Date:   Fri, 28 Jan 2022 10:36:27 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Vincent Ray <vray@...rayinc.com>, <vladimir.oltean@....com>,
        <kuba@...nel.org>, <davem@...emloft.ne>
CC:     Samuel Jones <sjones@...rayinc.com>, <netdev@...r.kernel.org>
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>
> 
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