netdev - Re: TCP many-connection regression (bisected to 4.5.0-rc2+)

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Date:   Tue, 23 Jan 2018 13:49:21 -0800
From:   Ben Greear <greearb@...delatech.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: TCP many-connection regression (bisected to 4.5.0-rc2+)

On 01/22/2018 10:16 AM, Eric Dumazet wrote:
> On Mon, 2018-01-22 at 09:28 -0800, Ben Greear wrote:
>> My test case is to have 6 processes each create 5000 TCP IPv4 connections to each other
>> on a system with 16GB RAM and send slow-speed data.  This works fine on a 4.7 kernel, but
>> will not work at all on a 4.13.  The 4.13 first complains about running out of tcp memory,
>> but even after forcing those values higher, the max connections we can get is around 15k.
>>
>> Both kernels have my out-of-tree patches applied, so it is possible it is my fault
>> at this point.
>>
>> Any suggestions as to what this might be caused by, or if it is fixed in more recent kernels?
>>
>> I will start bisecting in the meantime...
>>
>
> Hi Ben
>
> Unfortunately I have no idea.
>
> Are you using loopback flows, or have I misunderstood you ?
>
> How loopback connections can be slow-speed ?
>

Hello Eric, looks like it is one of your commits that causes the issue
I see.

Here are some more details on my specific test case I used to bisect:

I have two ixgbe ports looped back, configured on same subnet, but with different IPs.
Routing table rules, SO_BINDTODEVICE, binding to specific IPs on both client and server
side let me send-to-self over the external looped cable.

I have 2 mac-vlans on each physical interface.

I created 5 server-side connections on one physical port, and two more on one of the mac-vlans.

On the client-side, I create a process that spawns 5000 connections to the corresponding server side.

End result is 25,000 connections on one pair of real interfaces, and 10,000 connections on the
mac-vlan ports.

In the passing case, I get very close to all 5000 connections on all endpoints quickly.

In the failing case, I get a max of around 16k connections on the two physical ports.  The two mac-vlans have 10k connections
across them working reliably.  It seems to be an issue with 'connect' failing.

connect(2074, {sa_family=AF_INET, sin_port=htons(33012), sin_addr=inet_addr("10.1.1.5")}, 16) = -1 EINPROGRESS (Operation now in progress)
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 2075
fcntl(2075, F_GETFD)                    = 0
fcntl(2075, F_SETFD, FD_CLOEXEC)        = 0
setsockopt(2075, SOL_SOCKET, SO_BINDTODEVICE, "eth4\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
setsockopt(2075, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
bind(2075, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("10.1.1.4")}, 16) = 0
getsockopt(2075, SOL_SOCKET, SO_RCVBUF, [87380], [4]) = 0
getsockopt(2075, SOL_SOCKET, SO_SNDBUF, [16384], [4]) = 0
setsockopt(2075, SOL_TCP, TCP_NODELAY, [0], 4) = 0
fcntl(2075, F_GETFL)                    = 0x2 (flags O_RDWR)
fcntl(2075, F_SETFL, O_ACCMODE|O_NONBLOCK) = 0
connect(2075, {sa_family=AF_INET, sin_port=htons(33012), sin_addr=inet_addr("10.1.1.5")}, 16) = -1 EINPROGRESS (Operation now in progress)
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 2076
fcntl(2076, F_GETFD)                    = 0
fcntl(2076, F_SETFD, FD_CLOEXEC)        = 0
setsockopt(2076, SOL_SOCKET, SO_BINDTODEVICE, "eth4\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
setsockopt(2076, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
bind(2076, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("10.1.1.4")}, 16) = 0
getsockopt(2076, SOL_SOCKET, SO_RCVBUF, [87380], [4]) = 0
getsockopt(2076, SOL_SOCKET, SO_SNDBUF, [16384], [4]) = 0
setsockopt(2076, SOL_TCP, TCP_NODELAY, [0], 4) = 0
fcntl(2076, F_GETFL)                    = 0x2 (flags O_RDWR)
fcntl(2076, F_SETFL, O_ACCMODE|O_NONBLOCK) = 0
connect(2076, {sa_family=AF_INET, sin_port=htons(33012), sin_addr=inet_addr("10.1.1.5")}, 16) = -1 EADDRNOTAVAIL (Cannot assign requested address)
....


ea8add2b190395408b22a9127bed2c0912aecbc8 is the first bad commit
commit ea8add2b190395408b22a9127bed2c0912aecbc8
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Thu Feb 11 16:28:50 2016 -0800

     tcp/dccp: better use of ephemeral ports in bind()

     Implement strategy used in __inet_hash_connect() in opposite way :

     Try to find a candidate using odd ports, then fallback to even ports.

     We no longer disable BH for whole traversal, but one bucket at a time.
     We also use cond_resched() to yield cpu to other tasks if needed.

     I removed one indentation level and tried to mirror the loop we have
     in __inet_hash_connect() and variable names to ease code maintenance.

     Signed-off-by: Eric Dumazet <edumazet@...gle.com>
     Signed-off-by: David S. Miller <davem@...emloft.net>

:040000 040000 3af4595c6eb6d331e1cba78a142d44e00f710d81 e0c014ae8b7e2867256eff60f6210821d36eacef M	net


I will be happy to test patches or try to get any other results that might help diagnose
this problem better.

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com