Re: [PATCH net-next v4 3/6] virtio-net: auto-tune mergeable rx buffer size for improved performance

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 16, 2014 at 11:52:27AM -0800, Michael Dalton wrote:
> Commit 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page frag
> allocators") changed the mergeable receive buffer size from PAGE_SIZE to
> MTU-size, introducing a single-stream regression for benchmarks with large
> average packet size. There is no single optimal buffer size for all
> workloads.  For workloads with packet size <= MTU bytes, MTU + virtio-net
> header-sized buffers are preferred as larger buffers reduce the TCP window
> due to SKB truesize. However, single-stream workloads with large average
> packet sizes have higher throughput if larger (e.g., PAGE_SIZE) buffers
> are used.
> 
> This commit auto-tunes the mergeable receiver buffer packet size by
> choosing the packet buffer size based on an EWMA of the recent packet
> sizes for the receive queue. Packet buffer sizes range from MTU_SIZE +
> virtio-net header len to PAGE_SIZE. This improves throughput for
> large packet workloads, as any workload with average packet size >=
> PAGE_SIZE will use PAGE_SIZE buffers.
> 
> These optimizations interact positively with recent commit
> ba275241030c ("virtio-net: coalesce rx frags when possible during rx"),
> which coalesces adjacent RX SKB fragments in virtio_net. The coalescing
> optimizations benefit buffers of any size.
> 
> Benchmarks taken from an average of 5 netperf 30-second TCP_STREAM runs
> between two QEMU VMs on a single physical machine. Each VM has two VCPUs
> with all offloads & vhost enabled. All VMs and vhost threads run in a
> single 4 CPU cgroup cpuset, using cgroups to ensure that other processes
> in the system will not be scheduled on the benchmark CPUs. Trunk includes
> SKB rx frag coalescing.
> 
> net-next w/ virtio_net before 2613af0ed18a (PAGE_SIZE bufs): 14642.85Gb/s
> net-next (MTU-size bufs):  13170.01Gb/s
> net-next + auto-tune: 14555.94Gb/s
> 
> Jason Wang also reported a throughput increase on mlx4 from 22Gb/s
> using MTU-sized buffers to about 26Gb/s using auto-tuning.
> 
> Signed-off-by: Michael Dalton <mwdalton@google.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> v2->v3: Remove per-receive queue metadata ring. Encode packet buffer
>         base address and truesize into an unsigned long by requiring a
>         minimum packet size alignment of 256. Permit attempts to fill
>         an already-full RX ring (reverting the change in v2).
> v1->v2: Add per-receive queue metadata ring to track precise truesize for
>         mergeable receive buffers. Remove all truesize approximation. Never
>         try to fill a full RX ring (required for metadata ring in v2).
> 
>  drivers/net/virtio_net.c | 99 ++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 74 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 36cbf06..3e82311 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -26,6 +26,7 @@
>  #include <linux/if_vlan.h>
>  #include <linux/slab.h>
>  #include <linux/cpu.h>
> +#include <linux/average.h>
>  
>  static int napi_weight = NAPI_POLL_WEIGHT;
>  module_param(napi_weight, int, 0444);
> @@ -36,11 +37,18 @@ module_param(gso, bool, 0444);
>  
>  /* FIXME: MTU in config. */
>  #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> -#define MERGE_BUFFER_LEN (ALIGN(GOOD_PACKET_LEN + \
> -                                sizeof(struct virtio_net_hdr_mrg_rxbuf), \
> -                                L1_CACHE_BYTES))
>  #define GOOD_COPY_LEN	128
>  
> +/* Weight used for the 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-term, transient changes in packet size.
