netdev - Re: [PATCH 0/5] net: Checksum offload changes

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Date:	Mon, 23 Jun 2014 09:29:35 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH 0/5] net: Checksum offload changes - Part V

On Mon, Jun 23, 2014 at 9:28 AM, Tom Herbert <therbert@...gle.com> wrote:
>>
>> > - Added GRO checksum validation functions
>> > - Call the GRO validations functions from TCP and GRE gro_receive
>> > - Perform checksum verification in the UDP gro_receive path using
>> >   GRO functions and add support for gro_receive in UDP6
>>  > What is in
>> What kind of speed regression / code bloat comes from all these
>> changes ?
>>
> Some cases should improve performance (like GRE w/csum), some cases will
> regress (like now that we're actually verifying UDP checksums instead of
> ignoring them). As for "code bloat", I think you can determine the extent of
> that by reading the code, but unifying all these separate ad hoc instances
> of checksum handling into one common set of functions still seems like a
> good thing to me :-).
>
>>
>> I see a lot of added complexity / code in GRO layer, but no performance
>> numbers...
>>
> These patches won't affect the simple case of GRO (plain TCP), but may have
> impact in presence of encapsulations. I'm happy to provide some performance
> numbers, but there are many possible combinations of encapsulation,
> configuration, hardware offload support-- are there any specific scenarios
> you'd like to see?
>
>>
>> __skb_gro_checksum_complete() for example calls skb_checksum() twice,
>> and we know this is expensive, particularly for IPv4 case where we know
>> the header checksum has to be 0.
>>
> GRE already does this and this ensures that we never compute the full packet
> checksum twice (one of the primary goals of my checksum patches). I believe
> this would worse than the incremental cost of making checksum complete.
>
> The alternative we could consider, is to never do the full checksum in GRO.
> It looks like there's some weak attempt in this direction (e.g. GRO can only
> be enabled with csum offload enabled, UDP encap enforcing complete or
> unnecessary). I suspect there will still be cases where the benefits of
> aggregation outweigh the costs of doing the the csum in GRO.
>
>> GRO performance is already not very good, lot of people just disable GRO
>> because of it being very slow. I'd like to not give them more
>> incentives...
>>
>> Without proper RFS support from the NIC, all encapsulated traffic tend
>> to land on a single RX queue, so performance of GRO layer is critical.
>
>
> Why would encapsulated traffic be any different than other traffic in this
> regard? RSS is still applicable and the point of all the work to define foo
> over UDP is leverage things like RSS and ECMP.
>
> Tom
>
>
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