Re: [PATCHv16 bpf-next 1/6] bpf: run devmap xdp_prog on flush instead of bulk enqueue

Maciej Fijalkowski
2021-01-22T11:54:56Z

On Fri, Jan 22, 2021 at 03:46:47PM +0800, 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:
>
> Using xdp_redirect_map(with a 2nd xdp_prog patch[1]) 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
>
> [1] https://lore.kernel.org/bpf/20210122025007.2968381-1-liuhangbin@gmail.com
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
>
> ---
> v16:
> a) refactor bq_xmit_all logic and remove error label
>
> v15:
> a) do not use unlikely when checking bq->xdp_prog
> b) return sent frames for dev_map_bpf_prog_run()
>
> v14: no update, only rebase the code
> v13: pass in xdp_prog through __xdp_enqueue()
> v2-v12: no this patch
> ---
> kernel/bpf/devmap.c | 136 ++++++++++++++++++++++++++------------------
> 1 file changed, 81 insertions(+), 55 deletions(-)
>
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index f6e9c68afdd4..c24fcffbbfad 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -57,6 +57,7 @@ struct xdp_dev_bulk_queue {
> struct list_head flush_node;
> struct net_device *dev;
> struct net_device *dev_rx;
> + struct bpf_prog *xdp_prog;
> unsigned int count;
> };
>
> @@ -327,46 +328,95 @@ bool dev_map_can_have_prog(struct bpf_map *map)
> return false;
> }
>
> +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_sent = cnt;

Hmm if I would be super picky then I'd like to have this variable as
"to_send", as we spoke. Could you change that?

With that, you can add my:

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

to next revision.

> + 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_sent = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
> + if (!to_sent) {
> + sent = 0;
> + goto out;
> + }
> + drops = cnt - to_sent;
> + }
> +
> + sent = dev->netdev_ops->ndo_xdp_xmit(dev, to_sent, 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);
> bq->dev_rx = NULL;
> + bq->xdp_prog = NULL;

One more question, do you really have to do that per each bq_xmit_all
call? Couldn't you clear it in __dev_flush ?

Or IOW - what's the rationale behind storing xdp_prog in
xdp_dev_bulk_queue. Why can't you propagate the dst->xdp_prog and rely on
that without that local pointer?

You probably have an answer for that, so maybe include it in commit
message.

BTW same question for clearing dev_rx. To me this will be the same for all
bq_xmit_all() calls that will happen within same napi.

> __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;
> }
>
> /* __dev_flush is called from xdp_do_flush() which _must_ be signaled
> @@ -408,7 +458,7 @@ struct bpf_dtab_netdev *__dev_map_lookup_elem(struct bpf_map *map, u32 key)
> * Thus, safe percpu variable access.
> */
> static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
> - struct net_device *dev_rx)
> + struct net_device *dev_rx, struct bpf_prog *xdp_prog)
> {
> struct list_head *flush_list = this_cpu_ptr(&dev_flush_list);
> struct xdp_dev_bulk_queue *bq = this_cpu_ptr(dev->xdp_bulkq);
> @@ -423,6 +473,14 @@ static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
> if (!bq->dev_rx)
> bq->dev_rx = dev_rx;
>
> + /* Store (potential) xdp_prog that run before egress to dev as
> + * part of bulk_queue. This will be same xdp_prog for all
> + * xdp_frame's in bulk_queue, because this per-CPU store must
> + * be flushed from net_device drivers NAPI func end.
> + */
> + if (!bq->xdp_prog)
> + bq->xdp_prog = xdp_prog;
> +
> bq->q[bq->count++] = xdpf;
>
> if (!bq->flush_node.prev)
> @@ -430,7 +488,8 @@ static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
> }
>
> static inline int __xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
> - struct net_device *dev_rx)
> + struct net_device *dev_rx,
> + struct bpf_prog *xdp_prog)
> {
> struct xdp_frame *xdpf;
> int err;
> @@ -446,42 +505,14 @@ static inline int __xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
> if (unlikely(!xdpf))
> return -EOVERFLOW;
>
> - bq_enqueue(dev, xdpf, dev_rx);
> + bq_enqueue(dev, xdpf, dev_rx, xdp_prog);
> return 0;
> }
>
> -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 +520,7 @@ 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;
> - }
> - return __xdp_enqueue(dev, xdp, dev_rx);
> + return __xdp_enqueue(dev, xdp, dev_rx, dst->xdp_prog);
> }
>
> int dev_map_generic_redirect(struct bpf_dtab_netdev *dst, struct sk_buff *skb,
> --
> 2.26.2
>