netdev - Re: [net PATCH v2 2/2] ipv4/GRO: Make GRO conform to RFC 6864

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Date:	Tue, 5 Apr 2016 16:07:49 +0100
From:	Edward Cree <ecree@...arflare.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>,
	Alexander Duyck <alexander.duyck@...il.com>
CC:	Alexander Duyck <aduyck@...antis.com>,
	Tom Herbert <tom@...bertland.com>,
	Jesse Gross <jesse@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [net PATCH v2 2/2] ipv4/GRO: Make GRO conform to RFC 6864

On 05/04/16 05:32, Herbert Xu wrote:
> On Mon, Apr 04, 2016 at 09:26:55PM -0700, Alexander Duyck wrote:
>> The question I would have is what are you really losing with increment
>> from 0 versus fixed 0?  From what I see it is essentially just garbage
>> in/garbage out.
> GRO is meant to be lossless, that is, you should not be able to
> detect its presence from the outside.  If you lose information then
> you're breaking this rule and people will soon start asking for it
> to be disabled in various situations.
>
> I'm not against doing this per se but it should not be part of the
> default configuration.
I'm certainly in favour of this being configurable - indeed IMHO it should
also be possible to configure GRO with the 'looser' semantics of LRO, so
that people who want that can get it without all the horrible "don't confuse
Slow Start" hacks, and so that LRO can go away (AIUI the only reasons it
exists are (a) improved performance from the 'loose' semantics and (b) old
kernels without GRO.  We may not be able to kill (b) but we can certainly
address (a)).

But I don't agree that the default has to be totally lossless; anyone who is
caring about the ID fields in atomic datagrams is breaking the RFCs, and can
be assumed to Know What They're Doing sufficiently to configure this.

On the gripping hand, I feel like GRO+TSO is the wrong model for speeding up
forwarding/routing workloads.  Instead we should be looking into having lists
of SKBs traverse the stack together, splitting the list whenever e.g. the
destination changes.  That seems like it ought to be much more efficient than
rewriting headers twice, once to coalesce a superframe and once to segment it
again - and it also means this worry about GRO being lossless can go away.
But until someone tries implementing skb batches, we won't know for sure if
it works (and I don't have time right now ;)

-Ed