netdev - Re: [net RFC] net/mlx4_en: Use frag stride in crossing page boundary condition

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Date:   Wed, 20 Jun 2018 17:28:00 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Saeed Mahameed <saeedm@...lanox.com>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "kafai@...com" <kafai@...com>, Tariq Toukan <tariqt@...lanox.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>
Subject: Re: [net RFC] net/mlx4_en: Use frag stride in crossing page boundary
 condition



On 06/20/2018 04:41 PM, Saeed Mahameed wrote:
> 
> I see, I wanted to use _stride_ as grantee for how much a page frag can
> grow, for example in mlx5 we need the whole stride to build_skb  around
> the frag, since we always need the trailer, but it is different in here
> and we can avoid resource waste.
> 
> so how a bout this: (As suggested by Martin).
> currently as mlx4_en_complete_rx_desc assumes that priv->rx_headroom
> is always 0 in non-XDP setup, hence:
> 
> frags->page_offset += sz_align;
> 
> where it really should be:
> frags->page_offset += sz_align + priv->rx_headroom;
> 
> we can use it as a hint to not reuse as below:
> what do you think ?
> 
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> index 9f54ccbddea7..f14c7a574cc8 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> @@ -474,10 +474,10 @@ static int mlx4_en_complete_rx_desc(struct
> mlx4_en_priv *priv,
>  {
>         const struct mlx4_en_frag_info *frag_info = priv->frag_info;
>         unsigned int truesize = 0;
> +       bool release = true;
>         int nr, frag_size;
>         struct page *page;
>         dma_addr_t dma;
> -       bool release;
> index 9f54ccbddea7..f14c7a574cc8 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> 
>         /* Collect used fragments while replacing them in the HW
> descriptors */
>         for (nr = 0;; frags++) {
> @@ -500,7 +500,7 @@ static int mlx4_en_complete_rx_desc(struct
> mlx4_en_priv *priv,
>                         release = page_count(page) != 1 ||
>                                   page_is_pfmemalloc(page) ||
>                                   page_to_nid(page) != numa_mem_id();
> -               } else {
> +               } elseif(!priv->rx_headroom) {
>                         u32 sz_align = ALIGN(frag_size,
> SMP_CACHE_BYTES);
> 
>                         frags->page_offset += sz_align;
> 

I guess that would work, please double check priv->rx_headroom wont need another cache line,
thanks !