Subject: Re: [PATCH net-next] virtio-net: switch to use build_skb() for small buffer

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 22 Feb 2017 05:42:29 +0200

On Wed, Feb 22, 2017 at 11:17:50AM +0800, Jason Wang wrote:
> 
> 
> On 2017年02月22日 11:06, Michael S. Tsirkin wrote:
> > On Wed, Feb 22, 2017 at 10:58:08AM +0800, Jason Wang wrote:
> > > 
> > > On 2017年02月21日 22:37, Michael S. Tsirkin wrote:
> > > > On Tue, Feb 21, 2017 at 04:46:28PM +0800, Jason Wang wrote:
> > > > > This patch switch to use build_skb() for small buffer which can have
> > > > > better performance for both TCP and XDP (since we can work at page
> > > > > before skb creation). It also remove lots of XDP codes since both
> > > > > mergeable and small buffer use page frag during refill now.
> > > > > 
> > > > >                          Before   | After
> > > > > XDP_DROP(xdp1) 64B  :  11.1Mpps | 14.4Mpps
> > > > > 
> > > > > Tested with xdp1/xdp2/xdp_ip_tx_tunnel and netperf.
> > > > > 
> > > > > Signed-off-by: Jason Wang<[email-masked]>
> > > > Thanks!
> > > > I had a similar patch for mergeable too, though it's trickier there
> > > > as host has a lot of flexibility in sizing buffers.
> > > > Looks like a good intermediate step to me.
> > > Yes, I think it's more tricky for the case of mergeable buffer:
> > > 
> > > 1) we need reserve NET_SKB_PAD + NET_IP_ALIGN for each buffer, this will
> > > break rx frag coalescing
> > > 2) need tailroom for skb_shinfo, so it won't work for all size of packet
> > > 
> > > Thanks
> > Have you seen my prototype?
> 
> Not yet, please give me a pointer.
> 
> >   It works with qemu in practice,
> > just needs to cover a bunch of corner cases.
> 
> Sounds good, if you wish I can help to finalize it.
> 
> Thanks
> 
> > 
> > > > 
> > > > Acked-by: Michael S. Tsirkin<[email-masked]>
> > > > 

Great! Here it is I think it's on top of 4.9 or so. Issues to address:
- when ring is very small this might cause us not to
  be able to fit even a single packet in the whole queue.
  Limit logic to when ring is big enough?
- If e.g. header is split across many buffers, this won't work
  correctly. Detect and copy?

Could be more issues that I forgot. It might be a good idea to
first finish my buffer ctx hack. Will simplify the logic.

commit 77c177091a7c8473e3f0ed148afa2b4765b8b0f7
Author: Michael S. Tsirkin <[email-masked]>
Date:   Sun Feb 21 20:22:31 2016 +0200

    virtio_net: switch to build_skb for mrg_rxbuf
    
    For small packets data copy was observed to
    take up about 15% CPU time. Switch to build_skb
    and avoid the copy when using mergeable rx buffers.
    
    As a bonus, medium-size skbs that fit in a page will be
    completely linear.
    
    Of course, we now need to lower the lower bound on packet size,
    to make sure a sane number of skbs fits in rx socket buffer.
    By how much? I don't know yet.
    
    It might also be useful to prefetch the packet buffer since
    net stack will likely use it soon.

    TODO: it appears that Linux won't handle correctly the case of first
    buffer being very small (or consisting exclusively of virtio header).
    This is already the case for current code, so don't bother
    testing yet.
    TODO: might be unfair to the last packet in a fragment as we include
    remaining space if any in its truesize.
    
    Signed-off-by: Michael S. Tsirkin <[email-masked]>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b425fa1..a6b996f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -38,6 +38,8 @@ module_param(gso, bool, 0444);
 
 /* FIXME: MTU in config. */
 #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
+//#define MIN_PACKET_ALLOC GOOD_PACKET_LEN
+#define MIN_PACKET_ALLOC 128
 #define GOOD_COPY_LEN	128
 
 /* RX packet size EWMA. The average packet size is used to determine the packet
@@ -246,6 +248,9 @@ static void *mergeable_ctx_to_buf_address(unsigned long mrg_ctx)
 static unsigned long mergeable_buf_to_ctx(void *buf, unsigned int truesize)
 {
 	unsigned int size = truesize / MERGEABLE_BUFFER_ALIGN;
+
+	BUG_ON((unsigned long)buf & (MERGEABLE_BUFFER_ALIGN - 1));
+	BUG_ON(size - 1 >= MERGEABLE_BUFFER_ALIGN);
 	return (unsigned long)buf | (size - 1);
 }
 
@@ -354,25 +359,54 @@ static struct sk_buff *receive_big(struct net_device *dev,
 	return NULL;
 }
 
