netdev - Re: [PATCH 1/1] mvneta: fix prefetch location

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Date:   Mon, 15 Jun 2020 11:10:14 +0200
From:   Sven Auhagen <sven.auhagen@...eatech.de>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, thomas.petazzoni@...tlin.com,
        lorenzo@...nel.org
Subject: Re: [PATCH 1/1] mvneta: fix prefetch location

On Sun, Jun 14, 2020 at 10:23:43AM +0200, Jesper Dangaard Brouer wrote:
> On Sun, 14 Jun 2020 09:11:28 +0200
> Sven Auhagen <sven.auhagen@...eatech.de> wrote:
> 
> > The packet header prefetch is at an offset
> > now. Correct the prefetch address.
> > 
> > Signed-off-by: Sven Auhagen <sven.auhagen@...eatech.de>
> > ---
> >  drivers/net/ethernet/marvell/mvneta.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> > index 51889770958d..344fc5f649b4 100644
> > --- a/drivers/net/ethernet/marvell/mvneta.c
> > +++ b/drivers/net/ethernet/marvell/mvneta.c
> > @@ -2229,7 +2229,7 @@ mvneta_swbm_rx_frame(struct mvneta_port *pp,
> >  				len, dma_dir);
> >  
> >  	/* Prefetch header */
> > -	prefetch(data);
> > +	prefetch(data + pp->rx_offset_correction + MVNETA_MH_SIZE);
> 
> The comment does say "header", so I guess your change is correct if we
> are talking about the packet header.
> 
> Currently this prefetch will help XDP-redirect, as it store xdp_frame
> in this area.
> 
> >  	xdp->data_hard_start = data;
> >  	xdp->data = data + pp->rx_offset_correction + MVNETA_MH_SIZE;
> 
> If you really need to prefetch the packet headers, it would be the same
> as calling prefetch(xdp->data), right?

Yes, that is correct.

> 
> Have you benchmarked that this prefetch is a benefit?

I just ran some forwarding tests and the difference is not measureable
in normal forwarding.
You are correct XDP Forwarding seems to be 1-2% slower.

I guess you can disregard the patch.

> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: https://eur03.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.linkedin.com%2Fin%2Fbrouer&amp;data=02%7C01%7Csven.auhagen%40voleatech.de%7C4f3ae28c82514407dfd508d8103c4d1b%7Cb82a99f679814a7295344d35298f847b%7C0%7C0%7C637277198467217085&amp;sdata=qIJ3tkU2mHFqZpArOGnQWSPEVtTBMXK8MpekSGcKavw%3D&amp;reserved=0
>