On Thu, Sep 4, 2025 at 4:30 PM Amery Hung [off-list ref] wrote:

On Thu, Sep 4, 2025 at 3:57 PM Christoph Paasch via B4 Relay
[off-list ref] wrote:

quoted

From: Christoph Paasch <redacted>

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.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Christoph Paasch <redacted>
---
 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..0ac31c7fb64cd60720d390de45a5b6b453ed0a3f 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(rq->netdev, head_addr, headlen);
+
                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(rq->netdev, mxbuf->xdp.data, headlen);
+

The size of mxbuf->xdp.data is most likely not headlen here.

The driver currently generates a xdp_buff with empty linear data, pass
it to the xdp program and assumes the layout If the xdp program does
not change the layout of the xdp_buff through bpf_xdp_adjust_head() or
bpf_xdp_adjust_tail(). The assumption is not correct and I am working
on a fix. But, if we keep that assumption for now, mxbuf->xdp.data
will not contain any headers or payload. The thing that you try to do
probably should be:

        skb_frag_t *frag = &sinfo->frags[0];

        headlen = eth_get_headlen(rq->netdev, skb_frag_address(frag),
skb_frag_size(frag));

Ok, I think I understand what you mean! Thanks for taking the time to explain!

I will do some tests on my side to make sure I get it right.

As your change goes to net and mine to netnext, I can wait until yours
is in the tree so that there aren't any conflicts that need to be
taken care of.


Christoph

quoted

                __pskb_pull_tail(skb, headlen);
        } else {
                if (xdp_buff_has_frags(&mxbuf->xdp)) {

--
2.50.1