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, 6 insertions, 6 deletions

diff --git a/net/core/skb_dma_map.c b/net/core/skb_dma_map.c
index 7adb623ef664..79687dfd6957 100644
--- a/net/core/skb_dma_map.c
+++ b/net/core/skb_dma_map.c
@@ -20,7 +20,7 @@ int skb_dma_map(struct device *dev, struct sk_buff *skb,
         if (dma_mapping_error(dev, map))
                 goto out_err;
 
-        sp->dma_maps[0] = map;
+        sp->dma_head = map;
         for (i = 0; i < sp->nr_frags; i++) {
                 skb_frag_t *fp = &sp->frags[i];
 
@@ -28,7 +28,7 @@ int skb_dma_map(struct device *dev, struct sk_buff *skb,
                                    fp->size, dir);
                 if (dma_mapping_error(dev, map))
                         goto unwind;
-                sp->dma_maps[i + 1] = map;
+                sp->dma_maps[i] = map;
         }
 
         return 0;
@@ -37,10 +37,10 @@ unwind:
         while (--i >= 0) {
                 skb_frag_t *fp = &sp->frags[i];
 
-                dma_unmap_page(dev, sp->dma_maps[i + 1],
+                dma_unmap_page(dev, sp->dma_maps[i],
                                fp->size, dir);
         }
-        dma_unmap_single(dev, sp->dma_maps[0],
+        dma_unmap_single(dev, sp->dma_head,
                          skb_headlen(skb), dir);
 out_err:
         return -ENOMEM;
@@ -53,12 +53,12 @@ void skb_dma_unmap(struct device *dev, struct sk_buff *skb,
         struct skb_shared_info *sp = skb_shinfo(skb);
         int i;
 
-        dma_unmap_single(dev, sp->dma_maps[0],
+        dma_unmap_single(dev, sp->dma_head,
                          skb_headlen(skb), dir);
         for (i = 0; i < sp->nr_frags; i++) {
                 skb_frag_t *fp = &sp->frags[i];
 
-                dma_unmap_page(dev, sp->dma_maps[i + 1],
+                dma_unmap_page(dev, sp->dma_maps[i],
                                fp->size, dir);
         }
 }