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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@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Kenneth Klette Jonassen <kennetkl@ifi.uio.no>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Kenneth Klette Jonassen 2015-05-01 01:10:58 +02:00 committed by David S. Miller
parent 196da97475
commit 31231a8a87
1 changed files with 22 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

40
net/ipv4/tcp_input.c
View File

@ -1130,7 +1130,12 @@ 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 stay conservative,
* but congestion control should still get an accurate delay signal.
*/
struct skb_mstamp first_sackt;
struct skb_mstamp last_sackt;
int flag;
};
@ -1233,14 +1238,9 @@ 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 +3049,8 @@ 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 +3058,9 @@ 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 +3149,11 @@ 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 +3184,8 @@ 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 +3470,7 @@ 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 +3559,7 @@ 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 */