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Date: Thu, 30 Apr 2015 18:23:31 +0200
From: Kenneth Klette Jonassen <kennetkl@....uio.no>
To: netdev@...r.kernel.org
Cc: Kenneth Klette Jonassen <kennetkl@....uio.no>,
Yuchung Cheng <ycheng@...gle.com>,
Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 2/3] tcp: improve RTT from SACK for CC
tcp_sacktag_one() always picks the earliest sequence SACKed for RTT.
This might not make sense for congestion control in cases where:
1. ACKs are lost, i.e. a SACK following a lost SACK covers both
new and old segments at the receiver.
2. The receiver disregards the RFC 5681 recommendation to immediately
ACK out-of-order segments.
Give congestion control a RTT for the latest segment SACKed, which is the
most accurate RTT estimate, but preserve the conservative RTT for RTO.
Removes the call to skb_mstamp_get() in tcp_sacktag_one().
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Kenneth Klette Jonassen <kennetkl@....uio.no>
---
net/ipv4/tcp_input.c | 42 ++++++++++++++++++++++++------------------
1 file changed, 24 insertions(+), 18 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9902cf1..32bac6a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1130,7 +1130,14 @@ static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
struct tcp_sacktag_state {
int reord;
int fack_count;
- long rtt_us; /* RTT measured by SACKing never-retransmitted data */
+ /* Timestamps for earliest and latest never-retransmitted segment
+ * that was SACKed. RTO needs the earliest RTT to be conservative
+ * against receivers that might delay SACKing (RFC 5681 does not
+ * require ACKing out-of-order segments immediately), but congestion
+ * control should still get an accurate delay signal.
+ */
+ struct skb_mstamp first_sackt;
+ struct skb_mstamp last_sackt;
int flag;
};
@@ -1233,14 +1240,9 @@ static u8 tcp_sacktag_one(struct sock *sk,
state->reord);
if (!after(end_seq, tp->high_seq))
state->flag |= FLAG_ORIG_SACK_ACKED;
- /* Pick the earliest sequence sacked for RTT */
- if (state->rtt_us < 0) {
- struct skb_mstamp now;
-
- skb_mstamp_get(&now);
- state->rtt_us = skb_mstamp_us_delta(&now,
- xmit_time);
- }
+ if (state->first_sackt.v64 == 0)
+ state->first_sackt = *xmit_time;
+ state->last_sackt = *xmit_time;
}
if (sacked & TCPCB_LOST) {
@@ -3049,7 +3051,8 @@ static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb,
* arrived at the other end.
*/
static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
- u32 prior_snd_una, long sack_rtt_us)
+ u32 prior_snd_una,
+ struct tcp_sacktag_state *sack)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
struct skb_mstamp first_ackt, last_ackt, now;
@@ -3057,8 +3060,9 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
u32 prior_sacked = tp->sacked_out;
u32 reord = tp->packets_out;
bool fully_acked = true;
- long ca_seq_rtt_us = -1L;
+ long sack_rtt_us = -1L;
long seq_rtt_us = -1L;
+ long ca_rtt_us = -1L;
struct sk_buff *skb;
u32 pkts_acked = 0;
bool rtt_update;
@@ -3147,7 +3151,11 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
skb_mstamp_get(&now);
if (likely(first_ackt.v64)) {
seq_rtt_us = skb_mstamp_us_delta(&now, &first_ackt);
- ca_seq_rtt_us = skb_mstamp_us_delta(&now, &last_ackt);
+ ca_rtt_us = skb_mstamp_us_delta(&now, &last_ackt);
+ }
+ if (sack->first_sackt.v64) {
+ sack_rtt_us = skb_mstamp_us_delta(&now, &sack->first_sackt);
+ ca_rtt_us = skb_mstamp_us_delta(&now, &sack->last_sackt);
}
rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us);
@@ -3178,10 +3186,8 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
tp->fackets_out -= min(pkts_acked, tp->fackets_out);
- if (ca_ops->pkts_acked) {
- long rtt_us = min_t(ulong, ca_seq_rtt_us, sack_rtt_us);
- ca_ops->pkts_acked(sk, pkts_acked, rtt_us);
- }
+ if (ca_ops->pkts_acked)
+ ca_ops->pkts_acked(sk, pkts_acked, ca_rtt_us);
} else if (skb && rtt_update && sack_rtt_us >= 0 &&
sack_rtt_us > skb_mstamp_us_delta(&now, &skb->skb_mstamp)) {
@@ -3466,7 +3472,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
const int prior_unsacked = tp->packets_out - tp->sacked_out;
int acked = 0; /* Number of packets newly acked */
- sack_state.rtt_us = -1L;
+ sack_state.first_sackt.v64 = 0;
/* We very likely will need to access write queue head. */
prefetchw(sk->sk_write_queue.next);
@@ -3555,7 +3561,7 @@ 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. */
acked = tp->packets_out;
flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una,
- sack_state.rtt_us);
+ &sack_state);
acked -= tp->packets_out;
/* Advance cwnd if state allows */
--
2.1.0
--
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