netdev - Re: [PATCH net-next 3/3] net/mlx4_en: RX, Add a prefetch command for small L1_CACHE

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Date:   Wed, 26 Aug 2020 17:09:09 +0000
From:   Saeed Mahameed <saeedm@...dia.com>
To:     "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "tariqt@...lanox.com" <tariqt@...lanox.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "moshe@...lanox.com" <moshe@...lanox.com>
Subject: Re: [PATCH net-next 3/3] net/mlx4_en: RX, Add a prefetch command for
 small L1_CACHE_BYTES

On Wed, 2020-08-26 at 07:55 -0700, Eric Dumazet wrote:
> 
> On 8/26/20 5:54 AM, Tariq Toukan wrote:
> > A single cacheline might not contain the packet header for
> > small L1_CACHE_BYTES values.
> > Use net_prefetch() as it issues an additional prefetch
> > in this case.
> > 
> > Signed-off-by: Tariq Toukan <tariqt@...lanox.com>
> > Reviewed-by: Saeed Mahameed <saeedm@...lanox.com>
> > ---
> >  drivers/net/ethernet/mellanox/mlx4/en_rx.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > index b50c567ef508..99d7737e8ad6 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > @@ -705,7 +705,7 @@ int mlx4_en_process_rx_cq(struct net_device
> > *dev, struct mlx4_en_cq *cq, int bud
> >  
> >  		frags = ring->rx_info + (index << priv->log_rx_info);
> >  		va = page_address(frags[0].page) +
> > frags[0].page_offset;
> > -		prefetchw(va);
> > +		net_prefetchw(va);
> >  		/*
> >  		 * make sure we read the CQE after we read the
> > ownership bit
> >  		 */
> > 
> 
> Why these cache lines would be written next ? Presumably we read the
> headers (pulled into skb->head)
> 

XDP

> Really using prefetch() for the about to be read packet is too late
> anyway for current cpus.
>