RE: [PATCHv17 bpf-next 1/6] bpf: run devmap xdp_prog on flush instead of bulk enqueue

John Fastabend
2021-01-27T07:42:35Z

Hangbin Liu wrote:
> From: Jesper Dangaard Brouer <brouer@redhat.com>
>
> This changes the devmap XDP program support to run the program when the
> bulk queue is flushed instead of before the frame is enqueued. This has
> a couple of benefits:
>
> - It "sorts" the packets by destination devmap entry, and then runs the
> same BPF program on all the packets in sequence. This ensures that we
> keep the XDP program and destination device properties hot in I-cache.
>
> - It makes the multicast implementation simpler because it can just
> enqueue packets using bq_enqueue() without having to deal with the
> devmap program at all.
>
> The drawback is that if the devmap program drops the packet, the enqueue
> step is redundant. However, arguably this is mostly visible in a
> micro-benchmark, and with more mixed traffic the I-cache benefit should
> win out. The performance impact of just this patch is as follows:
>
> The bq_xmit_all's logic is also refactored and error label is removed.
> When bq_xmit_all() is called from bq_enqueue(), another packet will
> always be enqueued immediately after, so clearing dev_rx, xdp_prog and
> flush_node in bq_xmit_all() is redundant. Let's move the clear to
> __dev_flush(), and only check them once in bq_enqueue() since they are
> all modified together.
>
> By using xdp_redirect_map in sample/bpf and send pkts via pktgen cmd:
> ./pktgen_sample03_burst_single_flow.sh -i eno1 -d $dst_ip -m $dst_mac -t 10 -s 64
>
> There are about +/- 0.1M deviation for native testing, the performance
> improved for the base-case, but some drop back with xdp devmap prog attached.
>
> Version | Test | Generic | Native | Native + 2nd xdp_prog
> 5.10 rc6 | xdp_redirect_map i40e->i40e | 2.0M | 9.1M | 8.0M
> 5.10 rc6 | xdp_redirect_map i40e->veth | 1.7M | 11.0M | 9.7M
> 5.10 rc6 + patch | xdp_redirect_map i40e->i40e | 2.0M | 9.5M | 7.5M
> 5.10 rc6 + patch | xdp_redirect_map i40e->veth | 1.7M | 11.6M | 9.1M
>

[...]

> +static int dev_map_bpf_prog_run(struct bpf_prog *xdp_prog,
> + struct xdp_frame **frames, int n,
> + struct net_device *dev)
> +{
> + struct xdp_txq_info txq = { .dev = dev };
> + struct xdp_buff xdp;
> + int i, nframes = 0;
> +
> + for (i = 0; i < n; i++) {
> + struct xdp_frame *xdpf = frames[i];
> + u32 act;
> + int err;
> +
> + xdp_convert_frame_to_buff(xdpf, &xdp);
> + xdp.txq = &txq;
> +
> + act = bpf_prog_run_xdp(xdp_prog, &xdp);
> + switch (act) {
> + case XDP_PASS:
> + err = xdp_update_frame_from_buff(&xdp, xdpf);
> + if (unlikely(err < 0))
> + xdp_return_frame_rx_napi(xdpf);
> + else
> + frames[nframes++] = xdpf;
> + break;
> + default:
> + bpf_warn_invalid_xdp_action(act);
> + fallthrough;
> + case XDP_ABORTED:
> + trace_xdp_exception(dev, xdp_prog, act);
> + fallthrough;
> + case XDP_DROP:
> + xdp_return_frame_rx_napi(xdpf);
> + break;
> + }
> + }
> + return nframes; /* sent frames count */
> +}
> +
> static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
> {
> struct net_device *dev = bq->dev;
> - int sent = 0, drops = 0, err = 0;
> + unsigned int cnt = bq->count;
> + int drops = 0, err = 0;
> + int to_send = cnt;
> + int sent = cnt;
> int i;
>
> - if (unlikely(!bq->count))
> + if (unlikely(!cnt))
> return;
>
> - for (i = 0; i < bq->count; i++) {
> + for (i = 0; i < cnt; i++) {
> struct xdp_frame *xdpf = bq->q[i];
>
> prefetch(xdpf);
> }
>
> - sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, flags);
> + if (bq->xdp_prog) {
> + to_send = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
> + if (!to_send) {
> + sent = 0;
> + goto out;
> + }
> + drops = cnt - to_send;
> + }

I might be missing something about how *bq works here. What happens when
dev_map_bpf_prog_run returns to_send < cnt?

So I read this as it will send [0, to_send] and [to_send, cnt] will be
dropped? How do we know the bpf prog would have dropped the set,
[to_send+1, cnt]?

> +
> + sent = dev->netdev_ops->ndo_xdp_xmit(dev, to_send, bq->q, flags);
> if (sent < 0) {
> err = sent;
> sent = 0;
> - goto 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 < cnt - drops; i++) {
> + struct xdp_frame *xdpf = bq->q[i];
> +
> + xdp_return_frame_rx_napi(xdpf);
> + }
> }
> - drops = bq->count - sent;
> out:
> + drops = cnt - sent;
> bq->count = 0;
>
> trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, drops, err);

I gather the remaining [to_send+1, cnt] packets are in fact dropped
because we set bq->count=0 and trace_xdp_devmap_xmit seems to think
they are dropped.

Also update the comment in trace_xdp_devmap_xmit,

/* Catch API error of drv ndo_xdp_xmit sent more than count */

so that it reads to account for devmap progs as well?

> - bq->dev_rx = NULL;
> - __list_del_clearprev(&bq->flush_node);
> return;
> -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];
> -
> - xdp_return_frame_rx_napi(xdpf);
> - drops++;
> - }
> - goto out;
> }

[...]

> -static struct xdp_buff *dev_map_run_prog(struct net_device *dev,
> - struct xdp_buff *xdp,
> - struct bpf_prog *xdp_prog)
> -{
> - struct xdp_txq_info txq = { .dev = dev };
> - u32 act;
> -
> - xdp_set_data_meta_invalid(xdp);
> - xdp->txq = &txq;
> -
> - act = bpf_prog_run_xdp(xdp_prog, xdp);
> - switch (act) {
> - case XDP_PASS:
> - return xdp;
> - case XDP_DROP:
> - break;
> - default:
> - bpf_warn_invalid_xdp_action(act);
> - fallthrough;
> - case XDP_ABORTED:
> - trace_xdp_exception(dev, xdp_prog, act);
> - break;
> - }
> -
> - xdp_return_buff(xdp);
> - return NULL;
> -}
> -
> int dev_xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
> struct net_device *dev_rx)
> {
> - return __xdp_enqueue(dev, xdp, dev_rx);
> + return __xdp_enqueue(dev, xdp, dev_rx, NULL);
> }
>
> int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
> @@ -489,12 +516,7 @@ int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
> {
> struct net_device *dev = dst->dev;
>
> - if (dst->xdp_prog) {
> - xdp = dev_map_run_prog(dev, xdp, dst->xdp_prog);
> - if (!xdp)
> - return 0;

So here it looks like dev_map_run_prog will not drop extra
packets, but will see every single packet.

Are we changing the semantics subtle here? This looks like
a problem to me. We should not drop packets in the new case
unless bpf program tells us to.

> - }
> - return __xdp_enqueue(dev, xdp, dev_rx);
> + return __xdp_enqueue(dev, xdp, dev_rx, dst->xdp_prog);
> }

Did I miss something? If not maybe a comment in the commit
message would help or in the code itself.

Thanks,
John