Message-ID: <1688984310.480753-2-xuanzhuo@linux.alibaba.com>
Date: Mon, 10 Jul 2023 18:18:30 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: virtualization@...ts.linux-foundation.org,
 Jason Wang <jasowang@...hat.com>,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 netdev@...r.kernel.org,
 bpf@...r.kernel.org,
 Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH vhost v11 10/10] virtio_net: merge dma operation for one page

On Mon, 10 Jul 2023 05:40:21 -0400, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Mon, Jul 10, 2023 at 11:42:37AM +0800, Xuan Zhuo wrote:
> > Currently, the virtio core will perform a dma operation for each
> > operation. Although, the same page may be operated multiple times.
> >
> > The driver does the dma operation and manages the dma address based the
> > feature premapped of virtio core.
> >
> > This way, we can perform only one dma operation for the same page. In
> > the case of mtu 1500, this can reduce a lot of dma operations.
> >
> > Tested on Aliyun g7.4large machine, in the case of a cpu 100%, pps
> > increased from 1893766 to 1901105. An increase of 0.4%.
>
> what kind of dma was there? an IOMMU? which vendors? in which mode
> of operation?


Do you mean this:

[    0.470816] iommu: Default domain type: Passthrough


>
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>
> This kind of difference is likely in the noise.

It's really not high, but this is because the proportion of DMA under perf top
is not high. Probably that much.

>
>
> > ---
> >  drivers/net/virtio_net.c | 283 ++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 267 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 486b5849033d..4de845d35bed 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -126,6 +126,27 @@ static const struct virtnet_stat_desc virtnet_rq_stats_desc[] = {
> >  #define VIRTNET_SQ_STATS_LEN	ARRAY_SIZE(virtnet_sq_stats_desc)
> >  #define VIRTNET_RQ_STATS_LEN	ARRAY_SIZE(virtnet_rq_stats_desc)
> >
> > +/* The bufs on the same page may share this struct. */
> > +struct virtnet_rq_dma {
> > +	struct virtnet_rq_dma *next;
> > +
> > +	dma_addr_t addr;
> > +
> > +	void *buf;
> > +	u32 len;
> > +
> > +	u32 ref;
> > +};
> > +
> > +/* Record the dma and buf. */
>
> I guess I see that. But why?
> And these two comments are the extent of the available
> documentation, that's not enough I feel.
>
>
> > +struct virtnet_rq_data {
> > +	struct virtnet_rq_data *next;
>
> Is manually reimplementing a linked list the best
> we can do?

Yes, we can use llist.

