netdev - Re: [PATCH net-next v3 2/2] ipv4: use dst hint for ipv4 list receive

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Date:   Tue, 19 Nov 2019 10:45:00 -0700
From:   David Ahern <dsahern@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Edward Cree <ecree@...arflare.com>
Subject: Re: [PATCH net-next v3 2/2] ipv4: use dst hint for ipv4 list receive

On 11/19/19 10:33 AM, Paolo Abeni wrote:
> On Tue, 2019-11-19 at 17:20 +0100, Paolo Abeni wrote:
>> On Tue, 2019-11-19 at 09:00 -0700, David Ahern wrote:
>>> On 11/19/19 7:38 AM, Paolo Abeni wrote:
>>>> @@ -535,11 +550,20 @@ static void ip_sublist_rcv_finish(struct list_head *head)
>>>>  	}
>>>>  }
>>>>  
>>>> +static struct sk_buff *ip_extract_route_hint(struct net *net,
>>>> +					     struct sk_buff *skb, int rt_type)
>>>> +{
>>>> +	if (fib4_has_custom_rules(net) || rt_type != RTN_LOCAL)
>>>
>>> Why the local only limitation for v4 but not v6? Really, why limit this
>>> to LOCAL at all? 
>>
>> The goal here was to simplify as much as possible the ipv4
>> ip_route_use_hint() helper, as its complexity raised some eyebrown.
>>
>> Yes, hints can be used also for forwarding. I'm unsure how much will
>> help, given the daddr contraint. If there is agreement I can re-add it.
> 
> Sorry, I forgot to ask: would you be ok enabling the route hint for
> !RTN_BROADCAST, as in the previous iteration? Covering RTN_BROADCAST
> will add quite a bit of complexity to ip_route_use_hint(), likely with
> no relevant use-case.
> 

It is a trade-off of too many checks which just add overhead to the
packets that can not benefit from re-use. I was trying to understand why
local delivery was given preference.