netdev - Re: [net-next V3 PATCH 3/5] bpf: cpumap xdp_buff to skb conversion and allocation

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Date:   Mon, 2 Oct 2017 18:02:46 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, jakub.kicinski@...ronome.com,
        "Michael S. Tsirkin" <mst@...hat.com>, pavel.odintsov@...il.com,
        Jason Wang <jasowang@...hat.com>, mchan@...adcom.com,
        John Fastabend <john.fastabend@...il.com>,
        peter.waskiewicz.jr@...el.com,
        Daniel Borkmann <borkmann@...earbox.net>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [net-next V3 PATCH 3/5] bpf: cpumap xdp_buff to skb conversion
 and allocation

On Mon, Oct 02, 2017 at 06:05:29PM +0200, Jesper Dangaard Brouer wrote:
> +		while ((xdp_pkt = __ptr_ring_consume(rcpu->queue))) {
> +			struct sk_buff *skb;
> +			int ret;
> +
> +			/* Allow busy polling again */
> +			empty_cnt = 0;
> +
> +			skb = cpu_map_build_skb(rcpu, xdp_pkt);
> +			if (!skb) {
> +				page_frag_free(xdp_pkt);
> +				continue;
> +			}
> +
> +			/* Inject into network stack */
> +			ret = netif_receive_skb(skb);
> +			if (ret == NET_RX_DROP)
> +				drops++;

I thought the whole thing is an alternative to RPS,
but netif_receive_skb_internal() will call into RPS logic.
So the user has to make sure it disabled or they will
conflict in some weird way?
Or you're calling netif_receive_skb() to be able to call
generic XDP on that cpu again ?
But that prog can do cpumap redirect again?
sort-of recursive redirect? Is it really useful?
May be call into __netif_receive_skb_core() directly?
not sure.
I'm asking all these questions to make sure we think through
these implications before it becomes an abi.