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Date: Tue, 8 Nov 2011 13:07:11 -0500
From: Neal Cardwell <ncardwell@...gle.com>
To: David Miller <davem@...emloft.net>
Cc: netdev@...r.kernel.org, ilpo.jarvinen@...sinki.fi,
Nandita Dukkipati <nanditad@...gle.com>,
Yuchung Cheng <ycheng@...gle.com>,
Tom Herbert <therbert@...gle.com>,
Neal Cardwell <ncardwell@...gle.com>
Subject: [PATCH] tcp: fixes for DSACK-based undo of cwnd reduction during fast recovery
Fixes for some issues that prevent DSACKs from allowing TCP senders to
undo cwnd reductions made during fast recovery.
There were a few related bugs/issues:
1) Senders ignored DSACKs after recovery when there were no
outstanding packets (a common scenario for HTTP servers).
2) When the ACK field is below snd_una (which can happen when ACKs are
reordered), senders ignored DSACKs (preventing undo) and passed up
chances to send out more packets based on any newly-SACKed packets.
3) Senders were overriding cwnd values picked during an undo by
calling tcp_moderate_cwnd() in tcp_try_to_open().
The fixes:
(1) When there are no outstanding packets (the "no_queue" goto label),
use DSACKs to undo congestion window reductions.
(2) When the ACK field is below snd_una (the "old_ack" goto label),
process any DSACKs and try to send out more packets based on
newly-SACKed packets.
(3) Don't moderate cwnd in tcp_try_to_open() if we're in TCP_CA_Open,
since doing so is generally unnecessary and specifically would
override a DSACK-based undo of a cwnd reduction made in fast recovery.
(4) Simplify the congestion avoidance state machine by removing the
behavior where SACK-enabled connections hung around in the
TCP_CA_Disorder state just waiting for DSACKs. Instead, when snd_una
advances to high_seq or beyond we typically move to TCP_CA_Open
immediately and allow an undo in either TCP_CA_Open or TCP_CA_Disorder
if we later receive enough DSACKs. Previously, SACK-enabled
connections hung around in TCP_CA_Disorder state while
snd_una==high_seq, just waiting to accumulate DSACKs and hopefully
undo a cwnd reduction. This could and did lead to the following
unfortunate scenario: if some incoming ACKs advance snd_una beyond
high_seq then we were setting undo_marker to 0 and moving to
TCP_CA_Open, so if (due to reordering in the ACK return path) we
shortly thereafter received a DSACK then we were no longer able to
undo the cwnd reduction.
Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
---
net/ipv4/tcp_input.c | 44 +++++++++++++++++++++++---------------------
1 files changed, 23 insertions(+), 21 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 52b5c2d..78dd38c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2858,7 +2858,7 @@ static void tcp_try_keep_open(struct sock *sk)
struct tcp_sock *tp = tcp_sk(sk);
int state = TCP_CA_Open;
- if (tcp_left_out(tp) || tcp_any_retrans_done(sk) || tp->undo_marker)
+ if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
state = TCP_CA_Disorder;
if (inet_csk(sk)->icsk_ca_state != state) {
@@ -2881,7 +2881,8 @@ static void tcp_try_to_open(struct sock *sk, int flag)
if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
tcp_try_keep_open(sk);
- tcp_moderate_cwnd(tp);
+ if (inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
+ tcp_moderate_cwnd(tp);
} else {
tcp_cwnd_down(sk, flag);
}
@@ -3009,11 +3010,11 @@ static void tcp_update_cwnd_in_recovery(struct sock *sk, int newly_acked_sacked,
* tcp_xmit_retransmit_queue().
*/
static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
- int newly_acked_sacked, int flag)
+ int newly_acked_sacked, bool is_dupack,
+ int flag)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
- int is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
int do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
(tcp_fackets_out(tp) > tp->reordering));
int fast_rexmit = 0, mib_idx;
@@ -3066,17 +3067,6 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
}
break;
- case TCP_CA_Disorder:
- tcp_try_undo_dsack(sk);
- if (!tp->undo_marker ||
- /* For SACK case do not Open to allow to undo
- * catching for all duplicate ACKs. */
- tcp_is_reno(tp) || tp->snd_una != tp->high_seq) {
- tp->undo_marker = 0;
- tcp_set_ca_state(sk, TCP_CA_Open);
- }
- break;
-
case TCP_CA_Recovery:
if (tcp_is_reno(tp))
tcp_reset_reno_sack(tp);
@@ -3117,7 +3107,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
tcp_add_reno_sack(sk);
}
- if (icsk->icsk_ca_state == TCP_CA_Disorder)
+ if (icsk->icsk_ca_state <= TCP_CA_Disorder)
tcp_try_undo_dsack(sk);
if (!tcp_time_to_recover(sk)) {
@@ -3681,10 +3671,12 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
u32 prior_snd_una = tp->snd_una;
u32 ack_seq = TCP_SKB_CB(skb)->seq;
u32 ack = TCP_SKB_CB(skb)->ack_seq;
+ bool is_dupack = false;
u32 prior_in_flight;
u32 prior_fackets;
int prior_packets;
int prior_sacked = tp->sacked_out;
+ int pkts_acked = 0;
int newly_acked_sacked = 0;
int frto_cwnd = 0;
@@ -3757,6 +3749,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
/* See if we can take anything off of the retransmit queue. */
flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);
+ pkts_acked = prior_packets - tp->packets_out;
newly_acked_sacked = (prior_packets - prior_sacked) -
(tp->packets_out - tp->sacked_out);
@@ -3771,8 +3764,9 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
if ((flag & FLAG_DATA_ACKED) && !frto_cwnd &&
tcp_may_raise_cwnd(sk, flag))
tcp_cong_avoid(sk, ack, prior_in_flight);
- tcp_fastretrans_alert(sk, prior_packets - tp->packets_out,
- newly_acked_sacked, flag);
+ is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
+ tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
+ is_dupack, flag);
} else {
if ((flag & FLAG_DATA_ACKED) && !frto_cwnd)
tcp_cong_avoid(sk, ack, prior_in_flight);
@@ -3784,6 +3778,10 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
return 1;
no_queue:
+ /* If data was DSACKed, see if we can undo a cwnd reduction. */
+ if (flag & FLAG_DSACKING_ACK)
+ tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
+ is_dupack, flag);
/* If this ack opens up a zero window, clear backoff. It was
* being used to time the probes, and is probably far higher than
* it needs to be for normal retransmission.
@@ -3797,10 +3795,14 @@ invalid_ack:
return -1;
old_ack:
+ /* If data was SACKed, tag it and see if we should send more data.
+ * If data was DSACKed, see if we can undo a cwnd reduction.
+ */
if (TCP_SKB_CB(skb)->sacked) {
- tcp_sacktag_write_queue(sk, skb, prior_snd_una);
- if (icsk->icsk_ca_state == TCP_CA_Open)
- tcp_try_keep_open(sk);
+ flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);
+ newly_acked_sacked = tp->sacked_out - prior_sacked;
+ tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
+ is_dupack, flag);
}
SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
--
1.7.3.1
--
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