[PATCH] net-xdp: netdev attribute to control xdpgeneric skb linearization

[Luigi Rizzo <lrizzo@google.com>]

Add a netdevice flag to control skb linearization in generic xdp mode.
Among the various mechanism to control the flag, the sysfs
interface seems sufficiently simple and self-contained.
The attribute can be modified through
	/sys/class/net/<DEVICE>/xdp_linearize
The default is 1 (on)

On a kernel instrumented to grab timestamps around the linearization
code in netif_receive_generic_xdp, and heavy netperf traffic with 1500b
mtu, I see the following times (nanoseconds/pkt)

The receiver generally sees larger packets so the difference is more
significant.

ns/pkt                   RECEIVER                 SENDER

                    p50     p90     p99       p50   p90    p99

LINEARIZATION:    600ns  1090ns  4900ns     149ns 249ns  460ns
NO LINEARIZATION:  40ns    59ns    90ns      40ns  50ns  100ns

Tested: run tests on an instrumented kernel
Change-Id: I69884661167ab86347c50bdece8cae1afa821956
Signed-off-by: Luigi Rizzo <lrizzo@google.com>
---
 include/linux/netdevice.h |  3 ++-
 net/core/dev.c            |  5 +++--
 net/core/net-sysfs.c      | 15 +++++++++++++++
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 2741aa35bec6..ae873fb5ec3c 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1958,7 +1958,8 @@ struct net_device {
 
 	struct netdev_rx_queue	*_rx;
 	unsigned int		num_rx_queues;
-	unsigned int		real_num_rx_queues;
+	unsigned int		real_num_rx_queues:31;
+	unsigned int		xdp_linearize : 1;
 
 	struct bpf_prog __rcu	*xdp_prog;
 	unsigned long		gro_flush_timeout;
diff --git a/net/core/dev.c b/net/core/dev.c
index 6368c94c9e0a..04c7c8ed1b4a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4484,8 +4484,8 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	 * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
 	 * native XDP provides, thus we need to do it here as well.
 	 */
-	if (skb_is_nonlinear(skb) ||
-	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
+	if (skb->dev->xdp_linearize && (skb_is_nonlinear(skb) ||
+	    skb_headroom(skb) < XDP_PACKET_HEADROOM)) {
 		int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
 		int troom = skb->tail + skb->data_len - skb->end;
 
@@ -9756,6 +9756,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 	dev->gso_max_segs = GSO_MAX_SEGS;
 	dev->upper_level = 1;
 	dev->lower_level = 1;
+	dev->xdp_linearize = 1;
 
 	INIT_LIST_HEAD(&dev->napi_list);
 	INIT_LIST_HEAD(&dev->unreg_list);
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 4c826b8bf9b1..ec59aa296664 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -442,6 +442,20 @@ static ssize_t proto_down_store(struct device *dev,
 }
 NETDEVICE_SHOW_RW(proto_down, fmt_dec);
 
+static int change_xdp_linearize(struct net_device *dev, unsigned long val)
+{
+	dev->xdp_linearize = !!val;
+	return 0;
+}
+
+static ssize_t xdp_linearize_store(struct device *dev,
+				   struct device_attribute *attr,
+				   const char *buf, size_t len)
+{
+	return netdev_store(dev, attr, buf, len, change_xdp_linearize);
+}
+NETDEVICE_SHOW_RW(xdp_linearize, fmt_dec);
+
 static ssize_t phys_port_id_show(struct device *dev,
 				 struct device_attribute *attr, char *buf)
 {
@@ -536,6 +550,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
 	&dev_attr_phys_port_name.attr,
 	&dev_attr_phys_switch_id.attr,
 	&dev_attr_proto_down.attr,
+	&dev_attr_xdp_linearize.attr,
 	&dev_attr_carrier_up_count.attr,
 	&dev_attr_carrier_down_count.attr,
 	NULL,
-- 
2.25.0.341.g760bfbb309-goog