BTW,before the version 3.5 kernel, the source code contains the logic. 2.6.32, for example, in arp_bind_neighbour function, there are the following logic:
__be32 nexthop = ((struct rtable *) DST) - > rt_gateway; if (dev - > flags & (IFF_LOOPBACK | IFF_POINTOPOINT)) nexthop = 0; n = __neigh_lookup_errno ( ... zhao ya said, at 2/27/2016 12:40 PM: > From: Zhao Ya <marywangran0...@gmail.com> > Date: Sat, 27 Feb 2016 10:06:44 +0800 > Subject: [PATCH] IPIP tunnel performance improvement > > bypass the logic of each packet's own neighbour creation when using > pointopint or loopback device. > > Recently, in our tests, met a performance problem. > In a large number of packets with different target IP address through > ipip tunnel, PPS will decrease sharply. > > The output of perf top are as follows, __write_lock_failed is of the first: > - 5.89% [kernel] [k] __write_lock_failed > -__write_lock_failed a > -_raw_write_lock_bh a > -__neigh_create a > -ip_finish_output a > -ip_output a > -ip_local_out a > > The neighbour subsystem will create a neighbour object for each target > when using pointopint device. When massive amounts of packets with diff- > erent target IP address to be xmit through a pointopint device, these > packets will suffer the bottleneck at write_lock_bh(&tbl->lock) after > creating the neighbour object and then inserting it into a hash-table > at the same time. > > This patch correct it. Only one or little amounts of neighbour objects > will be created when massive amounts of packets with different target IP > address through ipip tunnel. > > As the result, performance will be improved. > > > Signed-off-by: Zhao Ya <marywangran0...@gmail.com> > Signed-off-by: Zhaoya <gaiusz...@tencent.com> > --- > net/ipv4/ip_output.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c > index 64878ef..d7c0594 100644 > --- a/net/ipv4/ip_output.c > +++ b/net/ipv4/ip_output.c > @@ -202,6 +202,8 @@ static int ip_finish_output2(struct net *net, struct sock > *sk, struct sk_buff *s > > rcu_read_lock_bh(); > nexthop = (__force u32) rt_nexthop(rt, ip_hdr(skb)->daddr); > + if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT)) > + nexthop = 0; > neigh = __ipv4_neigh_lookup_noref(dev, nexthop); > if (unlikely(!neigh)) > neigh = __neigh_create(&arp_tbl, &nexthop, dev, false); > >