> + */
> +#define RECEIVE_AVG_WEIGHT 64
> +
> +/* Minimum alignment for mergeable packet buffers. */
> +#define MERGEABLE_BUFFER_ALIGN max(L1_CACHE_BYTES, 256)
> +
>  #define VIRTNET_DRIVER_VERSION "1.0.0"
>  
>  struct virtnet_stats {
> @@ -78,6 +86,9 @@ struct receive_queue {
>  	/* Chain pages by the private ptr. */
>  	struct page *pages;
>  
> +	/* Average packet length for mergeable receive buffers. */
> +	struct ewma mrg_avg_pkt_len;
> +
>  	/* Page frag for packet buffer allocation. */
>  	struct page_frag alloc_frag;
>  
> @@ -219,6 +230,23 @@ static void skb_xmit_done(struct virtqueue *vq)
>  	netif_wake_subqueue(vi->dev, vq2txq(vq));
>  }
>  
> +static unsigned int mergeable_ctx_to_buf_truesize(unsigned long mrg_ctx)
> +{
> +	unsigned int truesize = mrg_ctx & (MERGEABLE_BUFFER_ALIGN - 1);
> +	return truesize * MERGEABLE_BUFFER_ALIGN;
> +}
> +
> +static void *mergeable_ctx_to_buf_address(unsigned long mrg_ctx)
> +{
> +	return (void *)(mrg_ctx & -MERGEABLE_BUFFER_ALIGN);
> +
> +}
> +
> +static unsigned long mergeable_buf_to_ctx(void *buf, unsigned int truesize)
> +{
> +	return (unsigned long)buf | (truesize / MERGEABLE_BUFFER_ALIGN);
> +}
> +
>  /* Called from bottom half context */
>  static struct sk_buff *page_to_skb(struct receive_queue *rq,
>  				   struct page *page, unsigned int offset,
> @@ -327,31 +355,33 @@ err:
>  
>  static struct sk_buff *receive_mergeable(struct net_device *dev,
>  					 struct receive_queue *rq,
> -					 void *buf,
> +					 unsigned long ctx,
>  					 unsigned int len)
>  {
> +	void *buf = mergeable_ctx_to_buf_address(ctx);
>  	struct skb_vnet_hdr *hdr = buf;
>  	int num_buf = hdr->mhdr.num_buffers;
>  	struct page *page = virt_to_head_page(buf);
>  	int offset = buf - page_address(page);
> -	unsigned int truesize = max_t(unsigned int, len, MERGE_BUFFER_LEN);
> +	unsigned int truesize = max(len, mergeable_ctx_to_buf_truesize(ctx));
> +
>  	struct sk_buff *head_skb = page_to_skb(rq, page, offset, len, truesize);
>  	struct sk_buff *curr_skb = head_skb;
>  
>  	if (unlikely(!curr_skb))
>  		goto err_skb;
> -
>  	while (--num_buf) {
>  		int num_skb_frags;
>  
> -		buf = virtqueue_get_buf(rq->vq, &len);
> -		if (unlikely(!buf)) {
> +		ctx = (unsigned long)virtqueue_get_buf(rq->vq, &len);
> +		if (unlikely(!ctx)) {
>  			pr_debug("%s: rx error: %d buffers out of %d missing\n",
>  				 dev->name, num_buf, hdr->mhdr.num_buffers);
>  			dev->stats.rx_length_errors++;
>  			goto err_buf;
>  		}
>  
> +		buf = mergeable_ctx_to_buf_address(ctx);
>  		page = virt_to_head_page(buf);
>  		--rq->num;
>  
> @@ -369,7 +399,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  			head_skb->truesize += nskb->truesize;
>  			num_skb_frags = 0;
>  		}
> -		truesize = max_t(unsigned int, len, MERGE_BUFFER_LEN);
> +		truesize = max(len, mergeable_ctx_to_buf_truesize(ctx));
>  		if (curr_skb != head_skb) {
>  			head_skb->data_len += len;
>  			head_skb->len += len;
> @@ -386,19 +416,20 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  		}
>  	}
>  
> +	ewma_add(&rq->mrg_avg_pkt_len, head_skb->len);
>  	return head_skb;
>  
>  err_skb:
>  	put_page(page);
>  	while (--num_buf) {
> -		buf = virtqueue_get_buf(rq->vq, &len);
> -		if (unlikely(!buf)) {
> +		ctx = (unsigned long)virtqueue_get_buf(rq->vq, &len);
> +		if (unlikely(!ctx)) {
>  			pr_debug("%s: rx error: %d buffers missing\n",
>  				 dev->name, num_buf);
>  			dev->stats.rx_length_errors++;
>  			break;
