Re: [PATCHv7 bpf-next 0/3] xdp: add a new helper for dev map multicast support

On 7/14/20 6:29 AM, Toke Høiland-Jørgensen wrote:

Hangbin Liu [off-list ref] writes:

quoted

This patch is for xdp multicast support. which has been discussed before[0],
The goal is to be able to implement an OVS-like data plane in XDP, i.e.,
a software switch that can forward XDP frames to multiple ports.

To achieve this, an application needs to specify a group of interfaces
to forward a packet to. It is also common to want to exclude one or more
physical interfaces from the forwarding operation - e.g., to forward a
packet to all interfaces in the multicast group except the interface it
arrived on. While this could be done simply by adding more groups, this
quickly leads to a combinatorial explosion in the number of groups an
application has to maintain.

To avoid the combinatorial explosion, we propose to include the ability
to specify an "exclude group" as part of the forwarding operation. This
needs to be a group (instead of just a single port index), because there
may have multi interfaces you want to exclude.

Thus, the logical forwarding operation becomes a "set difference"
operation, i.e. "forward to all ports in group A that are not also in
group B". This series implements such an operation using device maps to
represent the groups. This means that the XDP program specifies two
device maps, one containing the list of netdevs to redirect to, and the
other containing the exclude list.

To achieve this, I re-implement a new helper bpf_redirect_map_multi()
to accept two maps, the forwarding map and exclude map. If user
don't want to use exclude map and just want simply stop redirecting back
to ingress device, they can use flag BPF_F_EXCLUDE_INGRESS.

The 2nd and 3rd patches are for usage sample and testing purpose, so there
is no effort has been made on performance optimisation. I did same tests
with pktgen(pkt size 64) to compire with xdp_redirect_map(). Here is the
test result(the veth peer has a dummy xdp program with XDP_DROP directly):

Version         | Test                                   | Native | Generic
5.8 rc1         | xdp_redirect_map       i40e->i40e      |  10.0M |   1.9M
5.8 rc1         | xdp_redirect_map       i40e->veth      |  12.7M |   1.6M
5.8 rc1 + patch | xdp_redirect_map       i40e->i40e      |  10.0M |   1.9M
5.8 rc1 + patch | xdp_redirect_map       i40e->veth      |  12.3M |   1.6M
5.8 rc1 + patch | xdp_redirect_map_multi i40e->i40e      |   7.2M |   1.5M
5.8 rc1 + patch | xdp_redirect_map_multi i40e->veth      |   8.5M |   1.3M
5.8 rc1 + patch | xdp_redirect_map_multi i40e->i40e+veth |   3.0M |  0.98M

The bpf_redirect_map_multi() is slower than bpf_redirect_map() as we loop
the arrays and do clone skb/xdpf. The native path is slower than generic
path as we send skbs by pktgen. So the result looks reasonable.

Last but not least, thanks a lot to Jiri, Eelco, Toke and Jesper for
suggestions and help on implementation.

[0] https://xdp-project.net/#Handling-multicast

v7: Fix helper flag check
    Limit the *ex_map* to use DEVMAP_HASH only and update function
    dev_in_exclude_map() to get better performance.

Did it help? The performance numbers in the table above are the same as
in v6...

If there is only 1 entry in the exclude map, then the numbers should be
about the same.