Hi folks,
I had a look at the graph dispatch mode and wondered if it could be
adapted to be used with event loops for "slow path" processing.
The use case I have in mind is having lcores dedicated for control plane
operations which are not latency sensitive. It would be nice to allow
data plane workers to send packets to these lcores while keeping the
packets in the graph.
The problem I have with this solution for now, is that all CPUs
processing the graph with rte_graph_walk_mcore_dispatch() are supposed
to do busy polling. This is not compatible with select/epoll based event
loops.
Would it be possible to have a way for busy-polling threads to signal
non-busy-polling threads that there are packets waiting for them in the
graph ring? In grout, we implemented something outside of the graph
using pthread_cond_signal().
https://github.com/DPDK/grout/blob/main/modules/infra/datapath/control_output.c
https://github.com/DPDK/grout/blob/main/modules/infra/control/control_output.c
Unless I am mistaken, this does not perform any syscall and does not
require acquiring any lock. Only the "slow" thread needs to acquire
locks and wait for the data path threads to call pthread_cond_signal().
This solution can only be used for busy-polling -> epoll-based
communication, for the other direction epoll-based -> busy-polling there
would not need any specific signaling required. Only posting the packets
to the graph ring would be enough.
What do you think? How would you integrate this into rte_graph? What
would the API look like in your opinion?
Thanks!
--
Robin
