[Intel-wired-lan] [RFC PATCH] ixgbe: delay tail write to every 'n' packets

[John Fastabend]

Current XDP implementation hits the tail on every XDP_TX return
code. This patch changes driver behavior to only hit the tail after
packet processing is complete.

RFC for now as I test this, it looks promising on my dev box but
want to do some more tests before official submission.

Signed-off-by: John Fastabend <john.r.fastabend at intel.com>
---
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index bef4e24..2c244b6 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -2282,6 +2282,7 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
 	unsigned int mss = 0;
 #endif /* IXGBE_FCOE */
 	u16 cleaned_count = ixgbe_desc_unused(rx_ring);
+	bool xdp_xmit = false;
 
 	while (likely(total_rx_packets < budget)) {
 		union ixgbe_adv_rx_desc *rx_desc;
@@ -2321,10 +2322,12 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
 		}
 
 		if (IS_ERR(skb)) {
-			if (PTR_ERR(skb) == -IXGBE_XDP_TX)
+			if (PTR_ERR(skb) == -IXGBE_XDP_TX) {
+				xdp_xmit = true;
 				ixgbe_rx_buffer_flip(rx_ring, rx_buffer, size);
-			else
+			} else {
 				rx_buffer->pagecnt_bias++;
+			}
 			total_rx_packets++;
 			total_rx_bytes += size;
 		} else if (skb) {
@@ -2392,6 +2395,12 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
 		total_rx_packets++;
 	}
 
+	if (xdp_xmit) {
+		struct ixgbe_ring *ring = adapter->xdp_ring[smp_processor_id()];
+
+		writel(ring->next_to_use, ring->tail);
+	}
+
 	u64_stats_update_begin(&rx_ring->syncp);
 	rx_ring->stats.packets += total_rx_packets;
 	rx_ring->stats.bytes += total_rx_bytes;
@@ -8251,7 +8260,6 @@ static int ixgbe_xmit_xdp_ring(struct ixgbe_adapter *adapter,
 	tx_buffer->next_to_watch = tx_desc;
 	ring->next_to_use = i;
 
-	writel(i, ring->tail);
 	return IXGBE_XDP_TX;
 }