xdp: change ndo_xdp_xmit API to support bulking

This patch change the API for ndo_xdp_xmit to support bulking
xdp_frames.

When kernel is compiled with CONFIG_RETPOLINE, XDP sees a huge slowdown.
Most of the slowdown is caused by DMA API indirect function calls, but
also the net_device->ndo_xdp_xmit() call.

Benchmarked patch with CONFIG_RETPOLINE, using xdp_redirect_map with
single flow/core test (CPU E5-1650 v4 @ 3.60GHz), showed
performance improved:
 for driver ixgbe: 6,042,682 pps -> 6,853,768 pps = +811,086 pps
 for driver i40e : 6,187,169 pps -> 6,724,519 pps = +537,350 pps

With frames avail as a bulk inside the driver ndo_xdp_xmit call,
further optimizations are possible, like bulk DMA-mapping for TX.

Testing without CONFIG_RETPOLINE show the same performance for
physical NIC drivers.

The virtual NIC driver tun sees a huge performance boost, as it can
avoid doing per frame producer locking, but instead amortize the
locking cost over the bulk.

V2: Fix compile errors reported by kbuild test robot <lkp@intel.com>
V4: Isolated ndo, driver changes and callers.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 18 insertions, 11 deletions

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index a9cd5c93dd2b..77908311ec98 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -232,24 +232,31 @@ static int bq_xmit_all(struct bpf_dtab_netdev *obj,
                 prefetch(xdpf);
         }
 
-        for (i = 0; i < bq->count; i++) {
-                struct xdp_frame *xdpf = bq->q[i];
-                int err;
-
-                err = dev->netdev_ops->ndo_xdp_xmit(dev, xdpf);
-                if (err) {
-                        drops++;
-                        xdp_return_frame_rx_napi(xdpf);
-                } else {
-                        sent++;
-                }
+        sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q);
+        if (sent < 0) {
+                sent = 0;
+                goto error;
         }
+        drops = bq->count - sent;
+out:
         bq->count = 0;
 
         trace_xdp_devmap_xmit(&obj->dtab->map, obj->bit,
                               sent, drops, bq->dev_rx, dev);
         bq->dev_rx = NULL;
         return 0;
+error:
+        /* If ndo_xdp_xmit fails with an errno, no frames have been
+         * xmit'ed and it's our responsibility to them free all.
+         */
+        for (i = 0; i < bq->count; i++) {
+                struct xdp_frame *xdpf = bq->q[i];
+
+                /* RX path under NAPI protection, can return frames faster */
+                xdp_return_frame_rx_napi(xdpf);
+                drops++;
+        }
+        goto out;
 }
 
 /* __dev_map_flush is called from xdp_do_flush_map() which _must_ be signaled