linux/drivers/net/wan
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
..
lmc
.gitignore
c101.c
cosa.c
cosa.h
cycx_drv.c
cycx_main.c
cycx_x25.c
dlci.c
dscc4.c
farsync.c
farsync.h
hd64570.c
hd64570.h
hd64572.c
hd64572.h
hdlc.c
hdlc_cisco.c
hdlc_fr.c
hdlc_ppp.c
hdlc_raw.c
hdlc_raw_eth.c
hdlc_x25.c
hostess_sv11.c
ixp4xx_hss.c
Kconfig
lapbether.c
Makefile
n2.c
pc300-falc-lh.h
pc300.h
pc300_drv.c
pc300_tty.c
pc300too.c
pci200syn.c
sbni.c
sbni.h
sdla.c
sealevel.c
wanxl.c
wanxl.h
wanxlfw.inc_shipped
wanxlfw.S
x25_asy.c
x25_asy.h
z85230.c
z85230.h