Subject: Re: [net-next, PATCH 1/2, v3] net: socionext: different approach on DMA

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 1 Oct 2018 11:26:31 +0200

On Sat, 29 Sep 2018 14:28:01 +0300 Ilias Apalodimas <[email-masked]> wrote:

> +static void *netsec_alloc_rx_data(struct netsec_priv *priv,
> +				  dma_addr_t *dma_handle, u16 *desc_len)
> +{
> +	size_t len = priv->ndev->mtu + ETH_HLEN + 2 * VLAN_HLEN + NET_SKB_PAD +
> +		NET_IP_ALIGN;
> +	dma_addr_t mapping;
> +	void *buf;
> +
> +	len += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +	len = SKB_DATA_ALIGN(len);
> +
> +	buf = napi_alloc_frag(len);

Using napi_alloc_frag here ^^^^

> +	if (!buf)
> +		return NULL;
> +
> +	mapping = dma_map_single(priv->dev, buf, len, DMA_FROM_DEVICE);
> +	if (unlikely(dma_mapping_error(priv->dev, mapping)))
> +		goto err_out;
> +
> +	*dma_handle = mapping;
> +	*desc_len = len;
> +
> +	return buf;
> +
> +err_out:
> +	skb_free_frag(buf);
> +	return NULL;
> +}

Hmmm, you are using napi_alloc_frag() in above code, which behind
your-back allocates order-3 pages (32 Kbytes memory in 8 order-0 pages).

This violates at-least two XDP principals:

#1: You are NOT using order-0 page based allocations for XDP.

Notice, I'm not saying 1-page per packet, as ixgbe + i40e violated
this, and it is now "per-practical-code-example" acceptable to split up
the order-0 page, and store two RX frames per order-0 page (4096 bytes).
(To make this fit you have to reduce XDP_HEADROOM to 192 bytes, which
killed the idea of placing the SKB in this area).

#2: You have allocations on the XDP fast-path.

The REAL secret behind the XDP performance is to avoid allocations on
the fast-path.  While I just told you to use the page-allocator and
order-0 pages, this will actually kill performance.  Thus, to make this
fast, you need a driver local recycle scheme that avoids going through
the page allocator, which makes XDP_DROP and XDP_TX extremely fast.
For the XDP_REDIRECT action (which you seems to be interested in, as
this is needed for AF_XDP), there is a xdp_return_frame() API that can
make this fast.

To avoid every driver inventing their own driver local page-recycle
cache (which many does today), we/I have created the page pool API.
See include/net/page_pool.h, and look at how mlx5 driver uses it
in v4.18 links[1][2][3].  Do notice, that mlx5 ALSO have a driver
recycle scheme on top, which Tariq is working on removing or
generalizing.  AND also that mlx5 does not use the DMA mapping feature
that page_pool also provide yet. (Contact me if you want to use
page_pool for handing DMA mapping, we might need to export
__page_pool_clean_page and call it before XDP_PASS action).

[1] https://github.com/torvalds/linux/blob/v4.18/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c#L226
[2] https://github.com/torvalds/linux/blob/v4.18/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c#L255
[3] https://github.com/torvalds/linux/blob/v4.18/drivers/net/ethernet/mellanox/mlx5/core/en_main.c#L598-L618
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer