netdev - Re: [RFC PATCH net-next] tcp: add a tracepoint for tcp_listen_queue

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Date: Wed, 12 Jul 2023 11:42:26 -0500
From: Yan Zhai <yan@...udflare.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Ivan Babrou <ivan@...udflare.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kernel-team@...udflare.com, 
	Eric Dumazet <edumazet@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	Paolo Abeni <pabeni@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, David Ahern <dsahern@...nel.org>
Subject: Re: [RFC PATCH net-next] tcp: add a tracepoint for tcp_listen_queue_drop

On Tue, Jul 11, 2023 at 9:36 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 10 Jul 2023 21:34:52 -0700 Ivan Babrou wrote:
> > There's already a way to count the overall numbers of queue overflows:
> >
> >     $ sudo netstat -s | grep 'listen queue'
> >     4 times the listen queue of a socket overflowed
> >
> > However, it's too coarse for monitoring and alerting when a user wants to
> > track errors per socket and route alerts to people responsible for those
> > sockets directly. For UDP there's udp_fail_queue_rcv_skb, which fills
> > a similar need for UDP sockets. This patch adds a TCP equivalent.
>
> Makes me want to revert your recent UDP tracepoint to be honest :(
> We can play whack a mole like this. You said that kfree_skb fires
> too often, why is that? Maybe it's an issue of someone using
> kfree_skb() when they should be using consume_skb() ?

Hi Jakub,

  The issue with kfree_skb is not that it fires too frequently (not in
the 6.x kernel now). Rather, it is unable to locate the socket info
when a SYN is dropped due to the accept queue being full. The sk is
stolen upon inet lookup, e.g. in tcp_v4_rcv. This makes it unable to
tell in kfree_skb which socket a SYN skb is targeting (when TPROXY or
socket lookup are used). A tracepoint with sk information will be more
useful to monitor accurately which service/socket is involved.

--
Yan