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>
This commit is contained in:
Eric Dumazet 2009-05-18 22:19:19 -07:00 committed by David S. Miller
parent 496a60cdcd
commit 93f154b594
10 changed files with 20 additions and 0 deletions

View file

@ -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)

View file

@ -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)

View file

@ -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)

View file

@ -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);
}

View file

@ -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

View file

@ -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,

View file

@ -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 = {

View file

@ -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

View file

@ -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;

View file

@ -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;