+#define VNET_SKB_PAD (NET_SKB_PAD + NET_IP_ALIGN)
+#define VNET_SKB_BUG (VNET_SKB_PAD < sizeof(struct virtio_net_hdr_mrg_rxbuf))
+#define VNET_SKB_LEN(len) ((len) - sizeof(struct virtio_net_hdr_mrg_rxbuf))
+#define VNET_SKB_OFF VNET_SKB_LEN(VNET_SKB_PAD)
+
+static struct sk_buff *vnet_build_skb(struct virtnet_info *vi,
+				      void *buf,
+				      unsigned int len, unsigned int truesize)
+{
+	struct sk_buff *skb = build_skb(buf, truesize);
+
+	if (!skb)
+		return NULL;
+
+	skb_reserve(skb, VNET_SKB_PAD);
+	skb_put(skb, VNET_SKB_LEN(len));
+
+	return skb;
+}
+
 static struct sk_buff *receive_mergeable(struct net_device *dev,
 					 struct virtnet_info *vi,
 					 struct receive_queue *rq,
 					 unsigned long ctx,
-					 unsigned int len)
+					 unsigned int len,
+					 struct virtio_net_hdr_mrg_rxbuf *hdr)
 {
 	void *buf = mergeable_ctx_to_buf_address(ctx);
-	struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
-	u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
+	u16 num_buf;
 	struct page *page = virt_to_head_page(buf);
-	int offset = buf - page_address(page);
-	unsigned int truesize = max(len, mergeable_ctx_to_buf_truesize(ctx));
+	unsigned int truesize = mergeable_ctx_to_buf_truesize(ctx);
+	int offset;
+	struct sk_buff *head_skb;
+	struct sk_buff *curr_skb;
+
+	BUG_ON(len > truesize);
 
-	struct sk_buff *head_skb = page_to_skb(vi, rq, page, offset, len,
-					       truesize);
-	struct sk_buff *curr_skb = head_skb;
+	/* copy header: build_skb will overwrite it */
+	memcpy(hdr, buf + VNET_SKB_OFF, sizeof *hdr);
+
+	head_skb = vnet_build_skb(vi, buf, len, truesize);
+	curr_skb = head_skb;
+
+	num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
 
 	if (unlikely(!curr_skb))
 		goto err_skb;
+
 	while (--num_buf) {
 		int num_skb_frags;
 
@@ -386,7 +420,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			goto err_buf;
 		}
 
-		buf = mergeable_ctx_to_buf_address(ctx);
+		buf = mergeable_ctx_to_buf_address(ctx) + VNET_SKB_OFF;
 		page = virt_to_head_page(buf);
 
 		num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
@@ -403,7 +437,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			head_skb->truesize += nskb->truesize;
 			num_skb_frags = 0;
 		}
-		truesize = max(len, mergeable_ctx_to_buf_truesize(ctx));
+		truesize = mergeable_ctx_to_buf_truesize(ctx);
+		BUG_ON(len > truesize);
 		if (curr_skb != head_skb) {
 			head_skb->data_len += len;
 			head_skb->len += len;
@@ -449,6 +484,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
 	struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
 	struct sk_buff *skb;
 	struct virtio_net_hdr_mrg_rxbuf *hdr;
+	struct virtio_net_hdr_mrg_rxbuf hdr0;
 
 	if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
 		pr_debug("%s: short packet %i\n", dev->name, len);
@@ -465,17 +501,24 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
 		return;
 	}
 
-	if (vi->mergeable_rx_bufs)
-		skb = receive_mergeable(dev, vi, rq, (unsigned long)buf, len);
-	else if (vi->big_packets)
+	if (vi->mergeable_rx_bufs) {
+		skb = receive_mergeable(dev, vi, rq, (unsigned long)buf, len,
+					&hdr0);
+		if (unlikely(!skb))
+			return;
+		hdr = &hdr0;
+	} else if (vi->big_packets) {
 		skb = receive_big(dev, vi, rq, buf, len);
-	else
+		if (unlikely(!skb))
+			return;
+		hdr = skb_vnet_hdr(skb);
+	} else {
 		skb = receive_small(vi, buf, len);
+		if (unlikely(!skb))
+			return;
+		hdr = skb_vnet_hdr(skb);
+	}
 
-	if (unlikely(!skb))
-		return;
-
-	hdr = skb_vnet_hdr(skb);
 
 	u64_stats_update_begin(&stats->rx_syncp);
 	stats->rx_bytes += skb->len;
@@ -581,11 +624,14 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
 
 static unsigned int get_mergeable_buf_len(struct ewma_pkt_len *avg_pkt_len)
 {
-	const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
+	unsigned int hdr;
 	unsigned int len;
 
-	len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
-			GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
+	hdr = ALIGN(VNET_SKB_PAD + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)),
+		    MERGEABLE_BUFFER_ALIGN);
+
+	len = hdr + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
+			    MIN_PACKET_ALLOC, PAGE_SIZE - hdr);
 	return ALIGN(len, MERGEABLE_BUFFER_ALIGN);
 }
 
@@ -601,8 +647,11 @@ static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
 	if (unlikely(!skb_page_frag_refill(len, alloc_frag, gfp)))
 		return -ENOMEM;
 
-	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
-	ctx = mergeable_buf_to_ctx(buf, len);
+	BUILD_BUG_ON(VNET_SKB_BUG);
+
+	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset +
+		VNET_SKB_OFF;
+	//ctx = mergeable_buf_to_ctx(buf - VNET_SKB_OFF, len);
 	get_page(alloc_frag->page);
 	alloc_frag->offset += len;
 	hole = alloc_frag->size - alloc_frag->offset;
@@ -615,8 +664,10 @@ static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
 		len += hole;
 		alloc_frag->offset += hole;
 	}
+	ctx = mergeable_buf_to_ctx(buf - VNET_SKB_OFF, len);
 
-	sg_init_one(rq->sg, buf, len);
+	sg_init_one(rq->sg, buf,
+		    len - VNET_SKB_OFF - SKB_DATA_ALIGN(sizeof(struct skb_shared_info)));
 	err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, (void *)ctx, gfp);
 	if (err < 0)
 		put_page(virt_to_head_page(buf));
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