>  		}
> -		page = virt_to_head_page(buf);
> +		page = virt_to_head_page(mergeable_ctx_to_buf_address(ctx));
>  		put_page(page);
>  		--rq->num;
>  	}
> @@ -419,17 +450,20 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
>  	if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
>  		pr_debug("%s: short packet %i\n", dev->name, len);
>  		dev->stats.rx_length_errors++;
> -		if (vi->mergeable_rx_bufs)
> -			put_page(virt_to_head_page(buf));
> -		else if (vi->big_packets)
> +		if (vi->mergeable_rx_bufs) {
> +			unsigned long ctx = (unsigned long)buf;
> +			void *base = mergeable_ctx_to_buf_address(ctx);
> +			put_page(virt_to_head_page(base));
> +		} else if (vi->big_packets) {
>  			give_pages(rq, buf);
> -		else
> +		} else {
>  			dev_kfree_skb(buf);
> +		}
>  		return;
>  	}
>  
>  	if (vi->mergeable_rx_bufs)
> -		skb = receive_mergeable(dev, rq, buf, len);
> +		skb = receive_mergeable(dev, rq, (unsigned long)buf, len);
>  	else if (vi->big_packets)
>  		skb = receive_big(dev, rq, buf, len);
>  	else
> @@ -572,25 +606,36 @@ static int add_recvbuf_big(struct receive_queue *rq, gfp_t gfp)
>  
>  static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
>  {
> +	const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>  	struct page_frag *alloc_frag = &rq->alloc_frag;
>  	char *buf;
> +	unsigned long ctx;
>  	int err;
>  	unsigned int len, hole;
>  
> -	if (unlikely(!skb_page_frag_refill(MERGE_BUFFER_LEN, alloc_frag, gfp)))
> +	len = hdr_len + clamp_t(unsigned int, ewma_read(&rq->mrg_avg_pkt_len),
> +				GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
> +	len = ALIGN(len, MERGEABLE_BUFFER_ALIGN);
> +	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);
>  	get_page(alloc_frag->page);
> -	len = MERGE_BUFFER_LEN;
>  	alloc_frag->offset += len;
>  	hole = alloc_frag->size - alloc_frag->offset;
> -	if (hole < MERGE_BUFFER_LEN) {
> +	if (hole < len) {
> +		/* To avoid internal fragmentation, if there is very likely not
> +		 * enough space for another buffer, add the remaining space to
> +		 * the current buffer. This extra space is not included in
> +		 * the truesize stored in ctx.
> +		 */
>  		len += hole;
>  		alloc_frag->offset += hole;
>  	}
>  
>  	sg_init_one(rq->sg, buf, len);
> -	err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, buf, gfp);
> +	err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, (void *)ctx, gfp);
>  	if (err < 0)
>  		put_page(virt_to_head_page(buf));
>  
> @@ -1394,12 +1439,15 @@ static void free_unused_bufs(struct virtnet_info *vi)
>  		struct virtqueue *vq = vi->rq[i].vq;
>  
>  		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL) {
> -			if (vi->mergeable_rx_bufs)
> -				put_page(virt_to_head_page(buf));
> -			else if (vi->big_packets)
> +			if (vi->mergeable_rx_bufs) {
> +				unsigned long ctx = (unsigned long)buf;
> +				void *base = mergeable_ctx_to_buf_address(ctx);
> +				put_page(virt_to_head_page(base));
> +			} else if (vi->big_packets) {
>  				give_pages(&vi->rq[i], buf);
> -			else
> +			} else {
>  				dev_kfree_skb(buf);
> +			}
>  			--vi->rq[i].num;
>  		}
>  		BUG_ON(vi->rq[i].num != 0);
> @@ -1509,6 +1557,7 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
>  			       napi_weight);
>  
>  		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
> +		ewma_init(&vi->rq[i].mrg_avg_pkt_len, 1, RECEIVE_AVG_WEIGHT);
>  		sg_init_table(vi->sq[i].sg, ARRAY_SIZE(vi->sq[i].sg));
>  	}
>  
> -- 
> 1.8.5.2