[PATCH v3 net-next] tcp: Change txhash on every SYN and RTO retransmit

The current code changes txhash (flowlables) on every retransmitted
SYN/ACK, but only after the 2nd retransmitted SYN and only after
tcp_retries1 RTO retransmits.

With this patch:
1) txhash is changed with every SYN retransmits
2) adds the option for the txhash to be changed before tcp_retries1
   RTO retransmits. A new sysctl tcp_rto_txhash_prob represents the
   probability that txhash will be changed. A value of 0 maintains
   previous behavior and a value of 100 will always change it.

The result is that we can start re-routing around failed (or very
congested paths) as soon as possible. Otherwise application health
checks may fail and the connection may be terminated before we start
to change txhash.

v3: Removed text saying default value of sysctl is 0 (it is 100)
v2: Added sysctl documentation and cleaned code

Tested with packetdrill tests

Signed-off-by: Lawrence Brakmo <redacted>
---
 Documentation/networking/ip-sysctl.txt | 11 +++++++++++
 include/net/tcp.h                      |  1 +
 net/ipv4/sysctl_net_ipv4.c             | 10 ++++++++++
 net/ipv4/tcp_input.c                   |  2 ++
 net/ipv4/tcp_timer.c                   |  4 ++++
 5 files changed, 28 insertions(+)

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 3db8c67..0e7f9ac 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt

@@ -472,6 +472,17 @@ tcp_max_reordering - INTEGER
        if paths are using per packet load balancing (like bonding rr mode)
        Default: 300

+tcp_rto_txhash_prob - INTEGER
+       Probability [0 to 100] that we will recalculate txhash when a
+       packet is resent due to an RTO and the RTO for this packet has
+       fired less than tcp_retries1 times. It is always recalculated
+       after tcp_retries_times.
+
+       Setting it to 100 helps re-route around failed (or very congested
+       paths) as soon as possible. Otherwise application health checks may
+       fail and the connection may be terminated before the txhash has
+       a chance to change.
+
 tcp_retrans_collapse - BOOLEAN
        Bug-to-bug compatibility with some broken printers.
        On retransmit try to send bigger packets to work around bugs in

diff --git a/include/net/tcp.h b/include/net/tcp.h
index f83b7f2..406d474 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h

@@ -271,6 +271,7 @@ extern int sysctl_tcp_autocorking;
 extern int sysctl_tcp_invalid_ratelimit;
 extern int sysctl_tcp_pacing_ss_ratio;
 extern int sysctl_tcp_pacing_ca_ratio;
+extern int sysctl_tcp_rto_txhash_prob;

 extern atomic_long_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 1cb67de..0b185a1 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c

@@ -28,6 +28,7 @@
 static int zero;
 static int one = 1;
 static int four = 4;
+static int hundred = 100;
 static int thousand = 1000;
 static int gso_max_segs = GSO_MAX_SEGS;
 static int tcp_retr1_max = 255;

@@ -624,6 +625,15 @@ static struct ctl_table ipv4_table[] = {
                .proc_handler   = proc_dointvec_ms_jiffies,
        },
        {
+               .procname       = "tcp_rto_txhash_prob",
+               .data           = &sysctl_tcp_rto_txhash_prob,
+               .maxlen         = sizeof(int),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = &zero,
+               .extra2         = &hundred,
+       },
+       {
                .procname       = "icmp_msgs_per_sec",
                .data           = &sysctl_icmp_msgs_per_sec,
                .maxlen         = sizeof(int),

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8c6ad2d..2fea29d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -101,6 +101,8 @@ int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
 int sysctl_tcp_early_retrans __read_mostly = 3;
 int sysctl_tcp_invalid_ratelimit __read_mostly = HZ/2;

+int sysctl_tcp_rto_txhash_prob __read_mostly = 100;
+
 #define FLAG_DATA              0x01 /* Incoming frame contained data.          */
 #define FLAG_WIN_UPDATE                0x02 /* Incoming ACK was a window update.       */
 #define FLAG_DATA_ACKED                0x04 /* This ACK acknowledged new data.         */

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index f712b41..8bdb215 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c

@@ -192,6 +192,8 @@ static int tcp_write_timeout(struct sock *sk)
                        if (tp->syn_data && icsk->icsk_retransmits == 1)
                                NET_INC_STATS(sock_net(sk),
                                              LINUX_MIB_TCPFASTOPENACTIVEFAIL);
+               } else if (!tp->syn_data && !tp->syn_fastopen) {

+                       sk_rethink_txhash(sk);
                }
                retry_until = icsk->icsk_syn_retries ? : net->ipv4.sysctl_tcp_syn_retries;
                syn_set = true;

@@ -213,6 +215,8 @@ static int tcp_write_timeout(struct sock *sk)
                        tcp_mtu_probing(icsk, sk);

                        dst_negative_advice(sk);
+               } else if (prandom_u32_max(100) < sysctl_tcp_rto_txhash_prob) {