netdev - Re: [RFC PATCH v2 net-next 06/12] net: core: propagate SKB lists through packet

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Date:   Wed, 27 Jun 2018 10:36:24 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     ecree@...arflare.com
Cc:     linux-net-drivers@...arflare.com,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH v2 net-next 06/12] net: core: propagate SKB lists
 through packet_type lookup

On Tue, Jun 26, 2018 at 8:19 PM Edward Cree <ecree@...arflare.com> wrote:
>
> __netif_receive_skb_taps() does a depressingly large amount of per-packet
>  work that can't easily be listified, because the another_round looping
>  makes it nontrivial to slice up into smaller functions.
> Fortunately, most of that work disappears in the fast path:
>  * Hardware devices generally don't have an rx_handler
>  * Unless you're tcpdumping or something, there is usually only one ptype
>  * VLAN processing comes before the protocol ptype lookup, so doesn't force
>    a pt_prev deliver
>  so normally, __netif_receive_skb_taps() will run straight through and return
>  the one ptype found in ptype_base[hash of skb->protocol].
>
> Signed-off-by: Edward Cree <ecree@...arflare.com>
> ---

> -static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc)
> +static int __netif_receive_skb_taps(struct sk_buff *skb, bool pfmemalloc,
> +                                   struct packet_type **pt_prev)

A lot of code churn can be avoided by keeping local variable pt_prev and
calling this ppt_prev or so, then assigning just before returning on success.

Also, this function does more than just process network taps.

>  {
> -       struct packet_type *ptype, *pt_prev;
>         rx_handler_func_t *rx_handler;
>         struct net_device *orig_dev;
>         bool deliver_exact = false;
> +       struct packet_type *ptype;
>         int ret = NET_RX_DROP;
>         __be16 type;
>
> @@ -4514,7 +4515,7 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc)
>                 skb_reset_transport_header(skb);
>         skb_reset_mac_len(skb);
>
> -       pt_prev = NULL;
> +       *pt_prev = NULL;
>
>  another_round:
>         skb->skb_iif = skb->dev->ifindex;
> @@ -4535,25 +4536,25 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc)
>                 goto skip_taps;
>
>         list_for_each_entry_rcu(ptype, &ptype_all, list) {
> -               if (pt_prev)
> -                       ret = deliver_skb(skb, pt_prev, orig_dev);
> -               pt_prev = ptype;
> +               if (*pt_prev)
> +                       ret = deliver_skb(skb, *pt_prev, orig_dev);
> +               *pt_prev = ptype;
>         }