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

On 2023/5/17 6:52, Lorenzo Bianconi wrote:

quoted

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On Mon, May 15, 2023 at 01:24:20PM +0200, Lorenzo Bianconi wrote:

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quoted

On 2023/5/12 21:08, Lorenzo Bianconi wrote:

quoted

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

Is there any performance improvement beside the memory saving? As it
should reduce TLB miss, I wonder if the TLB miss reducing can even
out the cost of the extra frag reference count handling for the
frag support?

reducing the requested headroom to 192 (from 256) we have a nice improvement in
the 1500B frame case while it is mostly the same in the case of paged skb
(e.g. MTU 8000B).

Can you define 'nice improvement' ? ;)
Show us numbers or improvement in %.

I am testing this RFC patch in the scenario reported below:

iperf tcp tx --> veth0 --> veth1 (xdp_pass) --> iperf tcp rx

- 6.4.0-rc1 net-next:
  MTU 1500B: ~ 7.07 Gbps
  MTU 8000B: ~ 14.7 Gbps

- 6.4.0-rc1 net-next + page_pool frag support in veth:
  MTU 1500B: ~ 8.57 Gbps
  MTU 8000B: ~ 14.5 Gbps

Thanks for sharing the data.
Maybe using the new frag interface introduced in [1] bring
back the performance for the MTU 8000B case.

1. https://patchwork.kernel.org/project/netdevbpf/cover/20230516124801.2465-1-linyunsheng@huawei.com/


I drafted a patch for veth to use the new frag interface, maybe that
will show how veth can make use of it. Would you give it a try to see
if there is any performance improvment for MTU 8000B case? Thanks.

--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c

@@ -737,8 +737,8 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
            skb_shinfo(skb)->nr_frags ||
            skb_headroom(skb) < XDP_PACKET_HEADROOM) {
                u32 size, len, max_head_size, off;
+               struct page_pool_frag *pp_frag;
                struct sk_buff *nskb;
-               struct page *page;
                int i, head_off;

                /* We need a private copy of the skb and data buffers since

@@ -752,14 +752,20 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
                if (skb->len > PAGE_SIZE * MAX_SKB_FRAGS + max_head_size)
                        goto drop;

+               size = min_t(u32, skb->len, max_head_size);
+               size += VETH_XDP_HEADROOM;
+               size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+
                /* Allocate skb head */
-               page = page_pool_dev_alloc_pages(rq->page_pool);
-               if (!page)
+               pp_frag = page_pool_dev_alloc_frag(rq->page_pool, size);
+               if (!pp_frag)
                        goto drop;

-               nskb = napi_build_skb(page_address(page), PAGE_SIZE);
+               nskb = napi_build_skb(page_address(pp_frag->page) + pp_frag->offset,
+                                     pp_frag->truesize);
                if (!nskb) {
-                       page_pool_put_full_page(rq->page_pool, page, true);
+                       page_pool_put_full_page(rq->page_pool, pp_frag->page,
+                                               true);
                        goto drop;
                }

@@ -782,16 +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);
-                       if (!page) {
+                       size = min_t(u32, len, PAGE_SIZE);
+
+                       pp_frag = page_pool_dev_alloc_frag(rq->page_pool, size);
+                       if (!pp_frag) {
                                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)) {
+                       skb_add_rx_frag(nskb, i, pp_frag->page, pp_frag->offset,
+                                       size, pp_frag->truesize);
+                       if (skb_copy_bits(skb, off, page_address(pp_frag->page) +
+                                         pp_frag->offset, size)) {
                                consume_skb(nskb);
                                goto drop;
                        }

@@ -1047,6 +1055,8 @@ static int veth_create_page_pool(struct veth_rq *rq)
                return err;
        }

+       page_pool_set_max_frag_size(rq->page_pool, PAGE_SIZE / 2);
+
        return 0;
 }