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 07:55:26 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Tariq Toukan <tariqt@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, Moshe Shemesh <moshe@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next 3/3] net/mlx4_en: RX, Add a prefetch command for
 small L1_CACHE_BYTES



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)

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