tcp: allow for bigger reordering level

While testing upcoming Yaogong patch (converting out of order queue
into an RB tree), I hit the max reordering level of linux TCP stack.

Reordering level was limited to 127 for no good reason, and some
network setups [1] can easily reach this limit and get limited
throughput.

Allow a new max limit of 300, and add a sysctl to allow admins to even
allow bigger (or lower) values if needed.

[1] Aggregation of links, per packet load balancing, fabrics not doing
 deep packet inspections, alternative TCP congestion modules...

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yaogong Wang <wygivan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/bonding.txt

@@ -2230,11 +2230,8 @@ balance-rr: This mode is the only mode that will permit a single
 
 	It is possible to adjust TCP/IP's congestion limits by
 	altering the net.ipv4.tcp_reordering sysctl parameter.  The
-	usual default value is 3, and the maximum useful value is 127.
+	usual default value is 3. But keep in mind TCP stack is able
-	For a four interface balance-rr bond, expect that a single
+	to automatically increase this when it detects reorders.
-	TCP/IP stream will utilize no more than approximately 2.3
-	interface's worth of throughput, even after adjusting
-	tcp_reordering.
 
 	Note that the fraction of packets that will be delivered out of
 	order is highly variable, and is unlikely to be zero.  The level

Documentation/networking/ip-sysctl.txt

@@ -376,9 +376,17 @@ tcp_orphan_retries - INTEGER
 	may consume significant resources. Cf. tcp_max_orphans.
 
 tcp_reordering - INTEGER
-	Maximal reordering of packets in a TCP stream.
+	Initial reordering level of packets in a TCP stream.
+	TCP stack can then dynamically adjust flow reordering level
+	between this initial value and tcp_max_reordering
 	Default: 3
 
+tcp_max_reordering - INTEGER
+	Maximal reordering level of packets in a TCP stream.
+	300 is a fairly conservative value, but you might increase it
+	if paths are using per packet load balancing (like bonding rr mode)
+	Default: 300
+
 tcp_retrans_collapse - BOOLEAN
 	Bug-to-bug compatibility with some broken printers.
 	On retransmit try to send bigger packets to work around bugs in

include/linux/tcp.h

@@ -204,10 +204,10 @@ struct tcp_sock {
 
 	u16	urg_data;	/* Saved octet of OOB data and control flags */
 	u8	ecn_flags;	/* ECN status bits.			*/
-	u8	reordering;	/* Packet reordering metric.		*/
+	u8	keepalive_probes; /* num of allowed keep alive probes	*/
+	u32	reordering;	/* Packet reordering metric.		*/
 	u32	snd_up;		/* Urgent pointer		*/
 
-	u8	keepalive_probes; /* num of allowed keep alive probes	*/
 /*
  *      Options received (usually on last packet, some only on SYN packets).
  */

include/net/tcp.h

@@ -70,9 +70,6 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 /* After receiving this amount of duplicate ACKs fast retransmit starts. */
 #define TCP_FASTRETRANS_THRESH 3
 
-/* Maximal reordering. */
-#define TCP_MAX_REORDERING	127
-
 /* Maximal number of ACKs sent quickly to accelerate slow-start. */
 #define TCP_MAX_QUICKACKS	16U
 
@@ -252,6 +249,7 @@ extern int sysctl_tcp_abort_on_overflow;
 extern int sysctl_tcp_max_orphans;
 extern int sysctl_tcp_fack;
 extern int sysctl_tcp_reordering;
+extern int sysctl_tcp_max_reordering;
 extern int sysctl_tcp_dsack;
 extern long sysctl_tcp_mem[3];
 extern int sysctl_tcp_wmem[3];

net/ipv4/sysctl_net_ipv4.c

@@ -495,6 +495,13 @@ static struct ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec
 	},
+	{
+		.procname	= "tcp_max_reordering",
+		.data		= &sysctl_tcp_max_reordering,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec
+	},
 	{
 		.procname	= "tcp_dsack",
 		.data		= &sysctl_tcp_dsack,

net/ipv4/tcp_input.c

@@ -81,6 +81,7 @@ int sysctl_tcp_window_scaling __read_mostly = 1;
 int sysctl_tcp_sack __read_mostly = 1;
 int sysctl_tcp_fack __read_mostly = 1;
 int sysctl_tcp_reordering __read_mostly = TCP_FASTRETRANS_THRESH;
+int sysctl_tcp_max_reordering __read_mostly = 300;
 EXPORT_SYMBOL(sysctl_tcp_reordering);
 int sysctl_tcp_dsack __read_mostly = 1;
 int sysctl_tcp_app_win __read_mostly = 31;
@@ -833,7 +834,7 @@ static void tcp_update_reordering(struct sock *sk, const int metric,
 	if (metric > tp->reordering) {
 		int mib_idx;
 
-		tp->reordering = min(TCP_MAX_REORDERING, metric);
+		tp->reordering = min(sysctl_tcp_max_reordering, metric);
 
 		/* This exciting event is worth to be remembered. 8) */
 		if (ts)