> Subject: Re: [PATCH v2 net-next 0/7] dpaa2-eth: add support for Rx traffic > classes > > On Wed, 20 May 2020 15:10:42 +0000 Ioana Ciornei wrote: > > DPAA2 has frame queues per each Rx traffic class and the decision from > > which queue to pull frames from is made by the HW based on the queue > > priority within a channel (there is one channel per each CPU). > > IOW you're reading the descriptor for the device memory/iomem address and > the HW will return the next descriptor based on configured priority?
That's the general idea but the decision is not made on a frame by frame bases but rather on a dequeue operation which can, at a maximum, return 16 frame descriptors at a time. > Presumably strict priority? Only the two highest traffic classes are in strict priority, while the other 6 TCs form two priority tiers - medium(4 TCs) and low (last two TCs). > > > If this should be modeled in software, then I assume there should be a > > NAPI instance for each traffic class and the stack should know in > > which order to call the poll() callbacks so that the priority is respected. > > Right, something like that. But IMHO not needed if HW can serve the right > descriptor upon poll. After thinking this through I don't actually believe that multiple NAPI instances would solve this in any circumstance at all: - If you have hardware prioritization with full scheduling on dequeue then job on the driver side is already done. - If you only have hardware assist for prioritization (ie hardware gives you multiple rings but doesn't tell you from which one to dequeue) then you can still use a single NAPI instance just fine and pick the highest priority non-empty ring on-the-fly basically. What I am having trouble understanding is how the fully software implementation of this possible new Rx qdisc should work. Somehow the skb->priority should be taken into account when the skb is passing though the stack (ie a higher priority skb should surpass another previously received skb even if the latter one was received first, but its priority queue is congested). I don't have a very deep understanding of the stack but I am thinking that the enqueue_to_backlog()/process_backlog() area could be a candidate place for sorting out bottlenecks. In case we do that I don't see why a qdisc would be necessary at all and not have everybody benefit from prioritization based on skb->priority. Ioana
