netdev - Re: [PATCH net v2] net/mlx5: Correctly set gso

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Message-ID: <CADg4-L8B0i0Nz0tPyQX3qX4TWJ1rKD7KXvo2qHQKo_yFP-=1Fw@mail.gmail.com>
Date: Tue, 29 Jul 2025 09:08:12 -0700
From: Christoph Paasch <cpaasch@...nai.com>
To: Gal Pressman <gal@...dia.com>
Cc: Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>, Mark Bloch <mbloch@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	linux-rdma@...r.kernel.org
Subject: Re: [PATCH net v2] net/mlx5: Correctly set gso_size when LRO is used

On Tue, Jul 29, 2025 at 7:03 AM Gal Pressman <gal@...dia.com> wrote:
>
> On 29/07/2025 4:01, Christoph Paasch wrote:
> > The below fixes it. The problem is that because gso_segs is not set,
> > NAPI_GRO_CB()->count is 0 when processing the packets.
> > So, if we have a non-LRO'ed packet followed by an LRO'ed packet being
> > processed, the first one will have NAPI_GRO_CB()->count set to 1 the
> > next one to 0 (in dev_gro_receive()).
>
> I was also suspecting something in this area, but LRO packets shouldn't
> really aggregate with other GRO skbs as they use different checksum
> offloads (UNNECESSARY vs. COMPLETE), so tcp_gro_receive() should flush GRO.

AFAICS, that's only for when NAPI_GRO_CB(p)->is_flist is set (meaning
for forwarded traffic).
skb_gro_checksum_validate() will make sure the checksum is ok when
entering tcp4_gro_receive().


Christoph

> > This means we end up in gro_complete() with count == 1 and thus don't
> > call inet_gro_complete().
> >
> > I'm still unclear why this only fails much later then when checking
> > the checksum, but I'm sure the below diff fixes it (and also gets rid
> > of all packet-loss, so throughput goes up)
> >
> > Will submit a proper patch tomorrow.
>
> Thank you for the quick fix!