net: skb_shared_info optimization

skb_dma_unmap() is quite expensive for small packets,
because we use two different cache lines from skb_shared_info.

One to access nr_frags, one to access dma_maps[0]

Instead of dma_maps being an array of MAX_SKB_FRAGS + 1 elements,
let dma_head alone in a new dma_head field, close to nr_frags,
to reduce cache lines misses.

Tested on my dev machine (bnx2 & tg3 adapters), nice speedup !

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 7485058125e3..aad484cd5863 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -189,6 +189,9 @@ struct skb_shared_info {
         atomic_t        dataref;
         unsigned short  nr_frags;
         unsigned short  gso_size;
+#ifdef CONFIG_HAS_DMA
+        dma_addr_t      dma_head;
+#endif
         /* Warning: this field is not always filled in (UFO)! */
         unsigned short  gso_segs;
         unsigned short  gso_type;
@@ -198,7 +201,7 @@ struct skb_shared_info {
         struct skb_shared_hwtstamps hwtstamps;
         skb_frag_t      frags[MAX_SKB_FRAGS];
 #ifdef CONFIG_HAS_DMA
-        dma_addr_t      dma_maps[MAX_SKB_FRAGS + 1];
+        dma_addr_t      dma_maps[MAX_SKB_FRAGS];
 #endif
         /* Intermediate layers must ensure that destructor_arg
          * remains valid until skb destructor */