Re: [PATCH net-next v3 1/3] ipv6: Select best matching nexthop object in fib6_table_lookup()

[David Ahern <dsahern@kernel.org>]

On 6/11/26 9:46 AM, Ido Schimmel wrote:
> Currently, when using multipath routes without nexthop objects,
> fib6_table_lookup() selects the nexthop with the highest score. This
> means that when both a source address and an oif are specified, the
> nexthop that is chosen is the one that matches in terms of oif:
> 
>  # sysctl -wq net.ipv6.conf.all.forwarding=1
>  # ip address add 2001:db8:2::1/64 dev lo
>  # ip route add 2001:db8:10::/64 nexthop via fe80::1 dev dummy1 nexthop via fe80::2 dev dummy2
> 
>  # perf record -e fib6:fib6_table_lookup -- bash -c "for i in {1..100}; do ip route get 2001:db8:10::${i} from 2001:db8:2::1 oif dummy1; done > /dev/null"
>  # perf script | grep -o dummy[0-9] | sort | uniq -c
>      100 dummy1
>  # perf record -e fib6:fib6_table_lookup -- bash -c "for i in {1..100}; do ip route get 2001:db8:10::${i} from 2001:db8:2::1 oif dummy2; done > /dev/null"
>  # perf script | grep -o dummy[0-9] | sort | uniq -c
>      100 dummy2
> 
> When using nexthop objects, fib6_table_lookup() selects the first
> matching nexthop and not necessarily the one with the highest score:
> 
>  # ip nexthop add id 1 via fe80::1 dev dummy1
>  # ip nexthop add id 2 via fe80::2 dev dummy2
>  # ip nexthop add id 3 group 1/2
>  # ip route add 2001:db8:20::/64 nhid 3
> 
>  # perf record -e fib6:fib6_table_lookup -- bash -c "for i in {1..100}; do ip route get 2001:db8:20::${i} from 2001:db8:2::1 oif dummy1; done > /dev/null"
>  # perf script | grep -o dummy[0-9] | sort | uniq -c
>      100 dummy1
>  # perf record -e fib6:fib6_table_lookup -- bash -c "for i in {1..100}; do ip route get 2001:db8:20::${i} from 2001:db8:2::1 oif dummy2; done > /dev/null"
>  # perf script | grep -o dummy[0-9] | sort | uniq -c
>      100 dummy1
> 
> This is not very significant right now because the nexthop is later
> overwritten during path selection in fib6_select_path(). However, the
> next patch is going to skip path selection when we have an oif match
> during output route lookup.
> 
> As a preparation for this change, align the nexthop object behavior with
> the legacy one and make sure that fib6_table_lookup() always selects the
> best matching nexthop. Do that by always returning 0 from
> rt6_nh_find_match() in order not to terminate the loop in
> nexthop_for_each_fib6_nh() and storing in arg->nh the best matching
> nexthop so far.
> 
> Behavior after the change:
> 
>  # perf record -e fib6:fib6_table_lookup -- bash -c "for i in {1..100}; do ip route get 2001:db8:20::${i} from 2001:db8:2::1 oif dummy1; done > /dev/null"
>  # perf script | grep -o dummy[0-9] | sort | uniq -c
>      100 dummy1
>  # perf record -e fib6:fib6_table_lookup -- bash -c "for i in {1..100}; do ip route get 2001:db8:20::${i} from 2001:db8:2::1 oif dummy2; done > /dev/null"
>  # perf script | grep -o dummy[0-9] | sort | uniq -c
>      100 dummy2
> 
> Signed-off-by: Ido Schimmel <idosch@nvidia.com>
> ---
>  net/ipv6/route.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>