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Message-ID: <98c8c7d7-4b1a-474f-86b6-884d79ea4e41@intel.com>
Date: Mon, 14 Jul 2025 16:20:14 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: <cpaasch@...nai.com>
CC: 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>, <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
From: Christoph Paasch Via B4 Relay <devnull+cpaasch.openai.com@...nel.org>
Date: Sun, 13 Jul 2025 16:33:07 -0700
> 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
How did you end up with ->head_frag not set? IIRC mlx5 uses
napi_build_skb(), which explicitly sets ->head_frag to true.
It should be false only for kmalloced linear parts.
> 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
>
> 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;
Can't Q-in-Q be here?
> +
> + 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);
You don't account extensions and stuff here.
> +
> + 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)
> {
> 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);
For your usecase, have you tried setting headlen to just ETH_HLEN here?
Fast GRO should still work for this case, then VLAN/IP/L4 layers will
do a couple memcpy()s to pull their headers, but even on 32-bit MIPS
this was faster than let's say eth_get_headlen() (which involves Flow
Dissector) or open-coded header length assumptions as above.
(the above was correct for 2020 when I last time played with router
drivers, but I hope nothing's been broken since then)
> +
> if (prog) {
> /* area for bpf_xdp_[store|load]_bytes */
> net_prefetchw(netmem_address(frag_page->netmem) + frag_offset);
Thanks,
Olek
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