net: release dst entry in dev_hard_start_xmit()

One point of contention in high network loads is the dst_release() performed
when a transmited skb is freed. This is because NIC tx completion calls
dev_kree_skb() long after original call to dev_queue_xmit(skb).

CPU cache is cold and the atomic op in dst_release() stalls. On SMP, this is
quite visible if one CPU is 100% handling softirqs for a network device,
since dst_clone() is done by other cpus, involving cache line ping pongs.

It seems right place to release dst is in dev_hard_start_xmit(), for most
devices but ones that are virtual, and some exceptions.

David Miller suggested to define a new device flag, set in alloc_netdev_mq()
(so that most devices set it at init time), and carefuly unset in devices
which dont want a NULL skb->dst in their ndo_start_xmit().

List of devices that must clear this flag is :

- loopback device, because it calls netif_rx() and quoting Patrick :
    "ip_route_input() doesn't accept loopback addresses, so loopback packets
     already need to have a dst_entry attached."
- appletalk/ipddp.c : needs skb->dst in its xmit function

- And all devices that call again dev_queue_xmit() from their xmit function
(as some classifiers need skb->dst) : bonding, vlan, macvlan, eql, ifb, hdlc_fr

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

10 files changed, 20 insertions, 0 deletions

diff --git a/drivers/net/appletalk/ipddp.c b/drivers/net/appletalk/ipddp.c
index da64ba88d7f8..f939e92fcf8a 100644
--- a/drivers/net/appletalk/ipddp.c
+++ b/drivers/net/appletalk/ipddp.c
@@ -71,6 +71,7 @@ static struct net_device * __init ipddp_init(void)
         if (!dev)
                 return ERR_PTR(-ENOMEM);
 
+        dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
         strcpy(dev->name, "ipddp%d");
 
         if (version_printed++ == 0)
diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 96d7689995cd..92a9d69c5650 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -5148,6 +5148,7 @@ int bond_create(char *name, struct bond_params *params)
                 goto out_rtnl;
         }
 
+        bond_dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
         if (!name) {
                 res = dev_alloc_name(bond_dev, "bond%d");
                 if (res < 0)
diff --git a/drivers/net/eql.c b/drivers/net/eql.c
index 5210bb1027cc..19b7dd983944 100644
--- a/drivers/net/eql.c
+++ b/drivers/net/eql.c
@@ -194,6 +194,7 @@ static void __init eql_setup(struct net_device *dev)
 
         dev->type               = ARPHRD_SLIP;
         dev->tx_queue_len       = 5;            /* Hands them off fast */
+        dev->priv_flags        &= ~IFF_XMIT_DST_RELEASE;
 }
 
 static int eql_open(struct net_device *dev)
diff --git a/drivers/net/ifb.c b/drivers/net/ifb.c
index 60a263001933..96713ef06298 100644
--- a/drivers/net/ifb.c
+++ b/drivers/net/ifb.c
@@ -156,6 +156,7 @@ static void ifb_setup(struct net_device *dev)
 
         dev->flags |= IFF_NOARP;
         dev->flags &= ~IFF_MULTICAST;
+        dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
         random_ether_addr(dev->dev_addr);
 }
 
diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 6f71157bea8e..da472c687481 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -170,6 +170,7 @@ static void loopback_setup(struct net_device *dev)
         dev->tx_queue_len       = 0;
         dev->type               = ARPHRD_LOOPBACK;      /* 0x0001*/
         dev->flags              = IFF_LOOPBACK;
+        dev->priv_flags        &= ~IFF_XMIT_DST_RELEASE;
         dev->features           = NETIF_F_SG | NETIF_F_FRAGLIST
                 | NETIF_F_TSO
                 | NETIF_F_NO_CSUM
diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index 329cd50d0e29..d5334b41e4b4 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -414,6 +414,7 @@ static void macvlan_setup(struct net_device *dev)
 {
         ether_setup(dev);
 
+        dev->priv_flags        &= ~IFF_XMIT_DST_RELEASE;
         dev->netdev_ops         = &macvlan_netdev_ops;
         dev->destructor         = free_netdev;
         dev->header_ops         = &macvlan_hard_header_ops,
diff --git a/drivers/net/wan/hdlc_fr.c b/drivers/net/wan/hdlc_fr.c
index 800530101093..bfa0161a02d3 100644
--- a/drivers/net/wan/hdlc_fr.c
+++ b/drivers/net/wan/hdlc_fr.c
@@ -1054,6 +1054,7 @@ static void pvc_setup(struct net_device *dev)
         dev->flags = IFF_POINTOPOINT;
         dev->hard_header_len = 10;
         dev->addr_len = 2;
+        dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
 }
 
 static const struct net_device_ops pvc_ops = {
diff --git a/include/linux/if.h b/include/linux/if.h
index 1108f3e099e3..b9a6229f3be7 100644
--- a/include/linux/if.h
+++ b/include/linux/if.h
@@ -67,6 +67,9 @@
 #define IFF_ISATAP      0x80            /* ISATAP interface (RFC4214)   */
 #define IFF_MASTER_ARPMON 0x100         /* bonding master, ARP mon in use */
 #define IFF_WAN_HDLC    0x200           /* WAN HDLC device              */
+#define IFF_XMIT_DST_RELEASE 0x400      /* dev_hard_start_xmit() is allowed to
+                                         * release skb->dst
+                                         */
 
 #define IF_GET_IFACE    0x0001          /* for querying only */
 #define IF_GET_PROTO    0x0002
diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index 8faacee68633..ff7572ac5481 100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c
@@ -739,6 +739,7 @@ void vlan_setup(struct net_device *dev)
         ether_setup(dev);
 
         dev->priv_flags         |= IFF_802_1Q_VLAN;
+        dev->priv_flags         &= ~IFF_XMIT_DST_RELEASE;
         dev->tx_queue_len       = 0;
 
         dev->netdev_ops         = &vlan_netdev_ops;
diff --git a/net/core/dev.c b/net/core/dev.c
index 6d3630d16271..92ebeca29901 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1688,6 +1688,14 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
                                 goto gso;
                 }
 
+                /*
+                 * If device doesnt need skb->dst, release it right now while
+                 * its hot in this cpu cache
+                 */
+                if ((dev->priv_flags & IFF_XMIT_DST_RELEASE) && skb->dst) {
+                        dst_release(skb->dst);
+                        skb->dst = NULL;
+                }
                 rc = ops->ndo_start_xmit(skb, dev);
                 /*
                  * TODO: if skb_orphan() was called by
@@ -5045,6 +5053,7 @@ struct net_device *alloc_netdev_mq(int sizeof_priv, const char *name,
         netdev_init_queues(dev);
 
         INIT_LIST_HEAD(&dev->napi_list);
+        dev->priv_flags = IFF_XMIT_DST_RELEASE;
         setup(dev);
         strcpy(dev->name, name);
         return dev;