < [ect0] . 3001:4001(1000) ack 3 win 257 0.200 > [ect01] . 3:3(0) ack 4001 0.210 < [ce] P. 4001:4501(500) ack 3 win 257 +0.001 read(4, ..., 4500) = 4500 +0 write(4, ..., 1) = 1 +0 > [ect01] PE. 3:4(1) ack 4501 +0.010 < [ect0] W. 4501:5501(1000) ack 4 win 257 // Previously the ACK sequence below would be 4501, causing a long RTO +0.040~+0.045 > [ect01] . 4:4(0) ack 5501 // delayed ack +0.311 < [ect0] . 5501:6501(1000) ack 4 win 257 // More data +0 > [ect01] . 4:4(0) ack 6501 // now acks everything +0.500 < F. 9501:9501(0) ack 4 win 257 Reported-by: Larry Brakmo Signed-off-by: Yuchung Cheng Signed-off-by: Eric Dumazet Acked-by: Neal Cardwell Acked-by: Lawrence Brakmo Signed-off-by: David S. Miller --- net/ipv4/tcp_dctcp.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c index 5f5e5936760e..89f88b0d8167 100644 --- a/net/ipv4/tcp_dctcp.c +++ b/net/ipv4/tcp_dctcp.c @@ -134,7 +134,8 @@ static void dctcp_ce_state_0_to_1(struct sock *sk) /* State has changed from CE=0 to CE=1 and delayed * ACK has not sent yet. */ - if (!ca->ce_state && ca->delayed_ack_reserved) { + if (!ca->ce_state && + inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER) { u32 tmp_rcv_nxt; /* Save current rcv_nxt. */ @@ -164,7 +165,8 @@ static void dctcp_ce_state_1_to_0(struct sock *sk) /* State has changed from CE=1 to CE=0 and delayed * ACK has not sent yet. */ - if (ca->ce_state && ca->delayed_ack_reserved) { + if (ca->ce_state && + inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER) { u32 tmp_rcv_nxt; /* Save current rcv_nxt. */ From a69258f7aa2623e0930212f09c586fd06674ad79 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Thu, 12 Jul 2018 06:04:53 -0700 Subject: [PATCH 2/2] tcp: remove DELAYED ACK events in DCTCP After fixing the way DCTCP tracking delayed ACKs, the delayed-ACK related callbacks are no longer needed Signed-off-by: Yuchung Cheng Signed-off-by: Eric Dumazet Acked-by: Neal Cardwell Acked-by: Lawrence Brakmo Signed-off-by: David S. Miller --- include/net/tcp.h | 2 -- net/ipv4/tcp_dctcp.c | 25 ------------------------- net/ipv4/tcp_output.c | 4 ---- 3 files changed, 31 deletions(-) diff --git a/include/net/tcp.h b/include/net/tcp.h index af3ec72d5d41..3482d13d655b 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -912,8 +912,6 @@ enum tcp_ca_event { CA_EVENT_LOSS, /* loss timeout */ CA_EVENT_ECN_NO_CE, /* ECT set, but not CE marked */ CA_EVENT_ECN_IS_CE, /* received CE marked IP packet */ - CA_EVENT_DELAYED_ACK, /* Delayed ack is sent */ - CA_EVENT_NON_DELAYED_ACK, }; /* Information about inbound ACK, passed to cong_ops->in_ack_event() */ diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c index 89f88b0d8167..5869f89ca656 100644 --- a/net/ipv4/tcp_dctcp.c +++ b/net/ipv4/tcp_dctcp.c @@ -55,7 +55,6 @@ struct dctcp { u32 dctcp_alpha; u32 next_seq; u32 ce_state; - u32 delayed_ack_reserved; u32 loss_cwnd; }; @@ -96,7 +95,6 @@ static void dctcp_init(struct sock *sk) ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA); - ca->delayed_ack_reserved = 0; ca->loss_cwnd = 0; ca->ce_state = 0; @@ -250,25 +248,6 @@ static void dctcp_state(struct sock *sk, u8 new_state) } } -static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev) -{ - struct dctcp *ca = inet_csk_ca(sk); - - switch (ev) { - case CA_EVENT_DELAYED_ACK: - if (!ca->delayed_ack_reserved) - ca->delayed_ack_reserved = 1; - break; - case CA_EVENT_NON_DELAYED_ACK: - if (ca->delayed_ack_reserved) - ca->delayed_ack_reserved = 0; - break; - default: - /* Don't care for the rest. */ - break; - } -} - static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) { switch (ev) { @@ -278,10 +257,6 @@ static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) case CA_EVENT_ECN_NO_CE: dctcp_ce_state_1_to_0(sk); break; - case CA_EVENT_DELAYED_ACK: - case CA_EVENT_NON_DELAYED_ACK: - dctcp_update_ack_reserved(sk, ev); - break; default: /* Don't care for the rest. */ break; diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 8e08b409c71e..00e5a300ddb9 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -3523,8 +3523,6 @@ void tcp_send_delayed_ack(struct sock *sk) int ato = icsk->icsk_ack.ato; unsigned long timeout; - tcp_ca_event(sk, CA_EVENT_DELAYED_ACK); - if (ato > TCP_DELACK_MIN) { const struct tcp_sock *tp = tcp_sk(sk); int max_ato = HZ / 2; @@ -3581,8 +3579,6 @@ void tcp_send_ack(struct sock *sk) if (sk->sk_state == TCP_CLOSE) return; - tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK); - /* We are not putting this on the write queue, so * tcp_transmit_skb() will set the ownership to this * sock.