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authorEric Dumazet <dada1@cosmosbay.com>2009-05-19 07:19:19 +0200
committerDavid S. Miller <davem@davemloft.net>2009-05-19 07:19:19 +0200
commit93f154b594fe47e4a7e5358b309add449a046cd3 (patch)
tree479d9f2db25922a2312547c97e73d5a11b68bb1c /samples/Makefile
parentnet: FIX bonding sysfs rtnl_lock deadlock (diff)
downloadlinux-93f154b594fe47e4a7e5358b309add449a046cd3.tar.xz
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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>
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