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

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Date:   Wed, 13 Apr 2022 18:54:04 +0800
From:   Pengcheng Yang <yangpc@...gsu.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>, netdev@...r.kernel.org
Cc:     Pengcheng Yang <yangpc@...gsu.com>
Subject: [PATCH net-next] tcp: ensure to use the most recently sent skb when filling the rate sample

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@...gsu.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