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

[Yan Zhai]

On Tue, Jul 11, 2023 at 9:36 PM Jakub Kicinski <kuba@xxxxxxxxxx> 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