Re: [PATCH net-next,RFC 00/13] New fast forwarding path

On Thu, Jun 14, 2018 at 01:52:03PM -0700, Tom Herbert wrote:

On Thu, Jun 14, 2018 at 7:19 AM, Pablo Neira Ayuso [off-list ref] wrote:

quoted

Hi,

This patchset proposes a new fast forwarding path infrastructure that
combines the GRO/GSO and the flowtable infrastructures. The idea is to
add a hook at the GRO layer that is invoked before the standard GRO
protocol offloads. This allows us to build custom packet chains that we
can quickly pass in one go to the neighbour layer to define fast
forwarding path for flows.

For each packet that gets into the GRO layer, we first check if there is
an entry in the flowtable, if so, the packet is placed in a list until
the GRO infrastructure decides to send the batch from gro_complete to
the neighbour layer. The first packet in the list takes the route from
the flowtable entry, so we avoid reiterative routing lookups.

In case no entry is found in the flowtable, the packet is passed up to
the classic GRO offload handlers. Thus, this packet follows the standard
forwarding path. Note that the initial packets of the flow always go
through the standard IPv4/IPv6 netfilter forward hook, that is used to
configure what flows are placed in the flowtable. Therefore, only a few
(initial) packets follow the standard forwarding path while most of the
follow up packets take this new fast forwarding path.

IIRC, there was a similar proposal a while back that want to bundle
packets of the same flow together (without doing GRO) so that they
could be processed by various functions by looking at just one
representative packet in the group. The concept had some promise, but
in the end it created quite a bit of complexity since at some point
the packet bundle needed to be undone to go back to processing the
individual packets.

With the way we chain the packets it is not too complicated to
undo this chaining (nft_skb_segment in patch 5 implements this).
After that, this looks like a chain of usual segments, so we
trigger xmit_more with every packet chain.