[NET]: Speed up __alloc_skb()

From: Benjamin LaHaise <bcrl@kvack.org>

In __alloc_skb(), the use of skb_shinfo() which casts a u8 * to the 
shared info structure results in gcc being forced to do a reload of the 
pointer since it has no information on possible aliasing.  Fix this by 
using a pointer to refer to skb_shared_info.

By initializing skb_shared_info sequentially, the write combining buffers 
can reduce the number of memory transactions to a single write.  Reorder 
the initialization in __alloc_skb() to match the structure definition.  
There is also an alignment issue on 64 bit systems with skb_shared_info 
by converting nr_frags to a short everything packs up nicely.

Also, pass the slab cache pointer according to the fclone flag instead 
of using two almost identical function calls.

This raises bw_unix performance up to a peak of 707KB/s when combined 
with the spinlock patch.  It should help other networking protocols, too.

Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 14 insertions, 15 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 971677178e0c..483cfc47ec34 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -133,7 +133,7 @@ struct skb_frag_struct {
  */
 struct skb_shared_info {
         atomic_t        dataref;
-        unsigned int    nr_frags;
+        unsigned short  nr_frags;
         unsigned short  tso_size;
         unsigned short  tso_segs;
         unsigned short  ufo_size;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 83fee37de38e..070f91cfde59 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -135,17 +135,13 @@ void skb_under_panic(struct sk_buff *skb, int sz, void *here)
 struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
                             int fclone)
 {
+        struct skb_shared_info *shinfo;
         struct sk_buff *skb;
         u8 *data;
 
         /* Get the HEAD */
-        if (fclone)
-                skb = kmem_cache_alloc(skbuff_fclone_cache,
-                                       gfp_mask & ~__GFP_DMA);
-        else
-                skb = kmem_cache_alloc(skbuff_head_cache,
-                                       gfp_mask & ~__GFP_DMA);
-
+        skb = kmem_cache_alloc(fclone ? skbuff_fclone_cache : skbuff_head_cache,
+                                gfp_mask & ~__GFP_DMA);
         if (!skb)
                 goto out;
 
@@ -162,6 +158,16 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
         skb->data = data;
         skb->tail = data;
         skb->end  = data + size;
+        /* make sure we initialize shinfo sequentially */
+        shinfo = skb_shinfo(skb);
+        atomic_set(&shinfo->dataref, 1);
+        shinfo->nr_frags  = 0;
+        shinfo->tso_size = 0;
+        shinfo->tso_segs = 0;
+        shinfo->ufo_size = 0;
+        shinfo->ip6_frag_id = 0;
+        shinfo->frag_list = NULL;
+
         if (fclone) {
                 struct sk_buff *child = skb + 1;
                 atomic_t *fclone_ref = (atomic_t *) (child + 1);
@@ -171,13 +177,6 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 
                 child->fclone = SKB_FCLONE_UNAVAILABLE;
         }
-        atomic_set(&(skb_shinfo(skb)->dataref), 1);
-        skb_shinfo(skb)->nr_frags  = 0;
-        skb_shinfo(skb)->tso_size = 0;
-        skb_shinfo(skb)->tso_segs = 0;
-        skb_shinfo(skb)->frag_list = NULL;
-        skb_shinfo(skb)->ufo_size = 0;
-        skb_shinfo(skb)->ip6_frag_id = 0;
 out:
         return skb;
 nodata: