Re: [RFC net-next] net: veth: reduce page_pool memory footprint using half page per-buffer

Jesper Dangaard Brouer
2023-05-16T16:12:19Z

Cc. Alex Duyck + Eric, please criticize my idea below.

On 12/05/2023 15.08, Lorenzo Bianconi wrote:
> In order to reduce page_pool memory footprint, rely on
> page_pool_dev_alloc_frag routine and reduce buffer size
> (VETH_PAGE_POOL_FRAG_SIZE) to PAGE_SIZE / 2 in order to consume one page
> for two 1500B frames. Reduce VETH_XDP_PACKET_HEADROOM to 192 from 256
> (XDP_PACKET_HEADROOM) to fit max_head_size in VETH_PAGE_POOL_FRAG_SIZE.
> Please note, using default values (CONFIG_MAX_SKB_FRAGS=17), maximum
> supported MTU is now reduced to 36350B.
>
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
> drivers/net/veth.c | 39 +++++++++++++++++++++++++--------------
> 1 file changed, 25 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 614f3e3efab0..0e648703cccf 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -31,9 +31,12 @@
> #define DRV_NAME "veth"
> #define DRV_VERSION "1.0"
>
> -#define VETH_XDP_FLAG BIT(0)
> -#define VETH_RING_SIZE 256
> -#define VETH_XDP_HEADROOM (XDP_PACKET_HEADROOM + NET_IP_ALIGN)
> +#define VETH_XDP_FLAG BIT(0)
> +#define VETH_RING_SIZE 256
> +#define VETH_XDP_PACKET_HEADROOM 192
> +#define VETH_XDP_HEADROOM (VETH_XDP_PACKET_HEADROOM + \
> + NET_IP_ALIGN)
> +#define VETH_PAGE_POOL_FRAG_SIZE 2048
>
> #define VETH_XDP_TX_BULK_SIZE 16
> #define VETH_XDP_BATCH 16
> @@ -736,7 +739,7 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
> if (skb_shared(skb) || skb_head_is_locked(skb) ||
> skb_shinfo(skb)->nr_frags ||
> skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> - u32 size, len, max_head_size, off;
> + u32 size, len, max_head_size, off, pp_off;
> struct sk_buff *nskb;
> struct page *page;
> int i, head_off;
> @@ -747,17 +750,20 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
> *
> * Make sure we have enough space for linear and paged area
> */
> - max_head_size = SKB_WITH_OVERHEAD(PAGE_SIZE -
> + max_head_size = SKB_WITH_OVERHEAD(VETH_PAGE_POOL_FRAG_SIZE -
> VETH_XDP_HEADROOM);
> - if (skb->len > PAGE_SIZE * MAX_SKB_FRAGS + max_head_size)
> + if (skb->len >
> + VETH_PAGE_POOL_FRAG_SIZE * MAX_SKB_FRAGS + max_head_size)
> goto drop;
>
> /* Allocate skb head */
> - page = page_pool_dev_alloc_pages(rq->page_pool);

It seems wasteful to allocate a full page PAGE_SIZE.

> + page = page_pool_dev_alloc_frag(rq->page_pool, &pp_off,
> + VETH_PAGE_POOL_FRAG_SIZE);

Allocating PAGE_SIZE/2 isn't much better.

At this point we already know the skb->len (and skb_headlen).

Why don't we allocated the size that we need?

See page_frag_alloc() system invented by Eric and Duyck.

> if (!page)
> goto drop;
>
> - nskb = napi_build_skb(page_address(page), PAGE_SIZE);
> + nskb = napi_build_skb(page_address(page) + pp_off,
> + VETH_PAGE_POOL_FRAG_SIZE);
> if (!nskb) {
> page_pool_put_full_page(rq->page_pool, page, true);
> goto drop;
> @@ -782,15 +788,18 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
> len = skb->len - off;
>
> for (i = 0; i < MAX_SKB_FRAGS && off < skb->len; i++) {
> - page = page_pool_dev_alloc_pages(rq->page_pool);
> + page = page_pool_dev_alloc_frag(rq->page_pool, &pp_off,
> + VETH_PAGE_POOL_FRAG_SIZE);
> if (!page) {
> consume_skb(nskb);
> goto drop;
> }
>
> - size = min_t(u32, len, PAGE_SIZE);
> - skb_add_rx_frag(nskb, i, page, 0, size, PAGE_SIZE);
> - if (skb_copy_bits(skb, off, page_address(page),
> + size = min_t(u32, len, VETH_PAGE_POOL_FRAG_SIZE);
> + skb_add_rx_frag(nskb, i, page, pp_off, size,
> + VETH_PAGE_POOL_FRAG_SIZE);
> + if (skb_copy_bits(skb, off,
> + page_address(page) + pp_off,
> size)) {
> consume_skb(nskb);
> goto drop; …