[dpdk-users] KNI Threads/Cores

Matt Laswell laswell at infinite.io
Wed Jun 8 18:45:26 CEST 2016

Hey Cliff,

I have a similar use case in my application.  If you're willing to dedicate
an lcore per socket, another way to approach what you're describing is to
create a KNI interface thread that talks to the other cores via message
rings.  That is, the cores that are interacting with the NIC read a bunch
of packets, determine if any of them need to go to KNI and, if so, enqueue
them using rte_ring_enqueue().  They also do a periodic rte_ring_dequeue()
on another queue to accept back any packets that come back from KNI.

The KNI interface process, meanwhile, just loops along, taking packets in
from the NIC interface threads via rte_ring_dequeue() and sending them to
KNI, and taking packets from KNI and returning them to the NIC interface
threads via rte_ring_enqueue().

I've found that this sort of scheme works well, and is reasonably clean
architecturally.  Also, I found that calls into KNI can at times be very
slow.  In my application, I would periodically see KNI calls take 50-100K
cycles, which can cause congestion if you're handling large volumes of
traffic.  Letting a non-critical thread handle this interface was a big win
for me.

This leaves the kernel side processing out, of course.  But if the traffic
going to the kernel is lightweight, you likely don't need a dedicated core
for the kernel-side RX and TX work.

Matt Laswell
Principal Software Engineer
infinite io

On Wed, Jun 8, 2016 at 11:30 AM, Cliff Burdick <shaklee3 at gmail.com> wrote:

> Hi, I have an application with two sockets where each core I'm planning to
> transmit and receive a fairly large amount of traffic per core. Each core
> right now handles a single queue of either TX or RX of a given port. Across
> all the cores, I may be processing up to 12 ports. I also need to handle
> things like ARP and ping, so I'm going to add in the KNI driver to handle
> that. Since the amount of traffic I'm expecting that I'll need to forward
> to Linux is very small, it seems like I should be able to dedicate one
> lcore per socket to handle this functionality and have the dataplane cores
> pass the traffic off to this core using rte_kni_tx_burst().
> My question is, first of all, is this possible? It seems like I can
> configure the KNI driver to start in "single thread" mode. From that point,
> I want to initialize one KNI device for each port, and have each kernel
> lcore on each processor handle that traffic. I believe if I call
> rte_kni_alloc with core_id set to the kernel lcore for each device, then in
> the end I'll have something like 6 KNI devices on socket one being handled
> by lcore 0, and 6 KNI devices on socket 2 being handled by lcore 31 as an
> example. Then my threads that are handling the dataplane tx/rx can simply
> be passed a pointer to their respective rte_kni device. Does this sound
> correct?
> Also, the sample says the core affinity needs to be set using taskset. Is
> that already taken care of with conf.core_id in rte_kni_alloc or do I still
> need to set it?
> Thanks

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