Commit 4947d3ef authored by Benjamin LaHaise's avatar Benjamin LaHaise Committed by David S. Miller
Browse files

[NET]: Speed up __alloc_skb()

From: Benjamin LaHaise <>

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: default avatarDavid S. Miller <>
parent 17ba15fb
......@@ -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;
......@@ -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);
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,
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;
return skb;
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