diff options
author | Michael Chan <mchan@broadcom.com> | 2010-07-19 18:15:04 +0400 |
---|---|---|
committer | David S. Miller <davem@davemloft.net> | 2010-07-20 07:30:06 +0400 |
commit | 11848b964777af9c68d9160582628c2eb11f46d5 (patch) | |
tree | 6b026650813b24b50da954c5978811246f811a76 /drivers/net | |
parent | 379b39a2ad613745bfbfe80256957d19689b7b94 (diff) | |
download | linux-11848b964777af9c68d9160582628c2eb11f46d5.tar.xz |
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>
Diffstat (limited to 'drivers/net')
-rw-r--r-- | drivers/net/bnx2.c | 10 |
1 files changed, 9 insertions, 1 deletions
diff --git a/drivers/net/bnx2.c b/drivers/net/bnx2.c index d44ecc30865f..2af570d7a2fb 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); } |