From mboxrd@z Thu Jan 1 00:00:00 1970 From: Venkat Venkatsubra Subject: Re: listen(2) backlog changes in or around Linux 3.1? Date: Thu, 18 Oct 2012 11:00:31 -0500 Message-ID: <5080279F.80008@oracle.com> References: <507C4401.7050500@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org To: enh Return-path: Received: from acsinet15.oracle.com ([141.146.126.227]:17587 "EHLO acsinet15.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752600Ab2JRQAf (ORCPT ); Thu, 18 Oct 2012 12:00:35 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: Hi Elliott, I see the same behavior with your test program. The connect() keeps succeeding even though accept() is not performed. It pauses after 4 connections for a while and then periodically keeps adding few (2 I think). But the server side end points are terminated too. You will see only the first 2 sessions on the server side. If you modify your test program to say read or poll the sockets you should get a termination notification on them I think . The behavior overall looks fine in my opinion. But it could be a change of behavior for your test program. Venkat On 10/16/2012 6:31 PM, enh wrote: > boiling things down to a short C++ program, i see that i can reproduce > the behavior even on 2.6 kernels. if i run this, i see 4 connections > immediately (3 + 1, as i'd expect)... but then about 10s later i see > another 2. and every few seconds after that, i see another 2. i've let > this run until i have hundreds of connect(2) calls that have returned, > despite my small listen(2) backlog and the fact that i'm not > accept(2)ing. > > so i guess the only thing that's changed with newer kernels is timing > (hell, since i only see newer kernels on newer hardware, it might just > be a hardware thing). > > and clearly i don't understand what the listen(2) backlog means any more. > > #include > #include > #include > #include > #include > #include > #include > #include > > void dump_ti(int fd) { > tcp_info ti; > socklen_t tcp_info_length = sizeof(tcp_info); > int rc = getsockopt(fd, SOL_IP, TCP_INFO,&ti,&tcp_info_length); > if (rc == -1) { > std::cout<< "getsockopt rc "<< rc<< ": "<< strerror(errno)<< "\n"; > return; > } > > std::cout<< "ti.tcpi_unacked="<< ti.tcpi_unacked<< "\n"; > std::cout<< "ti.tcpi_sacked="<< ti.tcpi_sacked<< "\n"; > } > > void connect_to(sockaddr_in& sa) { > int s = socket(AF_INET, SOCK_STREAM, 0); > if (s == -1) { > abort(); > } > > int rc = connect(s, (sockaddr*)&sa, sizeof(sockaddr_in)); > std::cout<< "connect = "<< rc<< "\n"; > } > > int main() { > int ss = socket(AF_INET, SOCK_STREAM, 0); > std::cout<< "socket fd "<< ss<< "\n"; > > sockaddr_in sa; > memset(&sa, 0, sizeof(sa)); > sa.sin_family = AF_INET; > sa.sin_addr.s_addr = htonl(INADDR_ANY); > sa.sin_port = htons(9877); > int rc = bind(ss, (sockaddr*)&sa, sizeof(sa)); > std::cout<< "bind rc "<< rc<< ": "<< strerror(errno)<< "\n"; > std::cout<< "bind port "<< sa.sin_port<< "\n"; > > rc = listen(ss, 1); > std::cout<< "listen rc "<< rc<< ": "<< strerror(errno)<< "\n"; > dump_ti(ss); > > while (true) { > connect_to(sa); > dump_ti(ss); > } > > return 0; > } > > > On Mon, Oct 15, 2012 at 10:26 AM, enh wrote: >> On Mon, Oct 15, 2012 at 10:12 AM, Venkat Venkatsubra >> wrote: >>> On 10/12/2012 6:40 PM, enh wrote: >>>> i used to use the following hack to unit test connect timeouts: i'd >>>> call listen(2) on a socket and then deliberately connect (backlog + 3) >>>> sockets without accept(2)ing any of the connections. (why 3? because >>>> Stevens told me so, and experiment backed him up. see figure 4.10 in >>>> his UNIX Network Programming.) >>>> >>>> with "old" kernels, 2.6.35-ish to 3.0-ish, this worked great. my next >>>> connect(2) to the same loopback port would hang indefinitely. i could >>>> even unblock the connect by calling accept(2) in another thread. this >>>> was awesome for testing. >>>> >>>> in 3.1 on ARM, 3.2 on x86 (Ubuntu desktop), and 3.4 on ARM, this no >>>> longer works. it doesn't seem to be as simple as "the constant is no >>>> longer 3". my tests are now flaky. sometimes they work like they used >>>> to, and sometimes an extra connect(2) will succeed. (or, if i'm in >>>> non-blocking mode, my poll(2) will return with the non-blocking socket >>>> that's trying to connect now ready.) >>>> >>>> i'm guessing if this changed in 3.1 and is still changed in 3.4, >>>> whatever's changed wasn't an accident. but i haven't been able to find >>>> the right search terms to RTFM. i also finally got around to grepping >>>> the kernel for the "+ 3", but wasn't able to find that. (so i'd be >>>> interested to know where the old behavior came from too.) >>>> >>>> my least worst workaround at the moment is to use one of RFC5737's >>>> test networks, but that requires that the device have a network >>>> connection, otherwise my connect(2)s fail immediately with >>>> ENETUNREACH, which is no use to me. also, unlike my old trick, i've >>>> got no way to suddenly "unblock" a slow connect(2) (this is useful for >>>> unit testing the code that does the poll(2) part of the usual >>>> connect-with-timeout implementation). >>>> https://android-review.googlesource.com/#/c/44563/ >>>> >>>> hopefully someone here can shed some light on this? ideally someone >>>> will have a workaround as good as my old trick. i realize i was >>>> relying on undocumented behavior, and i'm happy to have to check >>>> /proc/version and behave appropriately, but i'd really like a way to >>>> keep my unit tests! >>>> >>>> thanks, >>>> elliott >>>> -- >>>> To unsubscribe from this list: send the line "unsubscribe netdev" in >>>> the body of a message to majordomo@vger.kernel.org >>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>> Hi Elliott, >>> >>> In BSD I think the backlog used to be reset to 3/2 times that passed by the >>> user. So, 2 becomes 3. >>> Probably the 1/2 times increase was to accommodate the ones in >>> partial/incomplete queue. >>> In Linux is it possible you were getting the same behavior before the below >>> commit ? >>> Since the check used to be "backlog+1" a 2 will behave as 3 ? >> i don't think so, because with<= 3.0 kernels i used to have a backlog >> of 1 and be able to make _4_ connections before my next connect would >> hang. but this> to>= change is at least something for me to >> investigate... >> >>> commit 8488df894d05d6fa41c2bd298c335f944bb0e401 >>> Author: Wei Dong >>> Date: Fri Mar 2 12:37:26 2007 -0800 >>> >>> [NET]: Fix bugs in "Whether sock accept queue is full" checking >>> >>> when I use linux TCP socket, and find there is a bug in function >>> sk_acceptq_is_full(). >>> >>> When a new SYN comes, TCP module first checks its validation. If >>> valid, >>> send SYN,ACK to the client and add the sock to the syn hash table. Next >>> time if received the valid ACK for SYN,ACK from the client. server will >>> accept this connection and increase the sk->sk_ack_backlog -- which is >>> done in function tcp_check_req().We check wether acceptq is full in >>> function tcp_v4_syn_recv_sock(). >>> >>> Consider an example: >>> >>> After listen(sockfd, 1) system call, sk->sk_max_ack_backlog is set to >>> 1. As we know, sk->sk_ack_backlog is initialized to 0. Assuming accept() >>> system call is not invoked now. >>> >>> 1. 1st connection comes. invoke sk_acceptq_is_full(). >>> sk->sk_ack_backlog=0 sk->sk_max_ack_backlog=1, function return 0 accept >>> this connection. >>> Increase the sk->sk_ack_backlog >>> 2. 2nd connection comes. invoke sk_acceptq_is_full(). >>> sk->sk_ack_backlog=1 sk->sk_max_ack_backlog=1, function return 0 accept >>> this connection. >>> Increase the sk->sk_ack_backlog >>> 3. 3rd connection comes. invoke sk_acceptq_is_full(). >>> sk->sk_ack_backlog=2 sk->sk_max_ack_backlog=1, function return 1. >>> Refuse this connection. >>> >>> I think it has bugs. after listen system call. sk->sk_max_ack_backlog=1 >>> but now it can accept 2 connections. >>> >>> Signed-off-by: Wei Dong >>> Signed-off-by: David S. Miller >>> >>> Venkat > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html