net: introduce netdevice gso_min_segs attribute

Some TSO engines might have a too heavy setup cost, that impacts
performance on hosts sending small bursts (2 MSS per packet).

This patch adds a device gso_min_segs, allowing drivers to set
a minimum segment size for TSO packets, according to the NIC
performance.

Tested on a mlx4 NIC, this allows to get a ~110% increase of
throughput when sending 2 MSS per packet.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1416,6 +1416,8 @@ enum netdev_priv_flags {
  *	@gso_max_size:	Maximum size of generic segmentation offload
  *	@gso_max_segs:	Maximum number of segments that can be passed to the
  *			NIC for GSO
+ *	@gso_min_segs:	Minimum number of segments that can be passed to the
+ *			NIC for GSO
  *
  *	@dcbnl_ops:	Data Center Bridging netlink ops
  *	@num_tc:	Number of traffic classes in the net device
@@ -1666,7 +1668,7 @@ struct net_device {
 	unsigned int		gso_max_size;
 #define GSO_MAX_SEGS		65535
 	u16			gso_max_segs;
-
+	u16			gso_min_segs;
 #ifdef CONFIG_DCB
 	const struct dcbnl_rtnl_ops *dcbnl_ops;
 #endif

net/core/dev.c

@@ -2567,10 +2567,12 @@ static netdev_features_t harmonize_features(struct sk_buff *skb,
 
 netdev_features_t netif_skb_features(struct sk_buff *skb)
 {
+	const struct net_device *dev = skb->dev;
+	netdev_features_t features = dev->features;
+	u16 gso_segs = skb_shinfo(skb)->gso_segs;
 	__be16 protocol = skb->protocol;
-	netdev_features_t features = skb->dev->features;
 
-	if (skb_shinfo(skb)->gso_segs > skb->dev->gso_max_segs)
+	if (gso_segs > dev->gso_max_segs || gso_segs < dev->gso_min_segs)
 		features &= ~NETIF_F_GSO_MASK;
 
 	if (protocol == htons(ETH_P_8021Q) || protocol == htons(ETH_P_8021AD)) {
@@ -2581,7 +2583,7 @@ netdev_features_t netif_skb_features(struct sk_buff *skb)
 	}
 
 	features = netdev_intersect_features(features,
-					     skb->dev->vlan_features |
+					     dev->vlan_features |
 					     NETIF_F_HW_VLAN_CTAG_TX |
 					     NETIF_F_HW_VLAN_STAG_TX);
 
@@ -6661,6 +6663,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 
 	dev->gso_max_size = GSO_MAX_SIZE;
 	dev->gso_max_segs = GSO_MAX_SEGS;
+	dev->gso_min_segs = 0;
 
 	INIT_LIST_HEAD(&dev->napi_list);
 	INIT_LIST_HEAD(&dev->unreg_list);