4.9-stable review patch. If anyone has any objections, please let me know.
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From: Eric Dumazet edumazet@google.com
ip_defrag uses skb->cb[] to store the fragment offset, and unfortunately this integer is currently in a different cache line than skb->next, meaning that we use two cache lines per skb when finding the insertion point.
By aliasing skb->ip_defrag_offset and skb->dev, we pack all the fields in a single cache line and save precious memory bandwidth.
Note that after the fast path added by Changli Gao in commit d6bebca92c66 ("fragment: add fast path for in-order fragments") this change wont help the fast path, since we still need to access prev->len (2nd cache line), but will show great benefits when slow path is entered, since we perform a linear scan of a potentially long list.
Also, note that this potential long list is an attack vector, we might consider also using an rb-tree there eventually.
Signed-off-by: Eric Dumazet edumazet@google.com Signed-off-by: David S. Miller davem@davemloft.net (cherry picked from commit bf66337140c64c27fa37222b7abca7e49d63fb57) Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/skbuff.h | 5 +++++ 1 file changed, 5 insertions(+)
--- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -645,6 +645,11 @@ struct sk_buff { }; struct rb_node rbnode; /* used in netem & tcp stack */ }; + + union { + int ip_defrag_offset; + }; + struct sock *sk; struct net_device *dev;