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Message-ID: <befdca60-f9e5-486d-8df4-eafe4f338d79@gmail.com>
Date: Mon, 14 Jul 2025 10:29:13 +0300
From: Tariq Toukan <ttoukan.linux@...il.com>
To: cpaasch@...nai.com, Saeed Mahameed <saeedm@...dia.com>,
Leon Romanovsky <leon@...nel.org>, Tariq Toukan <tariqt@...dia.com>,
Mark Bloch <mbloch@...dia.com>, 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>
Cc: linux-rdma@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] net/mlx5: Avoid copying payload to the skb's
linear part
On 14/07/2025 2:33, 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. The
> goal here is to err on the side of caution and prefer to copy too little
> instead of copying too much (because once it has been copied over, we
> trigger the above described behavior in skb_gro_receive).
>
> So, we can do a rough estimate of the header-space by looking at
> cqe_l3/l4_hdr_type and kind of do a lower-bound estimate. This is now
> done in mlx5e_cqe_get_min_hdr_len(). We always assume that TCP timestamps
> are present, as that's the most common use-case.
>
> That header-len is then used in mlx5e_skb_from_cqe_mpwrq_nonlinear for
> the headlen (which defines what is being copied over). 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
>
Nice improvement.
Did you test impact on other archs?
Did you test impact on non-LRO flows?
Specifically:
a. Large MTU, tcp stream.
b. Large MTU, small UDP packets.
> Signed-off-by: Christoph Paasch <cpaasch@...nai.com>
> ---
> drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 33 ++++++++++++++++++++++++-
> 1 file changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index 2bb32082bfccdc85d26987f792eb8c1047e44dd0..2de669707623882058e3e77f82d74893e5d6fefe 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -1986,13 +1986,40 @@ mlx5e_shampo_fill_skb_data(struct sk_buff *skb, struct mlx5e_rq *rq,
> } while (data_bcnt);
> }
>
> +static u16
> +mlx5e_cqe_get_min_hdr_len(const struct mlx5_cqe64 *cqe)
> +{
> + u16 min_hdr_len = sizeof(struct ethhdr);
> + u8 l3_type = get_cqe_l3_hdr_type(cqe);
> + u8 l4_type = get_cqe_l4_hdr_type(cqe);
> +
> + if (cqe_has_vlan(cqe))
> + min_hdr_len += VLAN_HLEN;
> +
> + if (l3_type == CQE_L3_HDR_TYPE_IPV4)
> + min_hdr_len += sizeof(struct iphdr);
> + else if (l3_type == CQE_L3_HDR_TYPE_IPV6)
> + min_hdr_len += sizeof(struct ipv6hdr);
> +
> + if (l4_type == CQE_L4_HDR_TYPE_UDP)
> + min_hdr_len += sizeof(struct udphdr);
> + else if (l4_type & (CQE_L4_HDR_TYPE_TCP_NO_ACK |
> + CQE_L4_HDR_TYPE_TCP_ACK_NO_DATA |
> + CQE_L4_HDR_TYPE_TCP_ACK_AND_DATA))
> + /* Previous condition works because we know that
> + * l4_type != 0x2 (CQE_L4_HDR_TYPE_UDP)
> + */
> + min_hdr_len += sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
> +
> + return min_hdr_len;
> +}
> +
> static struct sk_buff *
> mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *wi,
> struct mlx5_cqe64 *cqe, u16 cqe_bcnt, u32 head_offset,
> u32 page_idx)
BTW, this function handles IPoIB as well, not only Eth.
> {
> struct mlx5e_frag_page *frag_page = &wi->alloc_units.frag_pages[page_idx];
> - u16 headlen = min_t(u16, MLX5E_RX_MAX_HEAD, cqe_bcnt);
> struct mlx5e_frag_page *head_page = frag_page;
> struct mlx5e_xdp_buff *mxbuf = &rq->mxbuf;
> u32 frag_offset = head_offset;
> @@ -2004,10 +2031,14 @@ mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *w
> u32 linear_frame_sz;
> u16 linear_data_len;
> u16 linear_hr;
> + u16 headlen;
> void *va;
>
> prog = rcu_dereference(rq->xdp_prog);
>
> + headlen = min3(mlx5e_cqe_get_min_hdr_len(cqe), cqe_bcnt,
> + (u16)MLX5E_RX_MAX_HEAD);
> +
> if (prog) {
> /* area for bpf_xdp_[store|load]_bytes */
> net_prefetchw(netmem_address(frag_page->netmem) + frag_offset);
>
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