linux/drivers/net/appletalk
Eric Dumazet 93f154b594 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>
2009-05-18 22:19:19 -07:00
..
cops.c appltetalk: convert cops device to net_device ops 2009-03-27 00:46:41 -07:00
cops.h
cops_ffdrv.h
cops_ltdrv.h
ipddp.c net: release dst entry in dev_hard_start_xmit() 2009-05-18 22:19:19 -07:00
ipddp.h drivers/net/appletalk: endianness 2007-10-10 16:52:05 -07:00
Kconfig
ltpc.c appletalk: convert LTPC to net_device_ops 2009-03-27 00:46:42 -07:00
ltpc.h
Makefile