From: "Matt Carlson" <mcarlson@broadcom•com>
To: davem@davemloft•net
Cc: netdev@vger•kernel.org, andy@greyhouse•net,
mcarlson@broadcom•com, "Michael Chan" <mchan@broadcom•com>
Subject: [PATCH net-next 09/14] tg3: Improve small packet performance
Date: Mon, 2 Aug 2010 14:26:03 -0700 [thread overview]
Message-ID: <1280784368-4226-9-git-send-email-mcarlson@broadcom.com> (raw)
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>
---
drivers/net/tg3.c | 24 +++++++++++++++++++++++-
1 files changed, 23 insertions(+), 1 deletions(-)
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 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);
}
--
1.7.1
next reply other threads:[~2010-08-02 21:26 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-02 21:26 Matt Carlson [this message]
2010-08-04 22:27 ` [PATCH net-next 09/14] tg3: Improve small packet performance Anton Blanchard
2010-08-04 22:46 ` Matt Carlson
2010-08-04 22:47 ` Michael Chan
2010-08-04 23:08 ` Anton Blanchard
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