Merge branch 'ipv6-avoid-taking-refcnt-on-dst-during-route-lookup'

Merge branch 'ipv6-avoid-taking-refcnt-on-dst-during-route-lookup'

Wei Wang says:

====================
ipv6: avoid taking refcnt on dst during route lookup

Ipv6 route lookup code always grabs refcnt on the dst for the caller.
But for certain cases, grabbing refcnt is not always necessary if the
call path is rcu protected and the caller does not cache the dst.
Another issue in the route lookup logic is:
When there are multiple custom rules, we have to do the lookup into
each table associated to each rule individually. And when we can't
find the route in one table, we grab and release refcnt on
net->ipv6.ip6_null_entry before going to the next table.
This operation is completely redundant, and causes false issue because
net->ipv6.ip6_null_entry is a shared object.

This patch set introduces a new flag RT6_LOOKUP_F_DST_NOREF for route
lookup callers to set, to avoid any manipulation on the dst refcnt. And
it converts the major input and output path to use it.

The performance gain is noticable.
I ran synflood tests between 2 hosts under the same switch. Both hosts
have 20G mlx NIC, and 8 tx/rx queues.
Sender sends pure SYN flood with random src IPs and ports using trafgen.
Receiver has a simple TCP listener on the target port.
Both hosts have multiple custom rules:
- For incoming packets, only local table is traversed.
- For outgoing packets, 3 tables are traversed to find the route.
The packet processing rate on the receiver is as follows:
- Before the fix: 3.78Mpps
- After the fix:  5.50Mpps

v2->v3:
- Handled fib6_rule_lookup() when CONFIG_IPV6_MULTIPLE_TABLES is not
  configured in patch 03 (suggested by David Ahern)
- Removed the renaming of l3mdev_link_scope_lookup() in patch 05
  (suggested by David Ahern)
- Moved definition of ip6_route_output_flags() from an inline function
  in /net/ipv6/route.c to net/ipv6/route.c in order to address kbuild
  error in patch 05

v1->v2:
- Added a helper ip6_rt_put_flags() in patch 3 suggested by David Miller
====================

Reviewed-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>