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

From: Lawrence Brakmo <[email-masked]>
Date: Tue, 27 Sep 2016 16:31:23 -0700

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 <[email-masked]>
---
 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) {
+			sk_rethink_txhash(sk);
 		}
 
 		retry_until = net->ipv4.sysctl_tcp_retries2;
-- 
2.9.3