Subject: Re: [net PATCH v5 6/6] virtio_net: XDP support for adjust_head

From: John Fastabend <[email-masked]>
Date: Mon, 23 Jan 2017 14:12:47 -0800

On 17-01-23 12:09 PM, Michael S. Tsirkin wrote:
> On Mon, Jan 23, 2017 at 09:22:36PM +0200, Michael S. Tsirkin wrote:
>> On Tue, Jan 17, 2017 at 02:22:59PM -0800, John Fastabend wrote:
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 62dbf4b..3b129b4 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -41,6 +41,9 @@
>>>  #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
>>>  #define GOOD_COPY_LEN	128
>>>  
>>> +/* Amount of XDP headroom to prepend to packets for use by xdp_adjust_head */
>>> +#define VIRTIO_XDP_HEADROOM 256
>>> +
>>>  /* RX packet size EWMA. The average packet size is used to determine the packet
>>>   * buffer size when refilling RX rings. As the entire RX ring may be refilled
>>>   * at once, the weight is chosen so that the EWMA will be insensitive to short-
>>
>> I wonder where does this number come from?  This is quite a lot and
>> means that using XDP_PASS will slow down any sockets on top of it.
>> Which in turn means people will try to remove XDP when not in use,
>> causing resets.  E.g. build_skb (which I have a patch to switch to) uses
>> a much more reasonable NET_SKB_PAD.

I just used the value Alexei (or someone?) came up with. I think it needs to be
large enough to avoid copy in header encap cases. So minimum

  VXLAN_HDR + OUTER_UDP + OUTER_IPV6_HDR + OUTER_MAC =
     8      +     8     +        40      +      14   =  70

The choice of VXLAN hdr was sort of arbitrary but seems good for estimates. For
what its worth there is also a ndo_set_rx_headroom could we use that to set it
and choose a reasonable default.

>>
>> -- 
>> MST
> 
> 
> Let me show you a patch that I've been cooking.  What is missing there
> is handling corner cases like e.g.  when ring size is ~4 entries so
> using smaller buffers might mean we no longer have enough space to store
> a full packet.  So it looks like I have to maintain the skb copy path
> for this hardware.
> 
> With this patch, standard configuration has NET_SKB_PAD + NET_IP_ALIGN
> bytes head padding. Would this be enough for XDP? If yes we do not
> need the resets.

Based on above seems a bit small (L1_CACHE_BYTES + 2)? How tricky would it
be to add support for ndo_set_rx_headroom.

> Thoughts?

I'll take a look at the patch this afternoon. Thanks.

> 
> --->
> 
> 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.
> 
> Lightly tested, in particular, I didn't yet test what this
> actually does to performance - sending this out for early
> feedback/flames.
> 
> 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, need to fix.
> 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));
>