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SubjectDoubts about listen backlog and tcp_max_syn_backlog
Hi, I'm having some problems with missing SYNs in a server with a high
rate of incoming connections and, even when far from understanding the
kernel, I ended up looking at the kernel's source to try to understand
better what's going on, because some stuff doesn't make a lot of sense
to me.

The path I followed is this (line numbers for Linux 3.7):
net/socket.c[3]
SYSCALL_DEFINE2(listen, int, fd, int, backlog)
backlog is truncated to sysctl_somaxconn and
sock->ops->listen(sock, backlog) is called, which I guess it
calls to inet_listen().

net/ipv4/af_inet.c[4]
int inet_listen(struct socket *sock, int backlog)
the backlog is assigned to sk->sk_max_ack_backlog and
inet_csk_listen_start(sk, backlog) is called (if the socket
wans't already in TCP_LISTEN state)

net/ipv4/inet_connection_sock.c[5]
int inet_csk_listen_start(struct sock *sk, const int nr_table_entries)
reqsk_queue_alloc(&icsk->icsk_accept_queue, nr_table_entries) is
called, which I guess it creates the actual queue

net/core/request_sock.c[6]
int reqsk_queue_alloc(struct request_sock_queue *queue,
unsigned int nr_table_entries)
nr_table_entries is first adjusted to satisfy:
8 <= nr_table_entries <= sysctl_max_syn_backlog
and then incremented by one and rounded up to the next power of
2.

So here are a couple of questions:

1. What's the relation between the socket backlog and the queue created
by reqsk_queue_alloc()? Because the backlog is only adjusted not to
be grater than sysctl_somaxconn, but the queue size can be quite
different.
2. The comment just above the definition of reqsk_queue_alloc() about
sysctl_max_syn_backlog says "Maximum number of SYN_RECV sockets in
queue per LISTEN socket.". But then nr_table_entries is not only
rounded up to the next power of 2, is incremented by one before that,
so a backlog of, for example, 128, would end up with 256 table
entries even if sysctl_max_syn_backlog is 128.
3. Why is there a nr_table_entries + 1 at all in there? Looking at the
commit that introduced this[1] I can't find any explanation and I've
read some big projects are using backlogs of 511 because of this[2].
(which BTW, ff the queue is really a hash table, looks like an awful
idea).
4. I found some places sk->sk_ack_backlog is checked against
sk->sk_max_ack_backlog to see if new requests should be dropped, but
I also saw checks like inet_csk_reqsk_queue_young(sk) > 1 or
inet_csk_reqsk_queue_is_full(sk), so I guess the queue is used too.


Thanks a lot.

[1] http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=commitdiff;h=72a3effaf633bcae9034b7e176bdbd78d64a71db
[2] http://blog.dubbelboer.com/2012/04/09/syn-cookies.html#a_reasonably_backlog_size
[3] http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/socket.c;h=2ca51c719ef984cdadef749008456cf7bd5e1ae4;hb=HEAD#l1544
[4] http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/ipv4/af_inet.c;h=24b384b7903ea7a59a11e7a4cbf06db996498924;hb=HEAD#l192
[5] http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/ipv4/inet_connection_sock.c;h=d0670f00d5243f95bec4536f60edf32fa2ded850;hb=HEAD#l729
[6] http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/core/request_sock.c;h=c31d9e8668c30346894adbf3be55eed4beeb1258;hb=HEAD#l23

--
Leandro Lucarella
sociomantic labs GmbH
http://www.sociomantic.com


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