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Date: Sun, 12 Apr 2015 19:51:57 +0200
From: Kenneth Klette Jonassen <kennetkl@....uio.no>
To: netdev@...r.kernel.org
Cc: ycheng@...gle.com, Kenneth Klette Jonassen <kennetkl@....uio.no>
Subject: [PATCH RFC net-next] tcp: improve SACK RTT 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 subsequent to a lost SACK covers both
a new and an old segment 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.
Also remove the dependency on skb_mstamp_get() in tcp_sacktag_one() which
potentially contributes to variance between RTT signals.
This is a request for comments. For full effect, a later patch would move
the call to pkts_acked() outside the scope of if (flag & FLAG_ACKED). As is,
this hook only gets called when sequentially acknowledging new data.
---
net/ipv4/tcp_input.c | 90 ++++++++++++++++++++++++++++++----------------------
1 file changed, 52 insertions(+), 38 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a7ef679..8f0bd3c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1130,7 +1130,16 @@ 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 */
+ /* Timestamp for earliest never-retransmitted segment that was
+ * SACKed and its usec delta to the corresponding latest segment.
+ * 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. Even without delayed SACKing,
+ * this helps mitigate the effect of ACK loss on delay-based CC.
+ */
+ struct skb_mstamp first_ackt;
+ u32 last_ack_us_delta;
int flag;
};
@@ -1233,14 +1242,13 @@ 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_ackt.v64 == 0)
+ state->first_ackt.v64 = xmit_time->v64;
+ else
+ state->last_ack_us_delta =
+ skb_mstamp_us_delta(xmit_time,
+ &state->first_ackt);
}
if (sacked & TCPCB_LOST) {
@@ -1634,7 +1642,7 @@ static int tcp_sack_cache_ok(const struct tcp_sock *tp, const struct tcp_sack_bl
static int
tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
- u32 prior_snd_una, long *sack_rtt_us)
+ u32 prior_snd_una, struct tcp_sacktag_state *state)
{
struct tcp_sock *tp = tcp_sk(sk);
const unsigned char *ptr = (skb_transport_header(ack_skb) +
@@ -1642,7 +1650,6 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
struct tcp_sack_block_wire *sp_wire = (struct tcp_sack_block_wire *)(ptr+2);
struct tcp_sack_block sp[TCP_NUM_SACKS];
struct tcp_sack_block *cache;
- struct tcp_sacktag_state state;
struct sk_buff *skb;
int num_sacks = min(TCP_NUM_SACKS, (ptr[1] - TCPOLEN_SACK_BASE) >> 3);
int used_sacks;
@@ -1650,9 +1657,9 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
int i, j;
int first_sack_index;
- state.flag = 0;
- state.reord = tp->packets_out;
- state.rtt_us = -1L;
+ state->flag = 0;
+ state->last_ack_us_delta = 0;
+ state->reord = tp->packets_out;
if (!tp->sacked_out) {
if (WARN_ON(tp->fackets_out))
@@ -1663,7 +1670,7 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
num_sacks, prior_snd_una);
if (found_dup_sack)
- state.flag |= FLAG_DSACKING_ACK;
+ state->flag |= FLAG_DSACKING_ACK;
/* Eliminate too old ACKs, but take into
* account more or less fresh ones, they can
@@ -1728,7 +1735,7 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
}
skb = tcp_write_queue_head(sk);
- state.fack_count = 0;
+ state->fack_count = 0;
i = 0;
if (!tp->sacked_out) {
@@ -1762,10 +1769,10 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
/* Head todo? */
if (before(start_seq, cache->start_seq)) {
- skb = tcp_sacktag_skip(skb, sk, &state,
+ skb = tcp_sacktag_skip(skb, sk, state,
start_seq);
skb = tcp_sacktag_walk(skb, sk, next_dup,
- &state,
+ state,
start_seq,
cache->start_seq,
dup_sack);
@@ -1776,7 +1783,7 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
