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

[Amery Hung <ameryhung@gmail.com>]

On 8/28/25 8:36 PM, Christoph Paasch via B4 Relay wrote:
> From: Christoph Paasch <cpaasch@openai.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@openai.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()


>   		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)) {
>