Re: [EXTERNAL] Re: [RFC PATCH 0/3] add feature arc in rte_graph

["Robin Jarry" <rjarry@redhat.com>]

Hi Nitin, all,

Nitin Saxena, Oct 17, 2024 at 09:03:
> Hi Robin/David and all,
>
> We realized the feature arc patch series is difficult to understand as 
> a new concept. Our objectives are following with feature arc changes
>
> 1. Allow reusability of standard DPDK nodes (defined in lib/nodes/*) 
>    with out-of-tree applications (like grout). Currently out-of-tree 
>    graph applications are duplicating standard nodes but not reusing 
>    the standard ones which are available. In the long term, we would 
>    like to mature standard DPDK nodes with flexibility of hooking them 
>    to out-of-tree application nodes.

It would be ideal if the in-built nodes could be reused. When we started 
working on grout, I tried multiple approaches where I could reuse these 
nodes, but all failed. The nodes public API seems tailored for app/graph 
but does not fit well with other control plane implementations.

One of the main issues I had is that the ethdev_rx and ethdev_tx nodes 
are cloned per rxq / txq associated with a graph worker. The rte_node 
API requires that every clone has a unique name. This in turn makes hot 
plugging of DPDK ports very complex, if not impossible.

For example, with the in-built nodes, it is not possible to change the 
number of ports or their number of RX queues without destroying the 
whole graph and creating a new one from scratch.

Also, the current implementation of "ip{4,6}-rewrite" handles writing 
ethernet header data. This would prevent it from using this node for an 
IP-in-IP tunnel interface as we did in grout.

Do you think we could change the in-built nodes to enforce OSI layer 
separation of concerns? It would make them much more flexible. It may 
cause a slight drop of performance because you'd be splitting processing 
in two different nodes. But I think flexibility is more important. 
Otherwise, the in-built nodes can only be used for very specific 
use-cases.

Finally, I would like to improve the rte_node API to allow defining and 
enforcing per-packet metadata that every node expects as input. The 
current in-built nodes rely on mbuf dynamic fields for this but this 
means you only have 9x32 bits available. And using all of these may 
break some drivers (ixgbe) that rely on dynfields to work. Have you 
considered using mbuf private data for this?

>
> 2. Flexibility to enable/disable sub-graphs per interface based on the 
>    runtime configuration updates. Protocol sub-graphs can be 
>    selectively enabled for few (or all interfaces) at runtime
>
> 3. More than one sub-graphs/features can be enabled on an interface. 
>    So a packet has to follow a sequential ordering node path on worker 
>    cores. Packets may need to move from one sub-graph to another 
>    sub-graph per interface
>
> 4. Last but not least, an optimized implementation which does not (or 
>    minimally) stop worker cores for any control plane runtime updates. 
>    Any performance regression should also be avoided
>
> I am planning to create a draft presentation on feature arc which 
> I can share, when ready, to discuss. If needed, I can also plan to 
> present that in one of the DPDK community meetings. Their we can also 
> discuss if there are any alternatives of achieving above objectives

Looking forward to this.

Thanks!