Re: [PATCHv15 bpf-next 1/6] bpf: run devmap xdp_prog on flush instead of bulk enqueue

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Wed, Jan 20, 2021 at 10:25:09AM +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://patchwork.ozlabs.org/project/netdev/patch/20201208120159.2278277-1-liuhangbin@gmail.com/

nit: probably would be good to update the link to patch, I see it's v8
already whereas link refers to v4.

> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
> 
> ---
> 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 | 116 +++++++++++++++++++++++++++-----------------
>  1 file changed, 71 insertions(+), 45 deletions(-)
> 
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index f6e9c68afdd4..13ed68c24aad 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;
>  };
>  
> @@ -327,44 +328,93 @@ 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);

Bump on John's question.

> +			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;

if sent is inited below, the assignment above is useless.
Couldn't we do sent = cnt at the declaration over here?

> +	unsigned int cnt = bq->count;
>  	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);
> +	/* Init sent to cnt in case there is no xdp_prog */
> +	sent = cnt;
> +	if (bq->xdp_prog) {
> +		sent = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
> +		if (!sent)
> +			goto out;

Sorry, but 'sent' is a bit confusing to me, actual sending happens below
via ndo_xdp_xmit, right? This hook will not actually send frames.
Can we do a subtle change to have it in separate variable 'to_send' ?

> +	}
> +
> +	/* Backup drops value before xmit as we may need it in error label */
> +	drops = cnt - sent;
> +	sent = dev->netdev_ops->ndo_xdp_xmit(dev, sent, bq->q, flags);
>  	if (sent < 0) {
>  		err = sent;
>  		sent = 0;
>  		goto error;
>  	}
> -	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;
>  	__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++) {
> +	for (i = 0; i < cnt - drops; i++) {
>  		struct xdp_frame *xdpf = bq->q[i];
> -
>  		xdp_return_frame_rx_napi(xdpf);
> -		drops++;
>  	}
>  	goto out;

Although I'm a huge goto advocate, I feel like this particular usage could
be simplified. Not sure why we had that in first place.

I gave a shot at rewriting/refactoring whole bq_xmit_all and I feel like
it's more readable. I introduced 'to_send' variable and got rid of 'error'
label.

Thoughts?

I might have missed something, though.

static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
{
	struct net_device *dev = bq->dev;
	unsigned int cnt = bq->count;
	int drops = 0, err = 0;
	int to_send = 0;
	int sent = cnt;
	int i;

	if (unlikely(!cnt))
		return;

	for (i = 0; i < cnt; i++) {
		struct xdp_frame *xdpf = bq->q[i];

		prefetch(xdpf);
	}

	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;
	sent = dev->netdev_ops->ndo_xdp_xmit(dev, to_send, bq->q, flags);
	if (sent < 0) {
		err = sent;
		sent = 0;

		/* 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);
		}
	}
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;
	__list_del_clearprev(&bq->flush_node);

	return;
}

>  }
> @@ -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
>