netdev - Re: [PATCH net-next v4 2/2] net/mlx5: Avoid copying payload to the skb's linear part

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Message-ID: <CADg4-L_83eNn9huME6tuZKeQWyG2xkKCUj9erqzMBGxWt=NKcA@mail.gmail.com>
Date: Wed, 3 Sep 2025 16:57:21 -0700
From: Christoph Paasch <cpaasch@...nai.com>
To: Amery Hung <ameryhung@...il.com>
Cc: Gal Pressman <gal@...dia.com>, Dragos Tatulea <dtatulea@...dia.com>, 
	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>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, Stanislav Fomichev <sdf@...ichev.me>, netdev@...r.kernel.org, 
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net-next v4 2/2] net/mlx5: Avoid copying payload to the
 skb's linear part

On Wed, Sep 3, 2025 at 4:39 PM Amery Hung <ameryhung@...il.com> wrote:
>
>
>
> On 8/28/25 8:36 PM, Christoph Paasch via B4 Relay wrote:
> > From: Christoph Paasch <cpaasch@...nai.com>
> >
> > mlx5e_skb_from_cqe_mpwrq_nonlinear() copies MLX5E_RX_MAX_HEAD (256)
> > bytes from the page-pool to the skb's linear part. Those 256 bytes
> > include part of the payload.
> >
> > When attempting to do GRO in skb_gro_receive, if headlen > data_offset
> > (and skb->head_frag is not set), we end up aggregating packets in the
> > frag_list.
> >
> > This is of course not good when we are CPU-limited. Also causes a worse
> > skb->len/truesize ratio,...
> >
> > So, let's avoid copying parts of the payload to the linear part. We use
> > eth_get_headlen() to parse the headers and compute the length of the
> > protocol headers, which will be used to copy the relevant bits ot the
> > skb's linear part.
> >
> > We still allocate MLX5E_RX_MAX_HEAD for the skb so that if the networking
> > stack needs to call pskb_may_pull() later on, we don't need to reallocate
> > memory.
> >
> > This gives a nice throughput increase (ARM Neoverse-V2 with CX-7 NIC and
> > LRO enabled):
> >
> > BEFORE:
> > =======
> > (netserver pinned to core receiving interrupts)
> > $ netperf -H 10.221.81.118 -T 80,9 -P 0 -l 60 -- -m 256K -M 256K
> >   87380  16384 262144    60.01    32547.82
> >
> > (netserver pinned to adjacent core receiving interrupts)
> > $ netperf -H 10.221.81.118 -T 80,10 -P 0 -l 60 -- -m 256K -M 256K
> >   87380  16384 262144    60.00    52531.67
> >
> > AFTER:
> > ======
> > (netserver pinned to core receiving interrupts)
> > $ netperf -H 10.221.81.118 -T 80,9 -P 0 -l 60 -- -m 256K -M 256K
> >   87380  16384 262144    60.00    52896.06
> >
> > (netserver pinned to adjacent core receiving interrupts)
> >   $ netperf -H 10.221.81.118 -T 80,10 -P 0 -l 60 -- -m 256K -M 256K
> >   87380  16384 262144    60.00    85094.90
> >
> > Additional tests across a larger range of parameters w/ and w/o LRO, w/
> > and w/o IPv6-encapsulation, different MTUs (1500, 4096, 9000), different
> > TCP read/write-sizes as well as UDP benchmarks, all have shown equal or
> > better performance with this patch.
> >
> > Signed-off-by: Christoph Paasch <cpaasch@...nai.com>
> > ---
> >   drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > index 8bedbda522808cbabc8e62ae91a8c25d66725ebb..792bb647ba28668ad7789c328456e3609440455d 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > @@ -2047,6 +2047,8 @@ mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *w
> >               dma_sync_single_for_cpu(rq->pdev, addr + head_offset, headlen,
> >                                       rq->buff.map_dir);
> >
> > +             headlen = eth_get_headlen(skb->dev, head_addr, headlen);
> > +
>
> Hi,
>
> I am building on top of this patchset and got a kernel crash. It was
> triggered by attaching an xdp program.
>
> I think the problem is skb->dev is still NULL here. It will be set later by:
> mlx5e_complete_rx_cqe() -> mlx5e_build_rx_skb() -> eth_type_trans()

Hmmm... Not sure what happened here...
I'm almost certain I tested with xdp as well...

I will try again later/tomorrow.

Thanks!
Christoph

>
>
> >               frag_offset += headlen;
> >               byte_cnt -= headlen;
> >               linear_hr = skb_headroom(skb);
> > @@ -2123,6 +2125,9 @@ mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *w
> >                               pagep->frags++;
> >                       while (++pagep < frag_page);
> >               }
> > +
> > +             headlen = eth_get_headlen(skb->dev, mxbuf->xdp.data, headlen);
> > +
> >               __pskb_pull_tail(skb, headlen);
> >       } else {
> >               if (xdp_buff_has_frags(&mxbuf->xdp)) {
> >
>