From: Eric Dumazet <edumazet@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
(cherry picked from commit bf66337140c64c27fa37222b7abca7e49d63fb57)
---
include/linux/skbuff.h | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 1f207dd22757..c7cca35fcf6d 100644
--- 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;
--
2.17.1
