net: mvneta: convert to build_skb()

Make use of build_skb() to allocate frags on the RX path. When frag size
is lower than a page size, we can use netdev_alloc_frag(), and we fall back
to kmalloc() for larger sizes. The frag size is stored into the mvneta_port
struct. The alloc/free functions check the frag size to decide what alloc/
free method to use. MTU changes are safe because the MTU change function
stops the device and clears the queues before applying the change.

With this patch, I observed a reproducible 2% performance improvement on
HTTP-based benchmarks, and 5% on small packet RX rate.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvneta.c

@@ -268,6 +268,7 @@ struct mvneta_pcpu_stats {
 
 struct mvneta_port {
 	int pkt_size;
+	unsigned int frag_size;
 	void __iomem *base;
 	struct mvneta_rx_queue *rxqs;
 	struct mvneta_tx_queue *txqs;
@@ -1332,28 +1333,43 @@ static int mvneta_txq_done(struct mvneta_port *pp,
 	return tx_done;
 }
 
+static void *mvneta_frag_alloc(const struct mvneta_port *pp)
+{
+	if (likely(pp->frag_size <= PAGE_SIZE))
+		return netdev_alloc_frag(pp->frag_size);
+	else
+		return kmalloc(pp->frag_size, GFP_ATOMIC);
+}
+
+static void mvneta_frag_free(const struct mvneta_port *pp, void *data)
+{
+	if (likely(pp->frag_size <= PAGE_SIZE))
+		put_page(virt_to_head_page(data));
+	else
+		kfree(data);
+}
+
 /* Refill processing */
 static int mvneta_rx_refill(struct mvneta_port *pp,
 			    struct mvneta_rx_desc *rx_desc)
 
 {
 	dma_addr_t phys_addr;
-	struct sk_buff *skb;
+	void *data;
 
-	skb = netdev_alloc_skb(pp->dev, pp->pkt_size);
-	if (!skb)
+	data = mvneta_frag_alloc(pp);
+	if (!data)
 		return -ENOMEM;
 
-	phys_addr = dma_map_single(pp->dev->dev.parent, skb->head,
+	phys_addr = dma_map_single(pp->dev->dev.parent, data,
 				   MVNETA_RX_BUF_SIZE(pp->pkt_size),
 				   DMA_FROM_DEVICE);
 	if (unlikely(dma_mapping_error(pp->dev->dev.parent, phys_addr))) {
-		dev_kfree_skb(skb);
+		mvneta_frag_free(pp, data);
 		return -ENOMEM;
 	}
 
-	mvneta_rx_desc_fill(rx_desc, phys_addr, (u32)skb);
-
+	mvneta_rx_desc_fill(rx_desc, phys_addr, (u32)data);
 	return 0;
 }
 
@@ -1407,9 +1423,9 @@ static void mvneta_rxq_drop_pkts(struct mvneta_port *pp,
 	rx_done = mvneta_rxq_busy_desc_num_get(pp, rxq);
 	for (i = 0; i < rxq->size; i++) {
 		struct mvneta_rx_desc *rx_desc = rxq->descs + i;
-		struct sk_buff *skb = (struct sk_buff *)rx_desc->buf_cookie;
+		void *data = (void *)rx_desc->buf_cookie;
 
-		dev_kfree_skb_any(skb);
+		mvneta_frag_free(pp, data);
 		dma_unmap_single(pp->dev->dev.parent, rx_desc->buf_phys_addr,
 				 MVNETA_RX_BUF_SIZE(pp->pkt_size), DMA_FROM_DEVICE);
 	}
@@ -1440,20 +1456,21 @@ static int mvneta_rx(struct mvneta_port *pp, int rx_todo,
 	while (rx_done < rx_todo) {
 		struct mvneta_rx_desc *rx_desc = mvneta_rxq_next_desc_get(rxq);
 		struct sk_buff *skb;
+		unsigned char *data;
 		u32 rx_status;
 		int rx_bytes, err;
 
 		rx_done++;
 		rx_filled++;
 		rx_status = rx_desc->status;
-		skb = (struct sk_buff *)rx_desc->buf_cookie;
+		data = (unsigned char *)rx_desc->buf_cookie;
 
 		if (!mvneta_rxq_desc_is_first_last(rx_status) ||
-		    (rx_status & MVNETA_RXD_ERR_SUMMARY)) {
+		    (rx_status & MVNETA_RXD_ERR_SUMMARY) ||
+		    !(skb = build_skb(data, pp->frag_size > PAGE_SIZE ? 0 : pp->frag_size))) {
 			dev->stats.rx_errors++;
 			mvneta_rx_error(pp, rx_desc);
-			mvneta_rx_desc_fill(rx_desc, rx_desc->buf_phys_addr,
-					    (u32)skb);
+			/* leave the descriptor untouched */
 			continue;
 		}
 
@@ -1466,7 +1483,7 @@ static int mvneta_rx(struct mvneta_port *pp, int rx_todo,
 		rcvd_bytes += rx_bytes;
 
 		/* Linux processing */
-		skb_reserve(skb, MVNETA_MH_SIZE);
+		skb_reserve(skb, MVNETA_MH_SIZE + NET_SKB_PAD);
 		skb_put(skb, rx_bytes);
 
 		skb->protocol = eth_type_trans(skb, dev);
@@ -2276,6 +2293,8 @@ static int mvneta_change_mtu(struct net_device *dev, int mtu)
 	mvneta_cleanup_rxqs(pp);
 
 	pp->pkt_size = MVNETA_RX_PKT_SIZE(pp->dev->mtu);
+	pp->frag_size = SKB_DATA_ALIGN(MVNETA_RX_BUF_SIZE(pp->pkt_size)) +
+	                SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 
 	ret = mvneta_setup_rxqs(pp);
 	if (ret) {
@@ -2423,6 +2442,8 @@ static int mvneta_open(struct net_device *dev)
 	mvneta_mac_addr_set(pp, dev->dev_addr, rxq_def);
 
 	pp->pkt_size = MVNETA_RX_PKT_SIZE(pp->dev->mtu);
+	pp->frag_size = SKB_DATA_ALIGN(MVNETA_RX_BUF_SIZE(pp->pkt_size)) +
+	                SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 
 	ret = mvneta_setup_rxqs(pp);
 	if (ret)