Message-ID: <5e814066-29b1-1968-f5e6-4e30067430b4@huawei.com>
Date:   Sat, 29 Jan 2022 14:53:59 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Vincent Ray <vray@...rayinc.com>
CC:     vladimir oltean <vladimir.oltean@....com>, kuba <kuba@...nel.org>,
        "Samuel Jones" <sjones@...rayinc.com>,
        netdev <netdev@...r.kernel.org>, davem <davem@...emloft.net>
Subject: Re: packet stuck in qdisc

On 2022/1/28 17:59, Vincent Ray wrote:
> Hmm ... sorry, actually I don't think I've seen this pb on a native host (because mine has an older kernel version and I did not change it), though I suppose it could happen too.
> 

Hi,
  I have tested NVME-over-TCP in a native arm64 host, which does not seems
to have the problem, the test log is in the attachment.

A debug patch in the attachment can be used to catch the packet_stuck_in_
qdisc problem, maybe give it a try.


> 
> ----- Original Message -----
> From: "Vincent Ray" <vray@...rayinc.com>
> To: "linyunsheng" <linyunsheng@...wei.com>
> Cc: "vladimir oltean" <vladimir.oltean@....com>, "kuba" <kuba@...nel.org>, "Samuel Jones" <sjones@...rayinc.com>, "netdev" <netdev@...r.kernel.org>, "davem" <davem@...emloft.net>
> Sent: Friday, January 28, 2022 9:58:05 AM
> Subject: Re: packet stuck in qdisc
> 
> Hello,
> 
> It seems the problem can be reproduced easily. Is there some testcase without
> hw dependency, so that I can reproduce the problem myself?
> 
> [VR]
> Well, in my case, I produced it with the setup described below, which does include the Kalray board.
> However, I am quite confident this has nothing to do with the board, not even with any particular Host NIC / cable, so that you can surely reproduce it in a different environment.
> I think you just need to send some NVME-over-TCP traffic to any target able to receive it. In fact I suspect that any TCP traffic will do.
> Attached is the fio test case we are running after doing the following nvme connect :
> nvme connect -n nqn.2014-06.com.kalrayinc:nvme:ecca9057-4b59-5332-8d75-5acdcdd8a88e -t tcp -i 4 -a 10.20.0.1'      
> And then simply
> fio ./fio_jobs_randwr4k_60s.spdk
> 
> 
> Which cpu is used in the testing? It seems the cpu arch's
> memory semantic is importance when handling the rare case.
> 
> [VR] 
> It's x86_64
> FWIW, I've seen the problem happen both on native host and whithin a VM (be vcpu pinned or not, be fio threads pinned or not).
> 
> 
> ----- Original Message -----
> From: "linyunsheng" <linyunsheng@...wei.com>
> To: "Vincent Ray" <vray@...rayinc.com>, "vladimir oltean" <vladimir.oltean@....com>, "kuba" <kuba@...nel.org>, "davem" <davem@...emloft.ne>
> Cc: "Samuel Jones" <sjones@...rayinc.com>, "netdev" <netdev@...r.kernel.org>
> Sent: Friday, January 28, 2022 3:36:27 AM
> Subject: Re: packet stuck in qdisc
> 
> On 2022/1/25 20:55, Vincent Ray wrote:
>> Dear kernel maintainers / developers,
>>
>> I work at Kalray where we are developping an NVME-over-TCP target controller board.
>> My setup is as such :
>> - a development workstation running Linux 5.x.y (the host)
>> - sending NVME-TCP traffic to our board, to which it is connected through a Mellanox NIC (Connect-X-5) and a 100G ETH cable
>>
>> While doing performance tests, using simple fio scenarios running over the regular kernel nvme-tcp driver on the host, we noticed important performance variations.
>> After some digging (using tcpdump on the host), we found that there were big "holes" in the tcp traffic sent by the host.
>> The scenario we observed is the following :
>> 1) a TCP segment gets lost (not sent by the host) on a particular TCP connection, leading to a gap in the seq numbers received by the board
>> 2) the board sends dup-acks and/or sacks (if configured) to signal this loss
>> 3) then, sometimes, the host stops emitting on that TCP connection for several seconds (as much as 14s observed)
>> 4) finally the host resumes emission, sending the missing packet
>> 5) then the TCP connection continues correctly with the appropriate throughput
