Re: [PATCH bpf-next v3 3/3] xsk: build skb by page

From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Date: Fri, 22 Jan 2021 23:36:29 +0800

On Fri, 22 Jan 2021 12:08:00 +0000, Alexander Lobakin [off-list ref] wrote:

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From: Alexander Lobakin <redacted>
Date: Fri, 22 Jan 2021 11:55:35 +0000

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From: Alexander Lobakin <redacted>
Date: Fri, 22 Jan 2021 11:47:45 +0000

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From: Eric Dumazet <redacted>
Date: Thu, 21 Jan 2021 16:41:33 +0100

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On 1/21/21 2:47 PM, Xuan Zhuo wrote:

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This patch is used to construct skb based on page to save memory copy
overhead.

This function is implemented based on IFF_TX_SKB_NO_LINEAR. Only the
network card priv_flags supports IFF_TX_SKB_NO_LINEAR will use page to
directly construct skb. If this feature is not supported, it is still
necessary to copy data to construct skb.

---------------- Performance Testing ------------

The test environment is Aliyun ECS server.
Test cmd:

xdpsock -i eth0 -t  -S -s <msg size>

Test result data:

size    64      512     1024    1500
copy    1916747 1775988 1600203 1440054
page    1974058 1953655 1945463 1904478
percent 3.0%    10.0%   21.58%  32.3%

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
---
 net/xdp/xsk.c | 104 ++++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 86 insertions(+), 18 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 4a83117..38af7f1 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c

@@ -430,6 +430,87 @@ static void xsk_destruct_skb(struct sk_buff *skb)
 	sock_wfree(skb);
 }

+static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
+					      struct xdp_desc *desc)
+{
+	u32 len, offset, copy, copied;
+	struct sk_buff *skb;
+	struct page *page;
+	void *buffer;
+	int err, i;
+	u64 addr;
+
+	skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);

Also,
maybe we should allocate it with NET_SKB_PAD so NIC drivers could
use some reserved space?

		skb = sock_alloc_send_skb(&xs->sk, NET_SKB_PAD, 1, &err);
		...
		skb_reserve(skb, NET_SKB_PAD);

Eric, what do you think?

I think you are right. Some space should be added to continuous equipment. This
space should also be added in the copy mode below. Is LL_RESERVED_SPACE more
appropriate?

No. If you look at __netdev_alloc_skb() and __napi_alloc_skb(), they
reserve NET_SKB_PAD at the beginning of linear area. Documentation of
__build_skb() also says that driver should reserve NET_SKB_PAD before
the actual frame, so it is a standartized hardware-independent
headroom.
Leaving that space in skb->head will allow developers to implement
IFF_TX_SKB_NO_LINEAR in a wider variety of drivers, especially when
a driver has to prepend some sort of data before the actual frame.
Since it's usually of a size of one cacheline, shouldn't be a big
deal.


[ I also had an idea of allocating an skb with a headroom of
NET_SKB_PAD + 256 bytes, so nearly all drivers could just call
pskb_pull_tail() to support such type of skbuffs without much
effort, but I think that it's better to teach drivers to support
xmitting of really headless ones. If virtio_net can do it, why
shouldn't the others ]

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+	if (unlikely(!skb))
+		return ERR_PTR(err);
+
+	addr = desc->addr;
+	len = desc->len;
+
+	buffer = xsk_buff_raw_get_data(xs->pool, addr);
+	offset = offset_in_page(buffer);
+	addr = buffer - xs->pool->addrs;
+
+	for (copied = 0, i = 0; copied < len; i++) {
+		page = xs->pool->umem->pgs[addr >> PAGE_SHIFT];
+
+		get_page(page);
+
+		copy = min_t(u32, PAGE_SIZE - offset, len - copied);
+
+		skb_fill_page_desc(skb, i, page, offset, copy);
+
+		copied += copy;
+		addr += copy;
+		offset = 0;
+	}
+
+	skb->len += len;
+	skb->data_len += len;

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+	skb->truesize += len;

This is not the truesize, unfortunately.

We need to account for the number of pages, not number of bytes.

The easiest solution is:

	skb->truesize += PAGE_SIZE * i;

i would be equal to skb_shinfo(skb)->nr_frags after exiting the loop.

Oops, pls ignore this. I forgot that XSK buffers are not
"one per page".
We need to count the number of pages manually and then do

	skb->truesize += PAGE_SIZE * npages;

Right.

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+
+	refcount_add(len, &xs->sk.sk_wmem_alloc);
+
+	return skb;
+}
+

Al

Thanks,
Al

Al