Subject: Re: [RFC PATCH net-next 00/12] drop_monitor: Capture dropped packets and metadata

From: Ido Schimmel <[email-masked]>
Date: Tue, 23 Jul 2019 09:46:59 +0300

On Mon, Jul 22, 2019 at 09:43:15PM +0200, Toke Høiland-Jørgensen wrote:
> Is there a mechanism for the user to filter the packets before they are
> sent to userspace? A bpf filter would be the obvious choice I guess...

Hi Toke,

Yes, it's on my TODO list to write an eBPF program that only lets
"unique" packets to be enqueued on the netlink socket. Where "unique" is
defined as {5-tuple, PC}. The rest of the copies will be counted in an
eBPF map, which is just a hash table keyed by {5-tuple, PC}.

I think it would be good to have the program as part of the bcc
repository [1]. What do you think?

> For integrating with XDP the trick would be to find a way to do it that
> doesn't incur any overhead when it's not enabled. Are you envisioning
> that this would be enabled separately for the different "modes" (kernel,
> hardware, XDP, etc)?

Yes. Drop monitor have commands to enable and disable tracing, but they
don't carry any attributes at the moment. My plan is to add an attribute
(e.g., 'NET_DM_ATTR_DROP_TYPE') that will specify the type of drops
you're interested in - SW/HW/XDP. If the attribute is not specified,
then current behavior is maintained and all the drop types are traced.
But if you're only interested in SW drops, then overhead for the rest
should be zero.

For HW drops I'm going to have devlink call into drop monitor. The
function call will just be a NOP in case user is not interested in HW
drops. I'm not sure if for XDP you want to register a probe on a
tracepoint or call into drop monitor. If you want to use the former,
then you can just have drop monitor unregister its probe from the
tracepoint, which is what drop monitor is currently doing with the
kfree_skb() tracepoint.

Thanks!

[1] https://github.com/iovisor/bcc/tree/master/examples/networking