netdev - Re: [PATCH net-next 2/3] net/mlx4: MLX4_TX_BOUNCE_BUFFER_SIZE depends on MAX_SKB

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Date:   Wed, 7 Dec 2022 14:40:18 +0200
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     Tariq Toukan <tariqt@...dia.com>, Wei Wang <weiwan@...gle.com>,
        netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next 2/3] net/mlx4: MLX4_TX_BOUNCE_BUFFER_SIZE depends
 on MAX_SKB_FRAGS



On 12/6/2022 7:50 AM, Eric Dumazet wrote:
> Google production kernel has increased MAX_SKB_FRAGS to 45
> for BIG-TCP rollout.
> 
> Unfortunately mlx4 TX bounce buffer is not big enough whenever
> an skb has up to 45 page fragments.
> 
> This can happen often with TCP TX zero copy, as one frag usually
> holds 4096 bytes of payload (order-0 page).
> 
> Tested:
>   Kernel built with MAX_SKB_FRAGS=45
>   ip link set dev eth0 gso_max_size 185000
>   netperf -t TCP_SENDFILE
> 
> I made sure that "ethtool -G eth0 tx 64" was properly working,
> ring->full_size being set to 16.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Wei Wang <weiwan@...gle.com>
> Cc: Tariq Toukan <tariqt@...dia.com>
> ---
>   drivers/net/ethernet/mellanox/mlx4/mlx4_en.h | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> index 7cc288db2a64f75ffe64882e3c25b90715e68855..120b8c361e91d443f83f100a1afabcabc776a92a 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> +++ b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> @@ -89,8 +89,18 @@
>   #define MLX4_EN_FILTER_HASH_SHIFT 4
>   #define MLX4_EN_FILTER_EXPIRY_QUOTA 60
>   
> -/* Typical TSO descriptor with 16 gather entries is 352 bytes... */
> -#define MLX4_TX_BOUNCE_BUFFER_SIZE 512
> +#define CTRL_SIZE	sizeof(struct mlx4_wqe_ctrl_seg)
> +#define DS_SIZE		sizeof(struct mlx4_wqe_data_seg)
> +
> +/* Maximal size of the bounce buffer:
> + * 256 bytes for LSO headers.
> + * CTRL_SIZE for control desc.
> + * DS_SIZE if skb->head contains some payload.
> + * MAX_SKB_FRAGS frags.
> + */
> +#define MLX4_TX_BOUNCE_BUFFER_SIZE (256 + CTRL_SIZE + DS_SIZE +		\
> +				    MAX_SKB_FRAGS * DS_SIZE)
> +
>   #define MLX4_MAX_DESC_TXBBS	   (MLX4_TX_BOUNCE_BUFFER_SIZE / TXBB_SIZE)
>  

Now as MLX4_TX_BOUNCE_BUFFER_SIZE might not be a multiple of TXBB_SIZE, 
simple integer division won't work to calculate the max num of TXBBs.
Roundup is needed.

>   /*
> @@ -217,9 +227,7 @@ struct mlx4_en_tx_info {
>   
>   
>   #define MLX4_EN_BIT_DESC_OWN	0x80000000
> -#define CTRL_SIZE	sizeof(struct mlx4_wqe_ctrl_seg)
>   #define MLX4_EN_MEMTYPE_PAD	0x100
> -#define DS_SIZE		sizeof(struct mlx4_wqe_data_seg)
>   
>   
>   struct mlx4_en_tx_desc {