netdev - Re: [PATCH net-next 3/4] vxlan: remove gro

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Date:	Fri, 8 Jul 2016 11:55:09 -0400
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Jesse Gross <jesse@...nel.org>,
	Tom Herbert <tom@...bertland.com>, Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH net-next 3/4] vxlan: remove gro_cell support

On 08.07.2016 11:33, Eric Dumazet wrote:
> On Fri, 2016-07-08 at 11:12 -0400, Hannes Frederic Sowa wrote:
>> Hi Eric,
>>
>> On 07.07.2016 12:13, Eric Dumazet wrote:
>>> On Thu, 2016-07-07 at 17:58 +0200, Paolo Abeni wrote:
>>>> GRO is now handled entirely by the udp_offload layer and  there is no need
>>>> for trying it again at the device level. We can drop gro_cell usage,
>>>> simplifying the driver a bit, while maintaining the same performance for
>>>> TCP and improving slightly for UDP.
>>>> This basically reverts the commit 58ce31cca1ff ("vxlan: GRO support
>>>> at tunnel layer")
>>>
>>> Note that gro_cells provide GRO support after RPS, so this helps when we
>>> must perform TCP checksum computation, if NIC lacks CHECKSUM_COMPLETE
>>>
>>> (Say we receive packets all steered to a single RX queue due to RSS hash
>>> being computed on outer header only)
>>>
>>> Some people disable GRO on the physical device, but enable GRO on the
>>> tunnels.
>>
>> we are currently discussing your feedback and wonder how much it makes
>> sense to support such a scenario?
>>
>> We have part of the inner hash in the outer UDP source port. So even the
>> outer hash does provide enough entropy to get frames of one tunnel on
>> multiple CPUs via hardware hashing - given that you don't care about OoO
>> for UDP (I infer that from the fact that RPS will also reorder UDP
>> frames in case of fragmentation).
>>
>> I wonder why it makes sense to still take single RX queue nics into
>> consideration? We already provide support for multiqueue devices for
>> most VM-related interfaces as well. Can you describe why someone would
>> do such a scenario?
> 
> I was simply pointing out there are some use cases where the ability to
> split incoming traffic on multiple cpus can help, especially with dumb
> NIC.
> 
> Fact that GRO is already handled on the NIC itself is not something that
> is hard coded. GRO can be enabled or disabled.
>
> If you remove GRO support at tunnel, then you remove some flexibility.
>
> For example, when GRO for GRE was added by Jerry Chu, we did not remove
> GRO on GRE devices, because mlx4 NICs for example are unable to compute
> TCP checksum when GRE encapsulation is used. A single CPU can not decap
> at line rate on 40Gbit NIC without RX checksum offloading. An admin can
> choose to use RPS to  split traffic coming on a single RX queue to X
> cpus, and enable GRO after RPS, instead of before.
>
> UDP might be different, if the sender properly adds entropy on outer
> header (which is not something you can do with GRE)

Exactly, thus we are also only touching UDP tunneling protocols at the
moment. Did you nack the removal of gro_cell support from the udp
protocols or are you fine with it, given that we won't take away the
functionality to spread out skb_checksum to mulitple CPUs during GRO for
other protocols and didn't plan to do so?

> You probably could default GRO on tunnels to off, since by default GRO
> would already happen at the physical interface.

Thanks!