Re: [PATCH bpf-next] bpf: Export rx queue info for reuseport ebpf prog

[Yonghong Song <yhs@meta.com>]

On 5/24/23 8:37 PM, Joe Damato wrote:
> BPF_PROG_TYPE_SK_REUSEPORT / sk_reuseport ebpf programs do not have
> access to the queue_mapping or napi_id of the incoming skb. Having
> this information can help ebpf progs determine which listen socket to
> select.
> 
> This patch exposes both queue_mapping and napi_id so that
> sk_reuseport ebpf programs can use this information to direct incoming
> connections to the correct listen socket in the SOCKMAP.
> 
> For example:
> 
> A multi-threaded userland program with several threads accepting client
> connections via a reuseport listen socket group might want to direct
> incoming connections from specific receive queues (or NAPI IDs) to specific
> listen sockets to maximize locality or for use with epoll busy-poll.
> 
> Signed-off-by: Joe Damato <jdamato@fastly.com>
> ---
>   include/uapi/linux/bpf.h |  2 ++
>   net/core/filter.c        | 10 ++++++++++
>   2 files changed, 12 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 9273c654743c..31560b506535 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -6286,6 +6286,8 @@ struct sk_reuseport_md {
>   	 */
>   	__u32 eth_protocol;
>   	__u32 ip_protocol;	/* IP protocol. e.g. IPPROTO_TCP, IPPROTO_UDP */
> +	__u32 rx_queue_mapping; /* Rx queue associated with the skb */
> +	__u32 napi_id;          /* napi id associated with the skb */
>   	__u32 bind_inany;	/* Is sock bound to an INANY address? */
>   	__u32 hash;		/* A hash of the packet 4 tuples */

This won't work. You will need to append to the end of data structure
to keep it backward compatibility.

Also, recent kernel has a kfunc bpf_cast_to_kern_ctx() which converts
a ctx to a kernel ctx and you can then use tracing-coding-style to
access those fields. In this particular case, you can do

    struct sk_reuseport_kern *kctx = bpf_cast_to_kern_ctx(ctx);

We have

struct sk_reuseport_kern {
         struct sk_buff *skb;
         struct sock *sk;
         struct sock *selected_sk;
         struct sock *migrating_sk;
         void *data_end;
         u32 hash;
         u32 reuseport_id;
         bool bind_inany;
};

through sk and skb pointer, you should be access the fields presented in
this patch. You can access more fields too.

So using bpf_cast_to_kern_ctx(), there is no need for more uapi changes.
Please give a try.

>   	/* When reuse->migrating_sk is NULL, it is selecting a sk for the
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 968139f4a1ac..71826e1ef7dc 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -11134,6 +11134,8 @@ sk_reuseport_is_valid_access(int off, int size,
>   	case bpf_ctx_range(struct sk_reuseport_md, ip_protocol):
>   	case bpf_ctx_range(struct sk_reuseport_md, bind_inany):
>   	case bpf_ctx_range(struct sk_reuseport_md, len):
> +	case bpf_ctx_range(struct sk_reuseport_md, rx_queue_mapping):
> +	case bpf_ctx_range(struct sk_reuseport_md, napi_id):
>   		bpf_ctx_record_field_size(info, size_default);
>   		return bpf_ctx_narrow_access_ok(off, size, size_default);
>   
> @@ -11183,6 +11185,14 @@ static u32 sk_reuseport_convert_ctx_access(enum bpf_access_type type,
>   		SK_REUSEPORT_LOAD_SKB_FIELD(protocol);
>   		break;
>   
> +	case offsetof(struct sk_reuseport_md, rx_queue_mapping):
> +		SK_REUSEPORT_LOAD_SKB_FIELD(queue_mapping);
> +		break;
> +
> +	case offsetof(struct sk_reuseport_md, napi_id):
> +		SK_REUSEPORT_LOAD_SKB_FIELD(napi_id);
> +		break;
> +
>   	case offsetof(struct sk_reuseport_md, ip_protocol):
>   		SK_REUSEPORT_LOAD_SK_FIELD(sk_protocol);
>   		break;