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authorMichael Chan <mchan@broadcom.com>2010-07-19 14:15:04 +0000
committerDavid S. Miller <davem@davemloft.net>2010-07-19 20:30:06 -0700
commit11848b964777af9c68d9160582628c2eb11f46d5 (patch)
tree6b026650813b24b50da954c5978811246f811a76
parent379b39a2ad613745bfbfe80256957d19689b7b94 (diff)
bnx2: Remove some unnecessary smp_mb() in tx fast path.
smp_mb() inside bnx2_tx_avail() is used twice in the normal bnx2_start_xmit() path (see illustration below). The full memory barrier is only necessary during race conditions with tx completion. We can speed up the tx path by replacing smp_mb() in bnx2_tx_avail() with a compiler barrier. The compiler barrier is to force the compiler to fetch the tx_prod and tx_cons from memory. In the race condition between bnx2_start_xmit() and bnx2_tx_int(), we have the following situation: bnx2_start_xmit() bnx2_tx_int() if (!bnx2_tx_avail()) BUG(); ... if (!bnx2_tx_avail()) netif_tx_stop_queue(); update_tx_index(); smp_mb(); smp_mb(); if (bnx2_tx_avail()) if (netif_tx_queue_stopped() && netif_tx_wake_queue(); bnx2_tx_avail()) With smp_mb() removed from bnx2_tx_avail(), we need to add smp_mb() to bnx2_start_xmit() as shown above to properly order netif_tx_stop_queue() and bnx2_tx_avail() to check the ring index. If it is not strictly ordered, the tx queue can be stopped forever. This improves performance by about 5% with 2 ports running bi-directional 64-byte packets. Reviewed-by: Benjamin Li <benli@broadcom.com> Reviewed-by: Matt Carlson <mcarlson@broadcom.com> Signed-off-by: Michael Chan <mchan@broadcom.com> Signed-off-by: David S. Miller <davem@davemloft.net>
-rw-r--r--drivers/net/bnx2.c10
1 files changed, 9 insertions, 1 deletions
diff --git a/drivers/net/bnx2.c b/drivers/net/bnx2.c
index d44ecc30865..2af570d7a2f 100644
--- a/drivers/net/bnx2.c
+++ b/drivers/net/bnx2.c
@@ -253,7 +253,8 @@ static inline u32 bnx2_tx_avail(struct bnx2 *bp, struct bnx2_tx_ring_info *txr)
{
u32 diff;
- smp_mb();
+ /* Tell compiler to fetch tx_prod and tx_cons from memory. */
+ barrier();
/* The ring uses 256 indices for 255 entries, one of them
* needs to be skipped.
@@ -6534,6 +6535,13 @@ bnx2_start_xmit(struct sk_buff *skb, struct net_device *dev)
if (unlikely(bnx2_tx_avail(bp, txr) <= MAX_SKB_FRAGS)) {
netif_tx_stop_queue(txq);
+
+ /* netif_tx_stop_queue() must be done before checking
+ * tx index in bnx2_tx_avail() below, because in
+ * bnx2_tx_int(), we update tx index before checking for
+ * netif_tx_queue_stopped().
+ */
+ smp_mb();
if (bnx2_tx_avail(bp, txr) > bp->tx_wake_thresh)
netif_tx_wake_queue(txq);
}