goto advance_sp;
skb = tcp_maybe_skipping_dsack(skb, sk, next_dup,
- &state,
+ state,
cache->end_seq);
/* ...tail remains todo... */
@@ -1785,12 +1792,12 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
skb = tcp_highest_sack(sk);
if (!skb)
break;
- state.fack_count = tp->fackets_out;
+ state->fack_count = tp->fackets_out;
cache++;
goto walk;
}
- skb = tcp_sacktag_skip(skb, sk, &state, cache->end_seq);
+ skb = tcp_sacktag_skip(skb, sk, state, cache->end_seq);
/* Check overlap against next cached too (past this one already) */
cache++;
continue;
@@ -1800,12 +1807,12 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
skb = tcp_highest_sack(sk);
if (!skb)
break;
- state.fack_count = tp->fackets_out;
+ state->fack_count = tp->fackets_out;
}
- skb = tcp_sacktag_skip(skb, sk, &state, start_seq);
+ skb = tcp_sacktag_skip(skb, sk, state, start_seq);
walk:
- skb = tcp_sacktag_walk(skb, sk, next_dup, &state,
+ skb = tcp_sacktag_walk(skb, sk, next_dup, state,
start_seq, end_seq, dup_sack);
advance_sp:
@@ -1824,9 +1831,9 @@ advance_sp:
tcp_verify_left_out(tp);
- if ((state.reord < tp->fackets_out) &&
+ if ((state->reord < tp->fackets_out) &&
((inet_csk(sk)->icsk_ca_state != TCP_CA_Loss) || tp->undo_marker))
- tcp_update_reordering(sk, tp->fackets_out - state.reord, 0);
+ tcp_update_reordering(sk, tp->fackets_out - state->reord, 0);
out:
@@ -1836,8 +1843,7 @@ out:
WARN_ON((int)tp->retrans_out < 0);
WARN_ON((int)tcp_packets_in_flight(tp) < 0);
#endif
- *sack_rtt_us = state.rtt_us;
- return state.flag;
+ return state->flag;
}
/* Limits sacked_out so that sum with lost_out isn't ever larger than
@@ -3054,7 +3060,7 @@ 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;
@@ -3062,7 +3068,8 @@ 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 ca_rtt_us = -1L;
+ long sack_rtt_us = -1L;
long seq_rtt_us = -1L;
struct sk_buff *skb;
u32 pkts_acked = 0;
@@ -3152,7 +3159,14 @@ 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_ackt.v64) {
+ sack_rtt_us = skb_mstamp_us_delta(&now, &sack->first_ackt);
+ /* Pick the latest valid RTT for congestion control: */
+ ca_rtt_us = min_t(ulong, sack_rtt_us - sack->last_ack_us_delta,
+ ca_rtt_us);
}
rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us);
@@ -3183,10 +3197,8 @@ 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)) {
@@ -3443,6 +3455,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
+ struct tcp_sacktag_state sack_state;
u32 prior_snd_una = tp->snd_una;
u32 ack_seq = TCP_SKB_CB(skb)->seq;
u32 ack = TCP_SKB_CB(skb)->ack_seq;
@@ -3451,7 +3464,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
int prior_packets = tp->packets_out;
const int prior_unsacked = tp->packets_out - tp->sacked_out;
int acked = 0; /* Number of packets newly acked */
- long sack_rtt_us = -1L;
+
+ sack_state.first_ackt.v64 = 0;
/* We very likely will need to access write queue head. */
prefetchw(sk->sk_write_queue.next);
@@ -3515,7 +3529,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
if (TCP_SKB_CB(skb)->sacked)
flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
- &sack_rtt_us);
+ &sack_state);
if (tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb))) {
flag |= FLAG_ECE;
@@ -3540,7 +3554,7 @@ 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_rtt_us);
+ &sack_state);
acked -= tp->packets_out;
/* Advance cwnd if state allows */
@@ -3592,7 +3606,7 @@ old_ack:
*/
if (TCP_SKB_CB(skb)->sacked) {
flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
- &sack_rtt_us);
+ &sack_state);
tcp_fastretrans_alert(sk, acked, prior_unsacked,
is_dupack, flag);
}
--
2.1.0
--
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