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

[Yan Zhai]

On Wed, Jul 12, 2023 at 12:42 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Wed, 12 Jul 2023 11:42:26 -0500 Yan Zhai wrote:
> >   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.
>
> No doubt that kfree_skb isn't going to solve all our needs, but I'd
> really like you to clean up the unnecessary callers on your systems
> first, before adding further tracepoints. That way we'll have a clear
> picture of which points can be solved by kfree_skb and where we need
> further work.

Those are not unnecessary calls, e.g. a lot of those kfree_skb come
from iptables drops, tcp validation, ttl expires, etc. On a moderately
loaded server, it is called at a rate of ~10k/sec, which isn't
terribly awful given that we absorb millions of attack packets at each
data center. We used to have many consume skb noises at this trace
point with older versions of kernels, but those have gone ever since
the better separation between consume and drop.

That said, accessing sk information is still the critical piece to
address our use case. Is there any other possible way that we can get
this information at the accept queue overflow time?

--
Yan