net: fix the RTO timer retransmitting skb every 1ms if linear option is enabled

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 always 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@google.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 470f581..206418b 100644 (file)
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -591,7 +591,9 @@ out_reset_timer:
            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)) {