Date:   Tue, 23 Jan 2018 16:05:50 -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 01/23/2018 03:27 PM, Ben Greear wrote:
> On 01/23/2018 03:21 PM, Eric Dumazet wrote:
>> On Tue, 2018-01-23 at 15:10 -0800, Ben Greear wrote:
>>> On 01/23/2018 02:29 PM, Eric Dumazet wrote:
>>>> On Tue, 2018-01-23 at 14:09 -0800, Ben Greear wrote:
>>>>> On 01/23/2018 02:07 PM, Eric Dumazet wrote:
>>>>>> On Tue, 2018-01-23 at 13:49 -0800, Ben Greear wrote:
>>>>>>> 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.
>>>>>>
>>>>>> Problem is I do not see anything obvious here.
>>>>>>
>>>>>> Please provide /proc/sys/net/ipv4/ip_local_port_range
>>>>>
>>>>> [root@...003-e3v2-13100124-f20x64 ~]# cat /proc/sys/net/ipv4/ip_local_port_range
>>>>> 10000    61001
>>>>>
>>>>>>
>>>>>> Also you probably could use IP_BIND_ADDRESS_NO_PORT socket option
>>>>>> before the bind()
>>>>>
>>>>> I'll read up on that to see what it does...
>>>>
>>>> man 7 ip
>>>>
>>>>        IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
>>>>               Inform
>>>> the kernel to not reserve an ephemeral  port
>>>>               when  using
>>>> bind(2)  with a port number of 0.  The
>>>>               port will later be
>>>> automatically  chosen  at  con‐
>>>>               nect(2) time, in a way
>>>> that allows sharing a source
>>>>               port as long as the 4-tuple
>>>> is unique.
>>>>
>>>
>>> Yes, I found that.
>>>
>>> It appears this option works well for my case, and I see 30k connections across my pair of e1000e
>>> (though the NIC is wretching again, so I guess its issues are not fully resolved).
>>>
>>> I tested this on my 4.13.16+ kernel.
>>>
>>> But that said, maybe there is still some issue with the patch I bisected to, so if you have
>>> other suggestions, I can back out this IP_BIND_ADDRESS_NO_PORT feature and re-test.
>>>
>>> Also, I had to increase /proc/sys/net/ipv4/tcp_mem to get 30k connections to work without
>>> the kernel spamming:
>>>
>>> Jan 23 15:02:41 lf1003-e3v2-13100124-f20x64 kernel: TCP: out of memory -- consider tuning tcp_mem
>>> Jan 23 15:02:41 lf1003-e3v2-13100124-f20x64 kernel: TCP: out of memory -- consider tuning tcp_mem
>>>
>>> This is a 16 GB RAM system, and I did not have to tune this on the 4.5.0-rc2+ (good) kernels
>>> to get the similar performance.  I was testing on ixgbe there though, possibly that is part
>>> of it, or maybe I just need to force tcp_mem to be larger on more recent kernels??
>>
>>
>> Since linux-4.2 tcp_mem[0,1,2] defaults are 4.68%, 6.25%, 9.37% of
>> physical memory.
>>
>> It used to be twice that in older kernels.
>>
>> It is also possible that some change in TCP congestion or autotuning
>> allows each of your TCP flow to store more data in its write queue,
>> if your application is pushing bulk data as much as it can.
>>
>> It is virtually not possible to change anything in the kernel without
>> zero impact on very pathological use cases.
>
> Yes, but also pathological use cases may uncover a real issue that normal
> users will not often notice....  Based on the commit message, it seems you
> expected no real regressions with that patch, but at least in my case, I see
> large ones, so something might be off with it.
>
>>
>> tcp_wmem[2] is 4MB.
>>
>> 30,000 * 4MB = 120 GB
>>
>> Definitely more than your physical memory.
>
> I'll spend some time looking at the tcp-mem issue now that I have a work-around for
> the many connection issues...

Looks like when I use the ixgbe, I can do 70k connections (using 2 physical, plus two mac-vlans on each
to ensure no more than 30k connections per IP pair).  It runs solid, 3GB RAM free, and no tcp-mem warnings.

e1000e has lots of tx-hangs, and that exacerbates tcp memory pressure it seems.

So, seems I'm good to go on this as long as I stay away from e1000e.

Thanks,
Ben


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

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