nginx-devel Digest, Vol 162, Issue 26

[上勾拳]

Hi Roman,

Thanks a bunch for your quick response, it really made a difference for me.

I went through Patch 3, and it's pretty cool! And about Patch 4, which also
addresses the reload route issue, I would like to share an experience from
utilizing this solution in a production environment. Unfortunately, I
encountered a significant challenge that surpasses the race condition
between bind and connect. Specifically, this solution led to a notable
performance degradation in the kernel's UDP packet lookup under high
concurrency scenarios. The excessive number of client sockets caused the
UDP hash table lookup performance degrade into a linked list, because the
udp hashtable is based on target ip and target port hash. To address this
lookup performance issue, we implemented a proprietary kernel patch that
introduces a 4-tuple hash table for UDP socket lookup. Although effective,
it appears that the eBPF solution is more versatile and universal.

Big thanks again for your attention!

Best Regards,
Zhenzhong

<nginx-devel-request at nginx.org> 于2024年2月26日周一 20:00写道：

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> Today's Topics:
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>    1. Re: Inquiry Regarding Handling of QUIC Connections During
>       Nginx Reload (Roman Arutyunyan)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 26 Feb 2024 15:49:30 +0400
> From: Roman Arutyunyan <arut at nginx.com>
> To: nginx-devel at nginx.org
> Subject: Re: Inquiry Regarding Handling of QUIC Connections During
>         Nginx Reload
> Message-ID: <20240226114930.izp2quxwsp2usnvg at N00W24XTQX>
> Content-Type: text/plain; charset=utf-8
>
> Hi,
>
> On Sun, Feb 25, 2024 at 03:53:23PM +0800, 上勾拳 wrote:
> > Hello,
> >
> > I hope this email finds you well. I am writing to inquire about the
> status
> > of an issue I have encountered regarding the handling of QUIC connections
> > when Nginx is reloaded.
> >
> > Recently, I delved into the Nginx eBPF implementation, specifically
> > examining how QUIC connection packets are handled, especially during
> Nginx
> > reloads. My focus was on ensuring that existing QUIC connection packets
> are
> > correctly routed to the appropriate worker after a reload, and the Nginx
> > eBPF prog have done this job perfectly.
> >
> > However, I observed that during a reload, new QUIC connections might be
> > directed to the old worker. The underlying problem stems from the fact
> that
> > new QUIC connections fail to match the eBPF reuseport socket map. The
> > kernel default logic then routes UDP packets based on the hash UDP
> 4-tuple
> > in the reuseport group socket array. Since the old worker's listen FDs
> > persist in the reuseport group socket array (reuse->socks), there is a
> > possibility that the old worker may still be tasked with handling new
> QUIC
> > connections.
> >
> > Given that the old worker should not process new requests, it results in
> > the old worker not responding to the QUIC new connection. Consequently,
> > clients have to endure the handshake timeout and retry the connection,
> > potentially encountering the old worker again, leading to an ineffective
> > cycle.
> >
> > In my investigation, I came across a thread in the nginx-devel mailing
> list
> > [https://www.mail-archive.com/nginx-devel@nginx.org/msg10627.html],
> where
> > it was mentioned that there would be some work addressing this issue.
> >
> > Considering my limited experience with eBPF, I propose a potential
> > solution. The eBPF program could maintain another eBPF map containing
> only
> > the listen sockets of the new worker. When the eBPF program calls
> > `bpf_sk_select_reuseport` and receives `-ENOENT`, it could utilize this
> new
> > eBPF map with the hash UDP 4-tuple to route the new QUIC connection to
> the
> > new worker. This approach aims to circumvent the kernel logic routing the
> > packet to the shutting down worker since removing the old worker's listen
> > socket from the reuseport group socket array seems unfeasible. Not sure
> > about whether this solution is a good idea and I also wonder if there are
> > other solutions for this. I would appreciate any insights or updates you
> > could provide on this matter. Thank you for your time and consideration.
>
> It is true QUIC in nginx does not handle reloads well.  This is a known
> issue
> and we are working on improving it.  I made an effort a while back to
> address
> QUIC reloads in nginx:
>
>
> https://mailman.nginx.org/pipermail/nginx-devel/2023-January/thread.html#16073
>
> Here patch #3 implements ePBF-based solution and patch #4 implements
> client sockets-based solution.  The client sockets require extra worker
> process
> privileges to bind to listen port, which is a serious problem for a typical
> nginx configuration.  The ePBF solution does not seem to have any problems,
> but we still need more feedback to proceed with this.  If you apply all 4
> patches, make sure you disable client sockets using
> --without-quic_client_sockets.  Otherwise just apply the first 3 patches.
>
> Here's a relevant trac ticket:
>
> https://trac.nginx.org/nginx/ticket/2528
>
> --
> Roman Arutyunyan
>
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>
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> ------------------------------
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> End of nginx-devel Digest, Vol 162, Issue 26
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