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authorMatt Carlson <mcarlson@broadcom.com>2010-08-02 11:26:03 +0000
committerDavid S. Miller <davem@davemloft.net>2010-08-02 15:46:31 -0700
commitf65aac166fe10b96e64c233980a3522fc50fbecf (patch)
treebe56c446c8fd596a47d0c5c62a339f19a71a5fb9 /drivers/net/tg3.c
parenttg3: Remove 5720, 5750, and 5750M (diff)
downloadlinux-dev-f65aac166fe10b96e64c233980a3522fc50fbecf.tar.xz
linux-dev-f65aac166fe10b96e64c233980a3522fc50fbecf.zip
tg3: Improve small packet performance
smp_mb() inside tg3_tx_avail() is used twice in the normal tg3_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 tg3_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 tg3_start_xmit() and tg3_tx(), we have the following situation: tg3_start_xmit() tg3_tx() if (!tg3_tx_avail()) BUG(); ... if (!tg3_tx_avail()) netif_tx_stop_queue(); update_tx_index(); smp_mb(); smp_mb(); if (tg3_tx_avail()) if (netif_tx_queue_stopped() && netif_tx_wake_queue(); tg3_tx_avail()) With smp_mb() removed from tg3_tx_avail(), we need to add smp_mb() to tg3_start_xmit() as shown above to properly order netif_tx_stop_queue() and tg3_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 3% with 2 ports running bi-directional 64-byte packets. Reviewed-by: Benjamin Li <benli@broadcom.com> Signed-off-by: Michael Chan <mchan@broadcom.com> Signed-off-by: Matt Carlson <mcarlson@broadcom.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'drivers/net/tg3.c')
-rw-r--r--drivers/net/tg3.c24
1 files changed, 23 insertions, 1 deletions
diff --git a/drivers/net/tg3.c b/drivers/net/tg3.c
index 32e3a3de4c6d..820a7ddd7e09 100644
--- a/drivers/net/tg3.c
+++ b/drivers/net/tg3.c
@@ -4386,7 +4386,8 @@ static void tg3_tx_recover(struct tg3 *tp)
static inline u32 tg3_tx_avail(struct tg3_napi *tnapi)
{
- smp_mb();
+ /* Tell compiler to fetch tx indices from memory. */
+ barrier();
return tnapi->tx_pending -
((tnapi->tx_prod - tnapi->tx_cons) & (TG3_TX_RING_SIZE - 1));
}
@@ -5670,6 +5671,13 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb,
tnapi->tx_prod = entry;
if (unlikely(tg3_tx_avail(tnapi) <= (MAX_SKB_FRAGS + 1))) {
netif_tx_stop_queue(txq);
+
+ /* netif_tx_stop_queue() must be done before checking
+ * checking tx index in tg3_tx_avail() below, because in
+ * tg3_tx(), we update tx index before checking for
+ * netif_tx_queue_stopped().
+ */
+ smp_mb();
if (tg3_tx_avail(tnapi) > TG3_TX_WAKEUP_THRESH(tnapi))
netif_tx_wake_queue(txq);
}
@@ -5715,6 +5723,13 @@ static int tg3_tso_bug(struct tg3 *tp, struct sk_buff *skb)
/* Estimate the number of fragments in the worst case */
if (unlikely(tg3_tx_avail(&tp->napi[0]) <= frag_cnt_est)) {
netif_stop_queue(tp->dev);
+
+ /* netif_tx_stop_queue() must be done before checking
+ * checking tx index in tg3_tx_avail() below, because in
+ * tg3_tx(), we update tx index before checking for
+ * netif_tx_queue_stopped().
+ */
+ smp_mb();
if (tg3_tx_avail(&tp->napi[0]) <= frag_cnt_est)
return NETDEV_TX_BUSY;
@@ -5950,6 +5965,13 @@ static netdev_tx_t tg3_start_xmit_dma_bug(struct sk_buff *skb,
tnapi->tx_prod = entry;
if (unlikely(tg3_tx_avail(tnapi) <= (MAX_SKB_FRAGS + 1))) {
netif_tx_stop_queue(txq);
+
+ /* netif_tx_stop_queue() must be done before checking
+ * checking tx index in tg3_tx_avail() below, because in
+ * tg3_tx(), we update tx index before checking for
+ * netif_tx_queue_stopped().
+ */
+ smp_mb();
if (tg3_tx_avail(tnapi) > TG3_TX_WAKEUP_THRESH(tnapi))
netif_tx_wake_queue(txq);
}