tcp: increase tcp_max_syn_backlog max value

tcp_max_syn_backlog default value depends on memory size
and TCP ehash size. Before this patch, the max value
was 2048 [1], which is considered too small nowadays.

Increase it to 4096 to match the recent SOMAXCONN change.

[1] This is with TCP ehash size being capped to 524288 buckets.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Yue Cao <ycao009@ucr.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -408,11 +408,14 @@ tcp_max_orphans - INTEGER
 	up to ~64K of unswappable memory.
 
 tcp_max_syn_backlog - INTEGER
-	Maximal number of remembered connection requests, which have not
-	received an acknowledgment from connecting client.
+	Maximal number of remembered connection requests (SYN_RECV),
+	which have not received an acknowledgment from connecting client.
+	This is a per-listener limit.
 	The minimal value is 128 for low memory machines, and it will
 	increase in proportion to the memory of machine.
 	If server suffers from overload, try increasing this number.
+	Remember to also check /proc/sys/net/core/somaxconn
+	A SYN_RECV request socket consumes about 304 bytes of memory.
 
 tcp_max_tw_buckets - INTEGER
 	Maximal number of timewait sockets held by system simultaneously.

net/ipv4/tcp_ipv4.c

@@ -2681,7 +2681,7 @@ static int __net_init tcp_sk_init(struct net *net)
 	net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2;
 	net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo;
 
-	net->ipv4.sysctl_max_syn_backlog = max(128, cnt / 256);
+	net->ipv4.sysctl_max_syn_backlog = max(128, cnt / 128);
 	net->ipv4.sysctl_tcp_sack = 1;
 	net->ipv4.sysctl_tcp_window_scaling = 1;
 	net->ipv4.sysctl_tcp_timestamps = 1;