>>
>> Such a scenario can be observed in the attached tcpdump (+ comments).
> 
> Hi,
>     Thanks for reporting the problem.
> 
>>
>> We noticed that this was happening only in recent versions of the kernel, so we dichotomized until we found the culprit commits :
>> we believe that the bug was introduced in qdisc updates for 5.14.rc1 by this set of commits, more precisely the middle one :
>>
>> [2021-06-22] d3e0f57 Yunsheng Lin net: sched: remove qdisc->empty for lockless qdisc
>> [2021-06-22] c4fef01 Yunsheng Lin net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc    *=> KO*
>> [2021-06-22] dd25296 Yunsheng Lin net: sched: avoid unnecessary seqcount operation for lockless qdisc   *=> still OK*
>>
>> As far as I can tell, the bug is still present in the mainline (at least it was in 5.16-rc4).
>> From what I understand / guess, some optimizations in lockless qdiscs have lead to a race making the qdisc unaware that a packet has been enqueued and is waiting for emission.
>> I suspect that, when this happens with TCP packets "to be retransmitted", the TCP stack will not timeout and try to retransmitt again because it uses skb_still_in_host_queue() to avoid useless re-retransmissions
>> From net/ipv4/ tcp_output.c :
>> //* Thanks to skb fast clones, we can detect if a prior transmit of                                                                                                                                                   /
>> / * a packet is still in a qdisc or driver queue.                                                                                                                                                                     /
>> / * In this case, there is very little point doing a retransmit !                                                                                                                                                     /
>> / */  /
>> I guess this plays a role in making these holes grow up to 14s, and an other layer than TCP might not experience it (?).
>>
>> The interface through which my traffic is going is :
>> eth3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
>>     link/ether b8:ce:f6:60:c9:97 brd ff:ff:ff:ff:ff:ff
>>     inet 10.20.0.254/24 scope global eth3
>>
>> As you can see, it uses a mq qdisc. I did not try with other qdiscs yet.
>>
>> Finally, if/when troubleshooting this problem in kernels older than 5.14.7, it's a good idea to first cherry-pick this patch :
>> [2021-09-09] ae66447 Keith Busch nvme-tcp: fix io_work priority inversion
>> because it fixes a nvme-tcp bug whose performance impact is itself so big that it "hides" the one we've discovered (bringing itself lots of holes at the nvme-tcp layer ...)
>>
>> On impacted kernels, the "pkt_stuck_in_qdisc" bug shows up in the order of zero to a few occurences per minute per TCP connection.
> 
> It seems the problem can be reproduced easily. Is there some testcase without
> hw dependency, so that I can reproduce the problem myself?
> 
>>
>> I did not really have time to understand it thoroughly, nor am I a network stack expert, so I don't propose any particular patch for it but I'll be happy to help testing fix attempts if it can help.
>> Please feel free to ask any additional information.
> 
> Which cpu is used in the testing? It seems the cpu arch's
> memory semantic is importance when handling the rare case.
> 
> 
>> Best regards,
>>
>>
>> *Vincent Ray*
>> *Senior Architect • Kalray*
>> Phone: +33 6 43 94 87 65
>> _vray@...rayinc.com_ • _www.kalrayinc.com_ <https://www.kalrayinc.com>
>>
>> Kalray logo <https://www.kalrayinc.com>
>> 	
>> Intelligent Data Processing
>> From Cloud to Edge
>>
>>
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> 
> 
> 
> .
> 

View attachment "board server_2022-01-29_11-20-29-fio_test.log" of type "text/plain" (210450 bytes)

View attachment "packet_stuck_in_qdisc_debug.patch" of type "text/plain" (1915 bytes)

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