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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
---
 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


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