[PATCH net-next V6 2/3] net/mlx5e: Avoid copying payload to the skb's linear part

[Tariq Toukan <tariqt@nvidia.com>]

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 of 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 <cpaasch@openai.com>
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
---
 drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
index 75ccf40a7f17..301b33419207 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
@@ -1976,6 +1976,8 @@ mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *w
 					ALIGN(headlen, sizeof(long)),
 					rq->buff.map_dir);
 
+		headlen = eth_get_headlen(rq->netdev, head_addr, headlen);
+
 		frag_offset += headlen;
 		byte_cnt -= headlen;
 		linear_hr = skb_headroom(skb);
@@ -2012,9 +2014,13 @@ mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *w
 
 	if (prog) {
 		u8 nr_frags_free, old_nr_frags = sinfo->nr_frags;
+		skb_frag_t *frag = &sinfo->frags[0];
 		u8 new_nr_frags;
 		u32 len;
 
+		headlen = eth_get_headlen(rq->netdev, skb_frag_address(frag),
+					  skb_frag_size(frag));
+
 		if (mlx5e_xdp_handle(rq, prog, mxbuf)) {
 			if (__test_and_clear_bit(MLX5E_RQ_FLAG_XDP_XMIT, rq->flags)) {
 				struct mlx5e_frag_page *pfp;
@@ -2060,8 +2066,7 @@ mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *w
 				pagep->frags++;
 			while (++pagep < frag_page);
 
-			headlen = min_t(u16, MLX5E_RX_MAX_HEAD - len,
-					skb->data_len);
+			headlen = min_t(u16, headlen - len, skb->data_len);
 			__pskb_pull_tail(skb, headlen);
 		}
 	} else {
-- 
2.44.0