[PATCH 05/10] net: move destructor_arg to the front of sk_buff.

Ian Campbell
2012-04-10T14:32:39Z

As of the previous patch we align the end (rather than the start) of the struct
to a cache line and so, with 32 and 64 byte cache lines and the shinfo size
increase from the next patch, the first 8 bytes of the struct end up on a
different cache line to the rest of it so make sure it is something relatively
unimportant to avoid hitting an extra cache line on hot operations such as
kfree_skb.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
---
include/linux/skbuff.h | 15 ++++++++++-----
net/core/skbuff.c | 5 ++++-
2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0ad6a46..f0ae39c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -265,6 +265,15 @@ struct ubuf_info {
* the end of the header data, ie. at skb->end.
*/
struct skb_shared_info {
+ /* Intermediate layers must ensure that destructor_arg
+ * remains valid until skb destructor */
+ void *destructor_arg;
+
+ /*
+ * Warning: all fields from here until dataref are cleared in
+ * __alloc_skb()
+ *
+ */
unsigned char nr_frags;
__u8 tx_flags;
unsigned short gso_size;
@@ -276,14 +285,10 @@ struct skb_shared_info {
__be32 ip6_frag_id;

/*
- * Warning : all fields before dataref are cleared in __alloc_skb()
+ * Warning: all fields before dataref are cleared in __alloc_skb()
*/
atomic_t dataref;

- /* Intermediate layers must ensure that destructor_arg
- * remains valid until skb destructor */
- void * destructor_arg;
-
/* must be last field, see pskb_expand_head() */
skb_frag_t frags[MAX_SKB_FRAGS];
};
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d4e139e..b8a41d6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -214,7 +214,10 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,

/* make sure we initialize shinfo sequentially */
shinfo = skb_shinfo(skb);
- memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
+
+ memset(&shinfo->nr_frags, 0,
+ offsetof(struct skb_shared_info, dataref)
+ - offsetof(struct skb_shared_info, nr_frags));
atomic_set(&shinfo->dataref, 1);
kmemcheck_annotate_variable(shinfo->destructor_arg);

--
1.7.2.5