[PATCH net-next v2 1/2] tcp: ensure to use the most recently sent skb when filling the rate sample

[Pengcheng Yang <yangpc@wangsu.com>]

If an ACK (s)acks multiple skbs, we favor the information
from the most recently sent skb by choosing the skb with
the highest prior_delivered count. But in the interval
between receiving ACKs, we send multiple skbs with the same
prior_delivered, because the tp->delivered only changes
when we receive an ACK.

We used RACK's solution, copying tcp_rack_sent_after() as
tcp_skb_sent_after() helper to determine "which packet was
sent last?". Later, we will use tcp_skb_sent_after() instead
in RACK.

Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Cc: Neal Cardwell <ncardwell@google.com>
---
 include/net/tcp.h   |  6 ++++++
 net/ipv4/tcp_rate.c | 11 ++++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6d50a66..fcd69fc 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1042,6 +1042,7 @@ struct rate_sample {
 	int  losses;		/* number of packets marked lost upon ACK */
 	u32  acked_sacked;	/* number of packets newly (S)ACKed upon ACK */
 	u32  prior_in_flight;	/* in flight before this ACK */
+	u32  last_end_seq;	/* end_seq of most recently ACKed packet */
 	bool is_app_limited;	/* is sample from packet with bubble in pipe? */
 	bool is_retrans;	/* is sample from retransmission? */
 	bool is_ack_delayed;	/* is this (likely) a delayed ACK? */
@@ -1158,6 +1159,11 @@ void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost,
 		  bool is_sack_reneg, struct rate_sample *rs);
 void tcp_rate_check_app_limited(struct sock *sk);
 
+static inline bool tcp_skb_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
+{
+	return t1 > t2 || (t1 == t2 && after(seq1, seq2));
+}
+
 /* These functions determine how the current flow behaves in respect of SACK
  * handling. SACK is negotiated with the peer, and therefore it can vary
  * between different flows.
diff --git a/net/ipv4/tcp_rate.c b/net/ipv4/tcp_rate.c
index 617b818..a8f6d9d 100644
--- a/net/ipv4/tcp_rate.c
+++ b/net/ipv4/tcp_rate.c
@@ -74,27 +74,32 @@ void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb)
  *
  * If an ACK (s)acks multiple skbs (e.g., stretched-acks), this function is
  * called multiple times. We favor the information from the most recently
- * sent skb, i.e., the skb with the highest prior_delivered count.
+ * sent skb, i.e., the skb with the most recently sent time and the highest
+ * sequence.
  */
 void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
 			    struct rate_sample *rs)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
+	u64 tx_tstamp;
 
 	if (!scb->tx.delivered_mstamp)
 		return;
 
+	tx_tstamp = tcp_skb_timestamp_us(skb);
 	if (!rs->prior_delivered ||
-	    after(scb->tx.delivered, rs->prior_delivered)) {
+	    tcp_skb_sent_after(tx_tstamp, tp->first_tx_mstamp,
+			       scb->end_seq, rs->last_end_seq)) {
 		rs->prior_delivered_ce  = scb->tx.delivered_ce;
 		rs->prior_delivered  = scb->tx.delivered;
 		rs->prior_mstamp     = scb->tx.delivered_mstamp;
 		rs->is_app_limited   = scb->tx.is_app_limited;
 		rs->is_retrans	     = scb->sacked & TCPCB_RETRANS;
+		rs->last_end_seq     = scb->end_seq;
 
 		/* Record send time of most recently ACKed packet: */
-		tp->first_tx_mstamp  = tcp_skb_timestamp_us(skb);
+		tp->first_tx_mstamp  = tx_tstamp;
 		/* Find the duration of the "send phase" of this window: */
 		rs->interval_us = tcp_stamp_us_delta(tp->first_tx_mstamp,
 						     scb->tx.first_tx_mstamp);
-- 
1.8.3.1