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>
diff --git a/drivers/net/tg3.c b/drivers/net/tg3.c
index 32e3a3d..820a7dd 100644
--- a/drivers/net/tg3.c
+++ b/drivers/net/tg3.c
@@ -4386,7 +4386,8 @@
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 @@
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 @@
/* 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 @@
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);
}