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, 2 insertions, 2 deletions

diff --git a/drivers/net/e1000e/netdev.c b/drivers/net/e1000e/netdev.c
index 38694c79edcc..9043f1b845fe 100644
--- a/drivers/net/e1000e/netdev.c
+++ b/drivers/net/e1000e/netdev.c
@@ -3916,7 +3916,7 @@ static int e1000_tx_map(struct e1000_adapter *adapter,
                 buffer_info->length = size;
                 buffer_info->time_stamp = jiffies;
                 buffer_info->next_to_watch = i;
-                buffer_info->dma = map[0] + offset;
+                buffer_info->dma = skb_shinfo(skb)->dma_head + offset;
                 count++;
 
                 len -= size;
@@ -3947,7 +3947,7 @@ static int e1000_tx_map(struct e1000_adapter *adapter,
                         buffer_info->length = size;
                         buffer_info->time_stamp = jiffies;
                         buffer_info->next_to_watch = i;
-                        buffer_info->dma = map[f + 1] + offset;
+                        buffer_info->dma = map[f] + offset;
 
                         len -= size;
                         offset += size;