On Tue, 16 Jun 2026 12:27:26 +0200 Maxime Leroy <[email protected]> wrote:
> RTE_ETH_RX_OFFLOAD_VLAN_STRIP is advertised, but no hardware VLAN strip > backs it: when enabled, the Rx burst calls rte_vlan_strip() on every > frame, a software op masquerading as a hardware offload. > > It saves a forwarding application nothing: the datapath reads the L2 > header anyway to classify or strip. The offload does not remove that > read, it relocates it into the driver Rx burst, where it is far more > expensive. > > The cost is a matter of timing. rte_vlan_strip() reaches the L2 header > through rte_pktmbuf_mtod(), which dereferences mbuf->buf_addr. On a > freshly recycled buffer that mbuf cacheline is cold. eth_fd_to_mbuf() > has just written other fields of it (data_off, ol_flags), but buf_addr > is a persistent field it does not rewrite. A write does not stall: it > posts to the store buffer while the line fills in the background, and > the rewritten fields are forwarded straight from there. buf_addr has > nothing to forward, so it must be read from the line, whose fill is > still in flight, and the read stalls. The ethertype read that follows, > on the cold payload line, stalls again. Read later by the application, > when the fill has completed, the same read hits. The offload just > performs it at the worst possible moment. > > Measured on a single-core port-to-port forwarding test over two 10G > ports (one core at 2 GHz, 64-byte untagged frames): > > - throughput 4.22 -> 5.00 Mpps (+18 percent) > - IPC 0.93 -> 1.25: the cost was memory stall, not compute > - L3/DRAM-bound L2 refills 319M -> 200M over 10s (-37 percent) > > perf confirms it: with the offload, the buf_addr load (the cold mbuf > field) and the payload load account for about 84 percent of the Rx > burst's L2 refills; removing it, those vanish and only the inherent DQRR > dequeue misses remain. > > Stop advertising VLAN_STRIP and remove the rte_vlan_strip() calls from > every Rx path. This is a behavioural change: the tag is left in the > frame, so an application must strip it itself, on the L2 header it > already reads. > > Signed-off-by: Maxime Leroy <[email protected]> Acked-by: Stephen Hemminger <[email protected]>
