netdev - Re: [PATCH net-next] net/mlx4_en: avoid one cache line miss to ring doorbell

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Date:   Thu, 30 Sep 2021 17:10:52 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Tariq Toukan <tariqt@...dia.com>
Subject: Re: [PATCH net-next] net/mlx4_en: avoid one cache line miss to ring
 doorbell

On Thu, 30 Sep 2021 12:40:31 -0700 Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> This patch caches doorbell address directly in struct mlx4_en_tx_ring.
> 
> This removes the need to bring in cpu caches whole struct mlx4_uar
> in fast path.
> 
> Note that mlx4_uar is not guaranteed to be on a local node,
> because mlx4_bf_alloc() uses a single free list (priv->bf_list)
> regardless of its node parameter.
> 
> This kind of change does matter in presence of light/moderate traffic.
> In high stress, this read-only line would be kept hot in caches.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Tariq Toukan <tariqt@...dia.com>

>  	/* Following part should be mostly read */
> +	void			*doorbell_address;

We'll need to make sparse happy before applying:

drivers/net/ethernet/mellanox/mlx4/en_tx.c:133:32: warning: incorrect type in assignment (different address spaces)
drivers/net/ethernet/mellanox/mlx4/en_tx.c:133:32:    expected void *doorbell_address
drivers/net/ethernet/mellanox/mlx4/en_tx.c:133:32:    got void [noderef] __iomem *
drivers/net/ethernet/mellanox/mlx4/en_tx.c:757:56: warning: incorrect type in argument 2 (different address spaces)
drivers/net/ethernet/mellanox/mlx4/en_tx.c:757:56:    expected void [noderef] __iomem *
drivers/net/ethernet/mellanox/mlx4/en_tx.c:757:56:    got void *doorbell_address