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

[Felix Fietkau <nbd@openwrt.org>]

On 2012-05-29 2:59 PM, Eric Dumazet wrote:
> 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 ?
I don't have any real use case in mind, but it's not really adding an
extra NET_SKB_PAD, it simply fills up the headroom to NET_SKB_PAD, but I
guess that's probably unnecessary as well.
I'll resend the patch without the extra padding later.

- Felix