On 2021-03-01 17:08, Toke Høiland-Jørgensen wrote:
Björn Töpel <bjorn.topel@xxxxxxxxx> writes:From: Björn Töpel <bjorn.topel@xxxxxxxxx> Currently, the AF_XDP rings uses smp_{r,w,}mb() fences on the kernel-side. By updating the rings for load-acquire/store-release semantics, the full barrier on the consumer side can be replaced with improved performance as a nice side-effect. Note that this change does *not* require similar changes on the libbpf/userland side, however it is recommended [1]. On x86-64 systems, by removing the smp_mb() on the Rx and Tx side, the l2fwd AF_XDP xdpsock sample performance increases by 1%. Weakly-ordered platforms, such as ARM64 might benefit even more. [1] https://lore.kernel.org/bpf/20200316184423.GA14143@willie-the-truck/ Signed-off-by: Björn Töpel <bjorn.topel@xxxxxxxxx> --- net/xdp/xsk_queue.h | 27 +++++++++++---------------- 1 file changed, 11 insertions(+), 16 deletions(-) diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h index 2823b7c3302d..e24279d8d845 100644 --- a/net/xdp/xsk_queue.h +++ b/net/xdp/xsk_queue.h @@ -47,19 +47,18 @@ struct xsk_queue { u64 queue_empty_descs; };-/* The structure of the shared state of the rings are the same as the- * ring buffer in kernel/events/ring_buffer.c. For the Rx and completion - * ring, the kernel is the producer and user space is the consumer. For - * the Tx and fill rings, the kernel is the consumer and user space is - * the producer. +/* The structure of the shared state of the rings are a simple + * circular buffer, as outlined in + * Documentation/core-api/circular-buffers.rst. For the Rx and + * completion ring, the kernel is the producer and user space is the + * consumer. For the Tx and fill rings, the kernel is the consumer and + * user space is the producer. * * producer consumer * - * if (LOAD ->consumer) { LOAD ->producer - * (A) smp_rmb() (C) + * if (LOAD ->consumer) { (A) LOAD.acq ->producer (C)Why is LOAD.acq not needed on the consumer side?
You mean why LOAD.acq is not needed on the *producer* side, i.e. the ->consumer? The ->consumer is a control dependency for the store, so there is no ordering constraint for ->consumer at producer side. If there's no space, no data is written. So, no barrier is needed there -- at least that has been my perspective. This is very similar to the buffer in Documentation/core-api/circular-buffers.rst. Roping in Paul for some guidance. Björn
-Toke
