Re: [PATCH net-next] asix: avoid copies in tx path

From: Ming Lei <redacted>
Date: Fri, 6 Jul 2012 09:16:32 +0800

On Thu, Jul 5, 2012 at 10:31 PM, Eric Dumazet [off-list ref] wrote:

quoted

From: Eric Dumazet <edumazet@google.com>

I noticed excess calls to skb_copy_expand() or memmove() in asix driver.

This driver needs to push 4 bytes in front of frame (packet_len)
and maybe add 4 bytes after the end (if padlen is 4)

So it should set needed_headroom & needed_tailroom to avoid
copies. But its not enough, because many packets are cloned
before entering asix_tx_fixup() and this driver use skb_cloned()
as a lazy way to check if it can push and put additional bytes in frame.

Avoid skb_copy_expand() expensive call, using following rules :

- We are allowed to push 4 bytes in headroom if skb_header_cloned()
  is false (and if we have 4 bytes of headroom)

- We are allowed to put 4 bytes at tail if skb_cloned()
  is false (and if we have 4 bytes of tailroom)

TCP packets for example are cloned, but skb_header_release()
was called in tcp stack, allowing us to use headroom for our needs.

Signed-off-by: Eric Dumazet <edumazet@google.com>

 ...

Tested-by: Ming Lei <redacted>

Applied, thanks Eric.