[PATCH net-next] 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 prior_delivered may be equal, because tp->delivered only
changes when receiving, which requires further comparison of
skb timestamp.

Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
---
 net/ipv4/tcp_rate.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_rate.c b/net/ipv4/tcp_rate.c
index 617b818..ad893ad 100644
--- a/net/ipv4/tcp_rate.c
+++ b/net/ipv4/tcp_rate.c
@@ -86,7 +86,9 @@ void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
 		return;
 
 	if (!rs->prior_delivered ||
-	    after(scb->tx.delivered, rs->prior_delivered)) {
+	    after(scb->tx.delivered, rs->prior_delivered) ||
+	    (scb->tx.delivered == rs->prior_delivered &&
+	     tcp_skb_timestamp_us(skb) > tp->first_tx_mstamp)) {
 		rs->prior_delivered_ce  = scb->tx.delivered_ce;
 		rs->prior_delivered  = scb->tx.delivered;
 		rs->prior_mstamp     = scb->tx.delivered_mstamp;
-- 
1.8.3.1