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Commit 31231a8a authored by Kenneth Klette Jonassen's avatar Kenneth Klette Jonassen Committed by David S. Miller
Browse files

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: default avatarKenneth Klette Jonassen <kennetkl@ifi.uio.no>
Acked-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 196da974

net/ipv4/tcp_input.c

+22 −18
Original line number Diff line number Diff line
@@ -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 */