Re: [RFC] skb: avoid unnecessary reallocations in __skb_cow

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2012-05-29 at 14:41 +0200, Felix Fietkau wrote:
> On 2012-05-29 2:34 PM, Eric Dumazet wrote:
> > On Sun, 2012-05-27 at 17:26 +0200, Felix Fietkau wrote:
> >> At the beginning of __skb_cow, headroom gets set to a minimum of
> >> NET_SKB_PAD. This causes unnecessary reallocations if the buffer was not
> >> cloned and the headroom is just below NET_SKB_PAD, but still more than the
> >> amount requested by the caller.
> >> This was showing up frequently in my tests on VLAN tx, where
> >> vlan_insert_tag calls skb_cow_head(skb, VLAN_HLEN).
> >> 
> >> Fix this by only setting the headroom delta if either there is less
> >> headroom than specified by the caller, or if reallocation has to be done
> >> anyway because the skb was cloned.
> >> 
> >> Signed-off-by: Felix Fietkau <nbd@openwrt.org>
> >> ---
> >>  include/linux/skbuff.h |    9 ++++++---
> >>  1 files changed, 6 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> >> index 0e50171..1898471 100644
> >> --- a/include/linux/skbuff.h
> >> +++ b/include/linux/skbuff.h
> >> @@ -1894,12 +1894,15 @@ static inline int skb_clone_writable(const struct sk_buff *skb, unsigned int len
> >>  static inline int __skb_cow(struct sk_buff *skb, unsigned int headroom,
> >>  			    int cloned)
> >>  {
> >> +	unsigned int alloc_headroom = headroom;
> >>  	int delta = 0;
> >>  
> >>  	if (headroom < NET_SKB_PAD)
> >> -		headroom = NET_SKB_PAD;
> >> -	if (headroom > skb_headroom(skb))
> >> -		delta = headroom - skb_headroom(skb);
> >> +		alloc_headroom = NET_SKB_PAD;
> >> +	if (headroom > skb_headroom(skb) ||
> >> +	    (cloned && alloc_headroom > skb_headroom(skb))) {
> >> +		delta = alloc_headroom - skb_headroom(skb);
> >> +	}
> >>  
> >>  	if (delta || cloned)
> >>  		return pskb_expand_head(skb, ALIGN(delta, NET_SKB_PAD), 0,
> > 
> > Nice catch.
> > 
> > Scratching my head on this one. Why not the obvious fix ?
> If we're reallocating anyway, we might as well put in more headroom than
> requested, in case something else needs even more than that.


Locally generated packets should have enough headroom, and for forward
paths, we already have NET_SKB_PAD bytes of headroom.

Adding yet another NET_SKB_PAD extra space is overkill, unless you have
a real use case in mind ?