Date:   Tue, 14 Jul 2020 11:12:59 -0600
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Hangbin Liu <liuhangbin@...il.com>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Subject: 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 <liuhangbin@...il.com> writes:
> 
>> 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.

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