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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>
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| 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_cisco.c | ||
| hdlc_fr.c | ||
| hdlc_ppp.c | ||
| hdlc_raw_eth.c | ||
| hdlc_raw.c | ||
| hdlc_x25.c | ||
| hdlc.c | ||
| hostess_sv11.c | ||
| ixp4xx_hss.c | ||
| Kconfig | ||
| lapbether.c | ||
| Makefile | ||
| n2.c | ||
| pc300_drv.c | ||
| pc300_tty.c | ||
| pc300-falc-lh.h | ||
| pc300.h | ||
| 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 | ||