commit 042a53a9e437feaf2230dd2cadcecfae9c7bfe05
Author: Eric Dumazet <eric.dumazet@gmail.com>
Date:   Fri Jun 5 04:04:16 2009 +0000

    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>

Index: hurd-debian/libdde-linux26/contrib/include/linux/skbuff.h
===================================================================
--- hurd-debian.orig/libdde-linux26/contrib/include/linux/skbuff.h	2012-04-16 00:26:40.000000000 +0000
+++ hurd-debian/libdde-linux26/contrib/include/linux/skbuff.h	2012-04-16 00:34:56.000000000 +0000
@@ -142,6 +142,9 @@
 	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;
@@ -152,7 +155,7 @@
 	struct sk_buff	*frag_list;
 	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
 };
 
Index: hurd-debian/libdde-linux26/contrib/net/core/skb_dma_map.c
===================================================================
--- hurd-debian.orig/libdde-linux26/contrib/net/core/skb_dma_map.c	2012-04-16 00:26:40.000000000 +0000
+++ hurd-debian/libdde-linux26/contrib/net/core/skb_dma_map.c	2012-04-16 00:34:56.000000000 +0000
@@ -20,7 +20,7 @@
 	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 @@
 				   fp->size, dir);
 		if (dma_mapping_error(dev, map))
 			goto unwind;
-		sp->dma_maps[i + 1] = map;
+		sp->dma_maps[i] = map;
 	}
 	sp->num_dma_maps = i + 1;
 
@@ -38,10 +38,10 @@
 	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;
@@ -54,12 +54,12 @@
 	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);
 	}
 }