>
> > +
> > +	void *buf;
> > +
> > +	struct virtnet_rq_dma *dma;
> > +};
> > +
> >  /* Internal representation of a send virtqueue */
> >  struct send_queue {
> >  	/* Virtqueue associated with this send _queue */
> > @@ -175,6 +196,13 @@ struct receive_queue {
> >  	char name[16];
> >
> >  	struct xdp_rxq_info xdp_rxq;
> > +
> > +	struct virtnet_rq_data *data_array;
> > +	struct virtnet_rq_data *data_free;
> > +
> > +	struct virtnet_rq_dma *dma_array;
> > +	struct virtnet_rq_dma *dma_free;
> > +	struct virtnet_rq_dma *last_dma;
> >  };
> >
> >  /* This structure can contain rss message with maximum settings for indirection table and keysize
> > @@ -549,6 +577,176 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> >  	return skb;
> >  }
> >
> > +static void virtnet_rq_unmap(struct receive_queue *rq, struct virtnet_rq_dma *dma)
> > +{
> > +	struct device *dev;
> > +
> > +	--dma->ref;
> > +
> > +	if (dma->ref)
> > +		return;
> > +
>
> If you don't unmap there is no guarantee valid data will be
> there in the buffer.
>
> > +	dev = virtqueue_dma_dev(rq->vq);
> > +
> > +	dma_unmap_page(dev, dma->addr, dma->len, DMA_FROM_DEVICE);
>
>
>
>
>
> > +
> > +	dma->next = rq->dma_free;
> > +	rq->dma_free = dma;
> > +}
> > +
> > +static void *virtnet_rq_recycle_data(struct receive_queue *rq,
> > +				     struct virtnet_rq_data *data)
> > +{
> > +	void *buf;
> > +
> > +	buf = data->buf;
> > +
> > +	data->next = rq->data_free;
> > +	rq->data_free = data;
> > +
> > +	return buf;
> > +}
> > +
> > +static struct virtnet_rq_data *virtnet_rq_get_data(struct receive_queue *rq,
> > +						   void *buf,
> > +						   struct virtnet_rq_dma *dma)
> > +{
> > +	struct virtnet_rq_data *data;
> > +
> > +	data = rq->data_free;
> > +	rq->data_free = data->next;
> > +
> > +	data->buf = buf;
> > +	data->dma = dma;
> > +
> > +	return data;
> > +}
> > +
> > +static void *virtnet_rq_get_buf(struct receive_queue *rq, u32 *len, void **ctx)
> > +{
> > +	struct virtnet_rq_data *data;
> > +	void *buf;
> > +
> > +	buf = virtqueue_get_buf_ctx(rq->vq, len, ctx);
> > +	if (!buf || !rq->data_array)
> > +		return buf;
> > +
> > +	data = buf;
> > +
> > +	virtnet_rq_unmap(rq, data->dma);
> > +
> > +	return virtnet_rq_recycle_data(rq, data);
> > +}
> > +
> > +static void *virtnet_rq_detach_unused_buf(struct receive_queue *rq)
> > +{
> > +	struct virtnet_rq_data *data;
> > +	void *buf;
> > +
> > +	buf = virtqueue_detach_unused_buf(rq->vq);
> > +	if (!buf || !rq->data_array)
> > +		return buf;
> > +
> > +	data = buf;
> > +
> > +	virtnet_rq_unmap(rq, data->dma);
> > +
> > +	return virtnet_rq_recycle_data(rq, data);
> > +}
> > +
> > +static int virtnet_rq_map_sg(struct receive_queue *rq, void *buf, u32 len)
> > +{
> > +	struct virtnet_rq_dma *dma = rq->last_dma;
> > +	struct device *dev;
> > +	u32 off, map_len;
> > +	dma_addr_t addr;
> > +	void *end;
> > +
> > +	if (likely(dma) && buf >= dma->buf && (buf + len <= dma->buf + dma->len)) {
> > +		++dma->ref;
> > +		addr = dma->addr + (buf - dma->buf);
> > +		goto ok;
> > +	}
>
> So this is the meat of the proposed optimization. I guess that
> if the last buffer we allocated happens to be in the same page
> as this one then they can both be mapped for DMA together.

Since we use page_frag, the buffers we allocated are all continuous.

> Why last one specifically? Whether next one happens to
> be close depends on luck. If you want to try optimizing this
> the right thing to do is likely by using a page pool.
> There's actually work upstream on page pool, look it up.

As we discussed in another thread, the page pool is first used for xdp. Let's
transform it step by step.

Thanks.

