netdev - Re: [RFC] gro: Is it ok to share a single napi from several devs ?

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Message-Id: <20100830.095012.233695092.davem@davemloft.net>
Date:	Mon, 30 Aug 2010 09:50:12 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	jarkao2@...il.com, eric.dumazet@...il.com,
	herbert@...dor.apana.org.au, netdev@...r.kernel.org
Subject: Re: [RFC] gro: Is it ok to share a single napi from several devs ?

From: Stephen Hemminger <shemminger@...tta.com>
Date: Mon, 30 Aug 2010 08:57:21 -0700

> On Mon, 30 Aug 2010 06:42:31 +0000
> Jarek Poplawski <jarkao2@...il.com> wrote:
> 
>> On 2010-08-29 20:39, Eric Dumazet wrote:
>> > Le dimanche 29 aoĂťt 2010 Ă  10:06 -0700, David Miller a ĂŠcrit :
>> >> From: Jarek Poplawski <jarkao2@...il.com>
>> >> Date: Sun, 29 Aug 2010 11:59:51 +0200
>> >>
>> >>> Actually, when GRO compares napi->dev to skb->dev?
>> >>
>> >> Hmmm, I thought the code made a skb->dev comparison with the
>> >> existing SKBs in the list when checking for same-flow matches.
>> >>
>> >> It doesn't, probably based upon the assumption that a NAPI
>> >> instance maps to a unique device, the very topic we're
>> >> discussing right now :-/
>> >>
>> >>
>> > 
>> > It does the check, Stephen added it in the commit I mentioned to start
>> > this thread.
>> > 
>> > With net-next-2.6 this now reads :
>> > 
>> 
>> Since Stephen didn't seem to miss this too much it seems quite obvious
>> to me this check should be removed.
> 
> No. I just don't use that system much, breaking code for
> sake of one comparison is ridiculous.

It's not working to begin with.

I agree with Jarek that the check should be removed.  And GRO is one
of those places that, precisely, even one memory reference removal
can improve performance dramatically.

Herbert spent a lot of time doing micro-optimizations to make GRO
better and better, and the smallest things can turn out to make a huge
difference there.

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