Subject: Re: [net-next V4 PATCH 2/5] bpf: XDP_REDIRECT enable use of cpumap

From: Daniel Borkmann <[email-masked]>
Date: Thu, 05 Oct 2017 12:10:13 +0200

On 10/04/2017 02:03 PM, Jesper Dangaard Brouer wrote:
> This patch connects cpumap to the xdp_do_redirect_map infrastructure.
>
> Still no SKB allocation are done yet.  The XDP frames are transferred
> to the other CPU, but they are simply refcnt decremented on the remote
> CPU.  This served as a good benchmark for measuring the overhead of
> remote refcnt decrement.  If driver page recycle cache is not
> efficient then this, exposes a bottleneck in the page allocator.
>
> A shout-out to MST's ptr_ring, which is the secret behind is being so
> efficient to transfer memory pointers between CPUs, without constantly
> bouncing cache-lines between CPUs.
>
> V3: Handle !CONFIG_BPF_SYSCALL pointed out by kbuild test robot.
>
> V4: Make Generic-XDP aware of cpumap type, but don't allow redirect yet,
>   as implementation require a separate upstream discussion.
>
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
[...]
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index ae8e29352261..4926a9971f90 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -493,7 +493,8 @@ static int bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_pkt *xdp_pkt)
>   	return 0;
>   }
>
> -int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp)
> +int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp,
> +		    struct net_device *dev_rx)
>   {
>   	struct xdp_pkt *xdp_pkt;
>   	int headroom;
> @@ -505,7 +506,7 @@ int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp)
>   	xdp_pkt = xdp->data_hard_start;
>   	xdp_pkt->data = xdp->data;
>   	xdp_pkt->len  = xdp->data_end - xdp->data;
> -	xdp_pkt->headroom = headroom;
> +	xdp_pkt->headroom = headroom - sizeof(*xdp_pkt);

(Just a note, bit confusing that first two patches add and extend
  this, and only in the third you add the xdp->data_meta handling,
  makes it harder to review at least.)

[...]
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 9b6e7e84aafd..dbf2ae071108 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2521,10 +2521,36 @@ static int __bpf_tx_xdp(struct net_device *dev,
>   	err = dev->netdev_ops->ndo_xdp_xmit(dev, xdp);
>   	if (err)
>   		return err;
> -	if (map)
> +	dev->netdev_ops->ndo_xdp_flush(dev);
> +	return 0;
> +}
> +
> +static int __bpf_tx_xdp_map(struct net_device *dev_rx, void *fwd,
> +			    struct bpf_map *map,
> +			    struct xdp_buff *xdp,
> +			    u32 index)
> +{
> +	int err;
> +
> +	if (map->map_type == BPF_MAP_TYPE_DEVMAP) {
> +		struct net_device *dev = fwd;
> +
> +		if (!dev->netdev_ops->ndo_xdp_xmit)
> +			return -EOPNOTSUPP;
> +
> +		err = dev->netdev_ops->ndo_xdp_xmit(dev, xdp);
> +		if (err)
> +			return err;
>   		__dev_map_insert_ctx(map, index);
> -	else
> -		dev->netdev_ops->ndo_xdp_flush(dev);
> +
> +	} else if (map->map_type == BPF_MAP_TYPE_CPUMAP) {
> +		struct bpf_cpu_map_entry *rcpu = fwd;
> +
> +		err = cpu_map_enqueue(rcpu, xdp, dev_rx);
> +		if (err)
> +			return err;
> +		__cpu_map_insert_ctx(map, index);
> +	}
>   	return 0;
>   }
>
> @@ -2534,11 +2560,33 @@ void xdp_do_flush_map(void)
>   	struct bpf_map *map = ri->map_to_flush;
>
>   	ri->map_to_flush = NULL;
> -	if (map)
> -		__dev_map_flush(map);
> +	if (map) {
> +		switch (map->map_type) {
> +		case BPF_MAP_TYPE_DEVMAP:
> +			__dev_map_flush(map);
> +			break;
> +		case BPF_MAP_TYPE_CPUMAP:
> +			__cpu_map_flush(map);
> +			break;
> +		default:
> +			break;
> +		}
> +	}
>   }
>   EXPORT_SYMBOL_GPL(xdp_do_flush_map);
>
> +static void *__xdp_map_lookup_elem(struct bpf_map *map, u32 index)
> +{
> +	switch (map->map_type) {
> +	case BPF_MAP_TYPE_DEVMAP:
> +		return __dev_map_lookup_elem(map, index);
> +	case BPF_MAP_TYPE_CPUMAP:
> +		return __cpu_map_lookup_elem(map, index);
> +	default:
> +		return NULL;
> +	}

Should we just have a callback and instead of the above use
map->ptr_lookup_elem() (or however we name it) ... lot of it
is pretty much the same logic as with devmap.

> +}
> +
>   static inline bool xdp_map_invalid(const struct bpf_prog *xdp_prog,
>   				   unsigned long aux)
>   {
> @@ -2551,8 +2599,8 @@ static int xdp_do_redirect_map(struct net_device *dev, struct xdp_buff *xdp,
>   	struct redirect_info *ri = this_cpu_ptr(&redirect_info);
>   	unsigned long map_owner = ri->map_owner;
>   	struct bpf_map *map = ri->map;
> -	struct net_device *fwd = NULL;
>   	u32 index = ri->ifindex;
> +	void *fwd = NULL;
>   	int err;
>
>   	ri->ifindex = 0;