[PATCH RFC] veth: make veth aware of gso buffer size

[Solio Sarabia]

GSO buffer size supported by underlying devices is not propagated to
veth. In high-speed connections with hw TSO enabled, veth sends buffers
bigger than lower device's maximum GSO, forcing sw TSO and increasing
system CPU usage.

Signed-off-by: Solio Sarabia <solio.sarabia@xxxxxxxxx>
---
Exposing gso_max_size via sysfs is not advised [0]. This patch queries
available interfaces get this value. Reading dev_list is O(n), since it
can be large (e.g. hundreds of containers), only a subset of interfaces
is inspected.  _Please_ advise pointers how to make veth aware of lower
device's GSO value.

In a test scenario with Hyper-V, Ubuntu VM, Docker inside VM, and NTttcp
microworkload sending 40 Gbps from one container, this fix reduces 3x
sender host CPU overhead, since now all TSO is done on physical NIC.
Savings in CPU cycles benefit other use cases where veth is used, and
the GSO buffer size is properly set.

[0] https://lkml.org/lkml/2017/11/24/512

 drivers/net/veth.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index f5438d0..e255b51 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -298,6 +298,34 @@ static const struct net_device_ops veth_netdev_ops = {
 		       NETIF_F_HW_VLAN_CTAG_TX | NETIF_F_HW_VLAN_CTAG_RX | \
 		       NETIF_F_HW_VLAN_STAG_TX | NETIF_F_HW_VLAN_STAG_RX )
 
+static void veth_set_gso(struct net_device *dev)
+{
+	struct net_device *nd;
+	unsigned int size = GSO_MAX_SIZE;
+	u16 segs = GSO_MAX_SEGS;
+	unsigned int count = 0;
+	const unsigned int limit = 10;
+
+	/* Set default gso based on available physical/synthetic devices,
+	 * ignore virtual interfaces, and limit looping through dev_list
+	 * as the total number of interfaces can be large.
+	 */
+	read_lock(&dev_base_lock);
+	for_each_netdev(&init_net, nd) {
+		if (count >= limit)
+			break;
+		if (nd->dev.parent && nd->flags & IFF_UP) {
+			size = min(size, nd->gso_max_size);
+			segs = min(segs, nd->gso_max_segs);
+		}
+		count++;
+	}
+
+	read_unlock(&dev_base_lock);
+	netif_set_gso_max_size(dev, size);
+	dev->gso_max_segs = size ? size - 1 : 0;
+}
+
 static void veth_setup(struct net_device *dev)
 {
 	ether_setup(dev);
@@ -323,6 +351,8 @@ static void veth_setup(struct net_device *dev)
 	dev->hw_features = VETH_FEATURES;
 	dev->hw_enc_features = VETH_FEATURES;
 	dev->mpls_features = NETIF_F_HW_CSUM | NETIF_F_GSO_SOFTWARE;
+
+	veth_set_gso(dev);
 }
 
 /*
-- 
2.7.4