tcp: fix SNMP TCP timeout under-estimation

BugLink: https://bugs.launchpad.net/bugs/1857158
[ Upstream commit e1561fe2dd69dc5dddd69bd73aa65355bdfb048b ]

Previously the SNMP TCPTIMEOUTS counter has inconsistent accounting:
1. It counts all SYN and SYN-ACK timeouts
2. It counts timeouts in other states except recurring timeouts and
   timeouts after fast recovery or disorder state.

Such selective accounting makes analysis difficult and complicated. For
example the monitoring system needs to collect many other SNMP counters
to infer the total amount of timeout events. This patch makes TCPTIMEOUTS
counter simply counts all the retransmit timeout (SYN or data or FIN).

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 27e0cff5f83a704c28c5623b952a4561de66c60f..a78f782205af86728ae37c84155fc9c8e3e25cb7 100644 (file)
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -465,11 +465,12 @@ void tcp_retransmit_timer(struct sock *sk)
                goto out_reset_timer;
        }
 
+       __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTS);
        if (tcp_write_timeout(sk))
                goto out;
 
        if (icsk->icsk_retransmits == 0) {
-               int mib_idx;
+               int mib_idx = 0;
 
                if (icsk->icsk_ca_state == TCP_CA_Recovery) {
                        if (tcp_is_sack(tp))
@@ -484,10 +485,9 @@ void tcp_retransmit_timer(struct sock *sk)
                                mib_idx = LINUX_MIB_TCPSACKFAILURES;
                        else
                                mib_idx = LINUX_MIB_TCPRENOFAILURES;
-               } else {
-                       mib_idx = LINUX_MIB_TCPTIMEOUTS;
                }
-               __NET_INC_STATS(sock_net(sk), mib_idx);
+               if (mib_idx)
+                       __NET_INC_STATS(sock_net(sk), mib_idx);
        }
 
        tcp_enter_loss(sk);