netdev - Re: [PATCH net-next v2] net/mlx5e: Modify mlx5e_xdp

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Message-ID: <CAKgT0UdJX3T6UcmtbeYLRCNLtnF_=1Dx3RGwHSc_-Awk+cHwow@mail.gmail.com>
Date: Sun, 2 Nov 2025 16:13:22 -0800
From: Alexander Duyck <alexander.duyck@...il.com>
To: Tariq Toukan <ttoukan.linux@...il.com>
Cc: Zijian Zhang <zijianzhang@...edance.com>, netdev@...r.kernel.org, davem@...emloft.net, 
	kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com, 
	andrew+netdev@...n.ch, saeedm@...dia.com, gal@...dia.com, leonro@...dia.com, 
	witu@...dia.com, parav@...dia.com, tariqt@...dia.com, hkelam@...vell.com, 
	Alexander Lobakin <aleksander.lobakin@...el.com>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	Toke Høiland-Jørgensen <toke@...hat.com>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Jesse Brandeburg <jbrandeburg@...udflare.com>, 
	Salil Mehta <salil.mehta@...wei.com>
Subject: Re: [PATCH net-next v2] net/mlx5e: Modify mlx5e_xdp_xmit sq selection

On Sun, Nov 2, 2025 at 5:02 AM Tariq Toukan <ttoukan.linux@...il.com> wrote:
> On 01/11/2025 1:10, Zijian Zhang wrote:

...

> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> > index 5d51600935a6..6225734b256a 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> > @@ -855,13 +855,10 @@ int mlx5e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
> >       if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
> >               return -EINVAL;
> >
> > -     sq_num = smp_processor_id();
> > -
> > -     if (unlikely(sq_num >= priv->channels.num))
> > -             return -ENXIO;
> > -
> > +     sq_num = smp_processor_id() % priv->channels.num;
>
> Modulo is a costly operation.
> A while loop with subtraction would likely converge faster.

I agree. The modulo is optimizing for the worst exception case, and
heavily penalizing the case where it does nothing. A while loop in
most cases will likely just be a test and short jump which would be
two or three cycles whereas this would cost you somewhere in the 10s
of cycles for most processors as I recall.