netdev - [PATCH net] net: fix the RTO timer retransmitting skb every 1ms if linear option is enabled

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Date: Thu, 10 Aug 2023 19:21:48 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: edumazet@...gle.com,
	davem@...emloft.net,
	dsahern@...nel.org,
	kuba@...nel.org,
	pabeni@...hat.com
Cc: apetlund@...ula.no,
	netdev@...r.kernel.org,
	kerneljasonxing@...il.com,
	Jason Xing <kernelxing@...cent.com>
Subject: [PATCH net] net: fix the RTO timer retransmitting skb every 1ms if linear option is enabled

From: Jason Xing <kernelxing@...cent.com>

In the real workload, I encountered an issue which could cause the RTO
timer to retransmit the skb per 1ms with linear option enabled. The amount
of lost-retransmitted skbs can go up to 1000+ instantly.

The root cause is that if the icsk_rto happens to be zero in the 6th round
(which is the TCP_THIN_LINEAR_RETRIES value), then it will alway be zero
due to the changed calculation method in tcp_retransmit_timer() as follows:

icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);

Above line could be converted to
icsk->icsk_rto = min(0 << 1, TCP_RTO_MAX) = 0

Therefore, the timer expires so quickly without any doubt.

I read through the RFC 6298 and found that the RTO value can be rounded
up to a certain value, in Linux, say TCP_RTO_MIN as default, which is
regarded as the lower bound in this patch as suggested by Eric.

Fixes: 36e31b0af587 ("net: TCP thin linear timeouts")
Suggested-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Jason Xing <kernelxing@...cent.com>
---
 net/ipv4/tcp_timer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index d45c96c7f5a4..b2b25861355c 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -599,7 +599,9 @@ void tcp_retransmit_timer(struct sock *sk)
 	    tcp_stream_is_thin(tp) &&
 	    icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
 		icsk->icsk_backoff = 0;
-		icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
+		icsk->icsk_rto = clamp(__tcp_set_rto(tp),
+					    tcp_rto_min(sk),
+					    TCP_RTO_MAX);
 	} else if (sk->sk_state != TCP_SYN_SENT ||
 		   icsk->icsk_backoff >
 		   READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
-- 
2.37.3