linux-kernel - Re: [PATCH net-next] net/mlx5e: Reuse per-RQ XDP buffer to avoid stack zeroing overhead

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Message-ID: <CAADnVQLSMvk3uuzTCjqQKXs6hbZH9-_XeYo2Uvu2uHAiYrnkog@mail.gmail.com>
Date: Wed, 14 May 2025 17:26:22 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tariq Toukan <tariqt@...dia.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, Saeed Mahameed <saeedm@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, 
	Network Development <netdev@...r.kernel.org>, linux-rdma@...r.kernel.org, 
	LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Moshe Shemesh <moshe@...dia.com>, Mark Bloch <mbloch@...dia.com>, Gal Pressman <gal@...dia.com>, 
	Carolina Jubran <cjubran@...dia.com>
Subject: Re: [PATCH net-next] net/mlx5e: Reuse per-RQ XDP buffer to avoid
 stack zeroing overhead

On Wed, May 14, 2025 at 1:04 PM Tariq Toukan <tariqt@...dia.com> wrote:
>
> From: Carolina Jubran <cjubran@...dia.com>
>
> CONFIG_INIT_STACK_ALL_ZERO introduces a performance cost by
> zero-initializing all stack variables on function entry. The mlx5 XDP
> RX path previously allocated a struct mlx5e_xdp_buff on the stack per
> received CQE, resulting in measurable performance degradation under
> this config.
>
> This patch reuses a mlx5e_xdp_buff stored in the mlx5e_rq struct,
> avoiding per-CQE stack allocations and repeated zeroing.
>
> With this change, XDP_DROP and XDP_TX performance matches that of
> kernels built without CONFIG_INIT_STACK_ALL_ZERO.
>
> Performance was measured on a ConnectX-6Dx using a single RX channel
> (1 CPU at 100% usage) at ~50 Mpps. The baseline results were taken from
> net-next-6.15.
>
> Stack zeroing disabled:
> - XDP_DROP:
>     * baseline:                     31.47 Mpps
>     * baseline + per-RQ allocation: 32.31 Mpps (+2.68%)
>
> - XDP_TX:
>     * baseline:                     12.41 Mpps
>     * baseline + per-RQ allocation: 12.95 Mpps (+4.30%)

Looks good, but where are these gains coming from ?
The patch just moves mxbuf from stack to rq.
The number of operations should really be the same.

> Stack zeroing enabled:
> - XDP_DROP:
>     * baseline:                     24.32 Mpps
>     * baseline + per-RQ allocation: 32.27 Mpps (+32.7%)

This part makes sense.