Re: [PATCH net-next resubmit] skbuff: optimize the pull_pages code in __pskb_pull_tail()

[Eric Dumazet]

On Mon, 2017-07-17 at 10:22 +0800, Lin Zhang wrote:
> In the pull_pages code block, if the first frag size > eat,
> we can end the loop in advance to avoid extra copy.
> 
> Signed-off-by: Lin Zhang <xiaolou4...@gmail.com>
> ---
>  net/core/skbuff.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index f990eb8..c00a1df 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1720,6 +1720,10 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta)
>                               skb_shinfo(skb)->frags[k].page_offset += eat;
>                               skb_frag_size_sub(&skb_shinfo(skb)->frags[k], 
> eat);
>                               eat = 0;
> +                             if (!i) {
> +                                     k = skb_shinfo(skb)->nr_frags;
> +                                     break;
> +                             }
>                       }
>                       k++;
>               }

-CC lkml  (no need to spam lkml for netdev patches )

Why even reading skb_shinfo(skb)->nr_frags to write it again ?

I would use something like :

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 
8b11341ed69ad97d34dd3e9b73c8c44ef7c452ff..a562b0e61afa3faf7abdd7f9209ae9096310fb7d
 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1719,13 +1719,15 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta)
                        if (eat) {
                                skb_shinfo(skb)->frags[k].page_offset += eat;
                                skb_frag_size_sub(&skb_shinfo(skb)->frags[k], 
eat);
+                               if (!i)
+                                       goto end;
                                eat = 0;
                        }
                        k++;
                }
        }
        skb_shinfo(skb)->nr_frags = k;
-
+end:
        skb->tail     += delta;
        skb->data_len -= delta;