From 93f154b594fe47e4a7e5358b309add449a046cd3 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 18 May 2009 22:19:19 -0700 Subject: 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 Signed-off-by: David S. Miller --- include/linux/if.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'include/linux/if.h') diff --git a/include/linux/if.h b/include/linux/if.h index 1108f3e099e..b9a6229f3be 100644 --- a/include/linux/if.h +++ b/include/linux/if.h @@ -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 -- cgit v1.2.3-70-g09d2