Re: [PATCH] net: Check skb->rxhash in gro_receive

[David Miller <davem@davemloft.net>]

From: Tom Herbert <therbert@google.com>
Date: Fri, 10 Jan 2014 08:27:20 -0800

> On Thu, Jan 9, 2014 at 9:38 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>> On Thu, 2014-01-09 at 20:54 -0800, Tom Herbert wrote:
>>> When initializing a gro_list for a packet, first check the rxhash of
>>> the incoming skb against that of the skb's in the list. This should be
>>> a very strong inidicator of whether the flow is going to be matched,
>>> and potentially allows a lot of other checks to be short circuited.
>>>
>>
>> Hmm... this idea was discussed in the past. I used it when attempting to
>> use a hash table instead of a gro_list last year.
>>
>> Unfortunately this added lot of cycles when rxhash is not provided by
>> hardware, and my tests found it was not a win.
>>
>> Remember : in most cases, gro_list contains one flow, so this test does
>> nothing special but adds overhead.
> 
> I don't understand what your basis is that gro_list in most cases
> contains one flow

It also doesn't jive well with Eric's recent patch to adjust the GRO
overflow strategy (600adc18eba823f9fd8ed5fec8b04f11dddf3884 ("net:
gro: change GRO overflow strategy"))

:-)

I sort of like Tom's idea to optimistically compare the hash, if we
do in fact have one already.

Eric would the change be OK if Tom did it that way?