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

[Tariq Toukan]

On 15/05/2025 3:26, Alexei Starovoitov wrote:
> On Wed, May 14, 2025 at 1:04 PM Tariq Toukan <tariqt@xxxxxxxxxx> wrote:
> > From: Carolina Jubran <cjubran@xxxxxxxxxx>
> >
> > 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.

I guess it's cache related. Hot/cold areas, alignments, movement of 
other fields in the mlx5e_rq structure...

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