>
> > +
> > +	end = buf + len - 1;
> > +	off = offset_in_page(end);
> > +	map_len = len + PAGE_SIZE - off;
> > +
> > +	dev = virtqueue_dma_dev(rq->vq);
> > +
> > +	addr = dma_map_page_attrs(dev, virt_to_page(buf), offset_in_page(buf),
> > +				  map_len, DMA_FROM_DEVICE, 0);
> > +	if (addr == DMA_MAPPING_ERROR)
> > +		return -ENOMEM;
> > +
> > +	dma = rq->dma_free;
> > +	rq->dma_free = dma->next;
> > +
> > +	dma->ref = 1;
> > +	dma->buf = buf;
> > +	dma->addr = addr;
> > +	dma->len = map_len;
> > +
> > +	rq->last_dma = dma;
> > +
> > +ok:
> > +	sg_init_table(rq->sg, 1);
> > +	rq->sg[0].dma_address = addr;
> > +	rq->sg[0].length = len;
> > +
> > +	return 0;
> > +}
> > +
> > +static int virtnet_rq_merge_map_init(struct virtnet_info *vi)
> > +{
> > +	struct receive_queue *rq;
> > +	int i, err, j, num;
> > +
> > +	/* disable for big mode */
> > +	if (!vi->mergeable_rx_bufs && vi->big_packets)
> > +		return 0;
> > +
> > +	for (i = 0; i < vi->max_queue_pairs; i++) {
> > +		err = virtqueue_set_premapped(vi->rq[i].vq);
> > +		if (err)
> > +			continue;
> > +
> > +		rq = &vi->rq[i];
> > +
> > +		num = virtqueue_get_vring_size(rq->vq);
> > +
> > +		rq->data_array = kmalloc_array(num, sizeof(*rq->data_array), GFP_KERNEL);
> > +		if (!rq->data_array)
> > +			goto err;
> > +
> > +		rq->dma_array = kmalloc_array(num, sizeof(*rq->dma_array), GFP_KERNEL);
> > +		if (!rq->dma_array)
> > +			goto err;
> > +
> > +		for (j = 0; j < num; ++j) {
> > +			rq->data_array[j].next = rq->data_free;
> > +			rq->data_free = &rq->data_array[j];
> > +
> > +			rq->dma_array[j].next = rq->dma_free;
> > +			rq->dma_free = &rq->dma_array[j];
> > +		}
> > +	}
> > +
> > +	return 0;
> > +
> > +err:
> > +	for (i = 0; i < vi->max_queue_pairs; i++) {
> > +		struct receive_queue *rq;
> > +
> > +		rq = &vi->rq[i];
> > +
> > +		kfree(rq->dma_array);
> > +		kfree(rq->data_array);
> > +	}
> > +
> > +	return -ENOMEM;
> > +}
> > +
> >  static void free_old_xmit_skbs(struct send_queue *sq, bool in_napi)
> >  {
> >  	unsigned int len;
> > @@ -835,7 +1033,7 @@ static struct page *xdp_linearize_page(struct receive_queue *rq,
> >  		void *buf;
> >  		int off;
> >
> > -		buf = virtqueue_get_buf(rq->vq, &buflen);
> > +		buf = virtnet_rq_get_buf(rq, &buflen, NULL);
> >  		if (unlikely(!buf))
> >  			goto err_buf;
> >
> > @@ -1126,7 +1324,7 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
> >  		return -EINVAL;
> >
> >  	while (--*num_buf > 0) {
> > -		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> > +		buf = virtnet_rq_get_buf(rq, &len, &ctx);
> >  		if (unlikely(!buf)) {
> >  			pr_debug("%s: rx error: %d buffers out of %d missing\n",
> >  				 dev->name, *num_buf,
> > @@ -1351,7 +1549,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> >  	while (--num_buf) {
> >  		int num_skb_frags;
> >
> > -		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> > +		buf = virtnet_rq_get_buf(rq, &len, &ctx);
> >  		if (unlikely(!buf)) {
> >  			pr_debug("%s: rx error: %d buffers out of %d missing\n",
> >  				 dev->name, num_buf,
> > @@ -1414,7 +1612,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> >  err_skb:
> >  	put_page(page);
> >  	while (num_buf-- > 1) {
> > -		buf = virtqueue_get_buf(rq->vq, &len);
> > +		buf = virtnet_rq_get_buf(rq, &len, NULL);
> >  		if (unlikely(!buf)) {
> >  			pr_debug("%s: rx error: %d buffers missing\n",
> >  				 dev->name, num_buf);
> > @@ -1529,6 +1727,7 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
> >  	unsigned int xdp_headroom = virtnet_get_headroom(vi);
> >  	void *ctx = (void *)(unsigned long)xdp_headroom;
> >  	int len = vi->hdr_len + VIRTNET_RX_PAD + GOOD_PACKET_LEN + xdp_headroom;
> > +	struct virtnet_rq_data *data;
