netdev - Re: gro vs vlan in myri10ge

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Date:	Thu, 08 Nov 2012 18:41:10 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrew Gallatin <gallatin@...i.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: gro vs vlan in myri10ge

On Thu, 2012-11-08 at 21:20 -0500, Andrew Gallatin wrote:
> Hi,
> 
> I've wanted to convert myri10ge from LRO to GRO for quite a while.
> The problem I'm facing is that the NIC cannot perform hardware vlan
> tag offload, so GRO performance is far below LRO performance when
> receiving vlan tagged TCP traffic.
> 
> If a vlan tagged frame is passed to lro_receive_frags(), inet_lro will
> look at the encapsulated IPv4 frame and TCP aggregation will succeed.
> However, it appears that GRO will not do this.  When I patch the
> driver to use GRO, and configure a vlan interface, I see high CPU
> utilization and poor bandwidth when I'm receiving a netperf TCP stream
> on the vlan interface.  If I use LRO in an unpatched driver, then I
> see good receive performance in the same scenario.
> 
> What is the best way to "fix" this?
> 
> Unless I'm just using GRO wrong, it seems that the simplest thing for
> me to do is to claim NETIF_F_HW_VLAN_RX, but pop the tags in the
> driver so as to allow myri10ge to pass up a non-encapsulated frame the
> same way that (nearly?) every other 10GbE NIC does.  I've got a quick
> and dirty patch that confirms doing the vtag pop in the driver gives
> me roughly the same performance with GRO as I used to have with LRO.
> 
> Is this (popping vlan tags in the driver) acceptable, or is it
> too much of a layering violation?

Given GRO assumes NIC does hardware vlan offloading, I guess
I would chose to do that.

It seems unfortunate to add vlan decap in GRO path, already very
complex.



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