[PATCH 2.6.5] Re: Fw: Stack sends SYN/ACKs even though accept queue is full

[Nivedita Singhvi <niv@xxxxxxxxxx>]

Andrew Morton wrote:

> Begin forwarded message:
>
> Date: Thu, 29 Apr 2004 14:53:36 -0700
> From: Jan Olderdissen <jan@xxxxxxxxxxx>
> To: "'linux-kernel@xxxxxxxxxxxxxxx'" <linux-kernel@xxxxxxxxxxxxxxx>
> Subject: Stack sends SYN/ACKs even though accept queue is full
Attaching a patch which adds a sysctl to turn off this
behaviour.   Could you test this, please?  Patch against
2.6.5 vanilla kernel.  If you need a 2.4 version, let me
know.

> Because newly accepted connections are considered 'young', two such
> connections put on the synq will cause additional SYNs to be dropped until
> young connections age and additional connections are SYN/ACKed , etc. Since
> the initial TCP timeout is three seconds, you would expect two additional
> connections to be accepted every three seconds. However, experiments with
> 2.4.25 show that number to be two connections every four seconds for unclear
> reasons.
Normally, I think the expected behaviour was that connections
would be short-lived. This is a reasonable expectation for most
web-servers etc.   In which case, the accept queue would free
up frequently, and having the syn request right there would save
a full timeout and round trip over the Internet. i.e. useful in the
common case.

I don't think it is worthwhile for environments where connections
are long-lived and turnover is infrequent, added to a heavily
congested network.   I'd prefer to have this be a tunable option.

thanks,
Nivedita

diff -urN linux-2.6.5/include/linux/sysctl.h
linux-2.6.5synq/include/linux/sysctl.h
--- linux-2.6.5/include/linux/sysctl.h  2004-04-03 19:37:23.000000000
-0800
+++ linux-2.6.5synq/include/linux/sysctl.h      2004-04-30 15:24:12.000000000
-0700
@@ -323,6 +323,7 @@
        NET_IPV4_IPFRAG_SECRET_INTERVAL=94,
        NET_TCP_WESTWOOD=95,
        NET_IPV4_IGMP_MAX_MSF=96,
+       NET_TCP_PRELOAD_SYNQ=97,
};

enum {
diff -urN linux-2.6.5/include/net/tcp.h
linux-2.6.5synq/include/net/tcp.h
--- linux-2.6.5/include/net/tcp.h       2004-04-03 19:36:18.000000000 -0800
+++ linux-2.6.5synq/include/net/tcp.h   2004-04-30 13:55:18.000000000
-0700
@@ -583,6 +583,7 @@
extern int sysctl_tcp_frto;
extern int sysctl_tcp_low_latency;
extern int sysctl_tcp_westwood;
+extern int sysctl_tcp_preload_synq;

extern atomic_t tcp_memory_allocated;
extern atomic_t tcp_sockets_allocated;
diff -urN linux-2.6.5/net/ipv4/sysctl_net_ipv4.c
linux-2.6.5synq/net/ipv4/sysctl_net_ipv4.c
--- linux-2.6.5/net/ipv4/sysctl_net_ipv4.c	2004-04-03 19:37:37.000000000
-0800
+++ linux-2.6.5synq/net/ipv4/sysctl_net_ipv4.c	2004-04-30
16:16:57.000000000 -0700
@@ -601,6 +601,14 @@
		.mode		= 0644,
		.proc_handler	= &proc_dointvec,
	},
+	{
+		.ctl_name	= NET_TCP_PRELOAD_SYNQ, 
+		.procname	= "tcp_preload_synq",
+		.data		= &sysctl_tcp_preload_synq,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
	{ .ctl_name = 0 }
};

diff -urN linux-2.6.5/net/ipv4/tcp_ipv4.c
linux-2.6.5synq/net/ipv4/tcp_ipv4.c
--- linux-2.6.5/net/ipv4/tcp_ipv4.c     2004-04-03 19:36:55.000000000 -0800
+++ linux-2.6.5synq/net/ipv4/tcp_ipv4.c 2004-04-30 15:51:38.000000000
-0700
@@ -78,6 +78,7 @@
extern int sysctl_ip_dynaddr;
int sysctl_tcp_tw_reuse;
int sysctl_tcp_low_latency;
+int sysctl_tcp_preload_synq = 1;

/* Check TCP sequence numbers in ICMP packets. */
#define ICMP_MIN_LENGTH 8
@@ -1442,8 +1443,11 @@
         * clogging syn queue with openreqs with exponentially increasing
         * timeout.
         */
-       if (tcp_acceptq_is_full(sk) && tcp_synq_young(sk) > 1)
-               goto drop;
+       if (tcp_acceptq_is_full(sk)) {
+               if (!sysctl_tcp_preload_synq ||
+                  (sysctl_tcp_preload_synq && (tcp_synq_young(sk) > 1)))
+                       goto drop;
+       }

        req = tcp_openreq_alloc();
        if (!req)
@@ -2683,4 +2687,5 @@
EXPORT_SYMBOL(sysctl_local_port_range);
EXPORT_SYMBOL(sysctl_max_syn_backlog);
EXPORT_SYMBOL(sysctl_tcp_low_latency);
+EXPORT_SYMBOL(sysctl_tcp_preload_synq);
#endif