From f65aac166fe10b96e64c233980a3522fc50fbecf Mon Sep 17 00:00:00 2001 From: Matt Carlson Date: Mon, 2 Aug 2010 11:26:03 +0000 Subject: [PATCH] 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 Signed-off-by: Michael Chan Signed-off-by: Matt Carlson Signed-off-by: David S. Miller --- drivers/net/tg3.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) 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); } -- 2.20.1