linux-kernel - Re: [PATCH net-next v3 1/3] net/mlx4: Track RX allocation failures in a stat

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Date: Tue, 7 May 2024 17:44:27 +0200
From: Larysa Zaremba <larysa.zaremba@...el.com>
To: Joe Damato <jdamato@...tly.com>
CC: <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
	<tariqt@...dia.com>, <saeedm@...dia.com>, <mkarsten@...terloo.ca>,
	<gal@...dia.com>, <nalramli@...tly.com>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, "open list:MELLANOX MLX4
 core VPI driver" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH net-next v3 1/3] net/mlx4: Track RX allocation failures
 in a stat

On Thu, May 02, 2024 at 09:26:25PM +0000, Joe Damato wrote:
> mlx4_en_alloc_frags currently returns -ENOMEM when mlx4_alloc_page
> fails but does not increment a stat field when this occurs.
> 
> struct mlx4_en_rx_ring has a dropped field which is tabulated in
> mlx4_en_DUMP_ETH_STATS, but never incremented by the driver.
> 
> This change modifies mlx4_en_alloc_frags to increment mlx4_en_rx_ring's
> dropped field for the -ENOMEM case.
> 
> Signed-off-by: Joe Damato <jdamato@...tly.com>
> Tested-by: Martin Karsten <mkarsten@...terloo.ca>
> ---
>  drivers/net/ethernet/mellanox/mlx4/en_rx.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> index 8328df8645d5..573ae10300c7 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> @@ -82,8 +82,10 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
>  
>  	for (i = 0; i < priv->num_frags; i++, frags++) {
>  		if (!frags->page) {
> -			if (mlx4_alloc_page(priv, frags, gfp))
> +			if (mlx4_alloc_page(priv, frags, gfp)) {
> +				ring->dropped++;
>  				return -ENOMEM;
> +			}

Correct me if I'm wrong, but ring->dropped is added to rx_dropped stats in 
mlx4_en_DUMP_ETH_STATS(). You have already established with Jakub that 
allocation error does not mean dropped packet, but the counter contributes to 
dropped packets stats.

Also, I do not think that using a `dropped` counter for something that does not 
neccessarily result in a dropped packet is plain confusing.

>  			ring->rx_alloc_pages++;
>  		}
>  		rx_desc->data[i].addr = cpu_to_be64(frags->dma +
> -- 
> 2.25.1
> 
>