Subject: Re: [PATCH 3/4 v2 net-next] net: make GRO aware of skb->head_frag

From: Alexander Duyck <[email-masked]>
Date: Tue, 01 May 2012 12:45:45 -0700

On 05/01/2012 10:04 AM, Eric Dumazet wrote:
> On Tue, 2012-05-01 at 09:17 -0700, Alexander Duyck wrote:
>> On 04/30/2012 11:39 PM, Eric Dumazet wrote:
>>> On Mon, 2012-04-30 at 22:33 -0700, Alexander Duyck wrote:
>>>
>>>> The question I had was more specific to GRO.  As long as we have
>>>> skb->users == 1 and the skb isn't cloned we should be fine.   It just
>>>> hadn't occurred to me before that napi_gro_receive had the extra
>>>> requirement that the skb couldn't be cloned.
>>>>
>>> OK
>>>
>>> By the way, even if skb was cloned, we would be allowed to steal
>>> skb->head.
>>>
>>> When we clone an oskb we :
>>>
>>> 1) allocate a struct nskb sk_buff (or use the shadow in case of TCP)
>>> 2) increment dataref
>> The problem I have is with this piece right here.  So you increment
>> dataref.  Now you have an skb that is still pointing to the shared info
>> on this page and dataref is 2.  What about the side that is stealing the
>> head?  Is it going to be tracking the dataref as well and decrementing
>> it before put_page or does it just assume that dataref is 1 and call
>> put_page directly?  I am guessing the latter since I didn't see anything
>> that allowed for tracking the dataref of stolen heads.
> The only changed thing is the kfree() replaced by put_page()
>
> This kfree() was done when last reference to dataref was released.
>
> If we had a problem before, we have same problem after my patch.
>
> Truth is : In TCP (coalesce and splice()) and GRO, we owns skbs.
>
> (See the various __kfree_skb(skb) calls in net/ipv4/tcp_input.c
> There is one exception in ipv6 / treq->pktopts ) but its for SYN packet
> and this wont be merged with a previous packet.
I wasn't worried about the kfree vs put_page, I was worried about the
coalesce case.  However, it looks like you are correct and I am not
seeing any issues so everything seems to be working fine.

I have a hacked together ixgbe up and running now with the new build_skb
logic and RSC/LRO disabled.  It looks like it is giving me a 5%
performance boost for small packet routing, but I am using more CPU for
netperf TCP receive tests and I was wondering if you had seen anything
similar on the tg3 driver?

Thanks,

Alex