> >  	int err;
> >
> >  	len = SKB_DATA_ALIGN(len) +
> > @@ -1539,11 +1738,34 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
> >  	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
> >  	get_page(alloc_frag->page);
> >  	alloc_frag->offset += len;
> > -	sg_init_one(rq->sg, buf + VIRTNET_RX_PAD + xdp_headroom,
> > -		    vi->hdr_len + GOOD_PACKET_LEN);
> > -	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, buf, ctx, gfp);
> > +
> > +	if (rq->data_array) {
> > +		err = virtnet_rq_map_sg(rq, buf + VIRTNET_RX_PAD + xdp_headroom,
> > +					vi->hdr_len + GOOD_PACKET_LEN);
> > +		if (err)
> > +			goto map_err;
> > +
> > +		data = virtnet_rq_get_data(rq, buf, rq->last_dma);
> > +	} else {
> > +		sg_init_one(rq->sg, buf + VIRTNET_RX_PAD + xdp_headroom,
> > +			    vi->hdr_len + GOOD_PACKET_LEN);
> > +		data = (void *)buf;
> > +	}
> > +
> > +	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, data, ctx, gfp);
> >  	if (err < 0)
> > -		put_page(virt_to_head_page(buf));
> > +		goto add_err;
> > +
> > +	return err;
> > +
> > +add_err:
> > +	if (rq->data_array) {
> > +		virtnet_rq_unmap(rq, data->dma);
> > +		virtnet_rq_recycle_data(rq, data);
> > +	}
> > +
> > +map_err:
> > +	put_page(virt_to_head_page(buf));
> >  	return err;
> >  }
> >
> > @@ -1620,6 +1842,7 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
> >  	unsigned int headroom = virtnet_get_headroom(vi);
> >  	unsigned int tailroom = headroom ? sizeof(struct skb_shared_info) : 0;
> >  	unsigned int room = SKB_DATA_ALIGN(headroom + tailroom);
> > +	struct virtnet_rq_data *data;
> >  	char *buf;
> >  	void *ctx;
> >  	int err;
> > @@ -1650,12 +1873,32 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
> >  		alloc_frag->offset += hole;
> >  	}
> >
> > -	sg_init_one(rq->sg, buf, len);
> > +	if (rq->data_array) {
> > +		err = virtnet_rq_map_sg(rq, buf, len);
> > +		if (err)
> > +			goto map_err;
> > +
> > +		data = virtnet_rq_get_data(rq, buf, rq->last_dma);
> > +	} else {
> > +		sg_init_one(rq->sg, buf, len);
> > +		data = (void *)buf;
> > +	}
> > +
> >  	ctx = mergeable_len_to_ctx(len + room, headroom);
> > -	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, buf, ctx, gfp);
> > +	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, data, ctx, gfp);
> >  	if (err < 0)
> > -		put_page(virt_to_head_page(buf));
> > +		goto add_err;
> > +
> > +	return 0;
> > +
> > +add_err:
> > +	if (rq->data_array) {
> > +		virtnet_rq_unmap(rq, data->dma);
> > +		virtnet_rq_recycle_data(rq, data);
> > +	}
> >
> > +map_err:
> > +	put_page(virt_to_head_page(buf));
> >  	return err;
> >  }
> >
> > @@ -1775,13 +2018,13 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
> >  		void *ctx;
> >
> >  		while (stats.packets < budget &&
> > -		       (buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx))) {
> > +		       (buf = virtnet_rq_get_buf(rq, &len, &ctx))) {
> >  			receive_buf(vi, rq, buf, len, ctx, xdp_xmit, &stats);
> >  			stats.packets++;
> >  		}
> >  	} else {
> >  		while (stats.packets < budget &&
> > -		       (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
> > +		       (buf = virtnet_rq_get_buf(rq, &len, NULL)) != NULL) {
> >  			receive_buf(vi, rq, buf, len, NULL, xdp_xmit, &stats);
> >  			stats.packets++;
> >  		}
> > @@ -3514,6 +3757,9 @@ static void virtnet_free_queues(struct virtnet_info *vi)
> >  	for (i = 0; i < vi->max_queue_pairs; i++) {
> >  		__netif_napi_del(&vi->rq[i].napi);
> >  		__netif_napi_del(&vi->sq[i].napi);
> > +
> > +		kfree(vi->rq[i].data_array);
> > +		kfree(vi->rq[i].dma_array);
> >  	}
> >
> >  	/* We called __netif_napi_del(),
> > @@ -3591,9 +3837,10 @@ static void free_unused_bufs(struct virtnet_info *vi)
> >  	}
> >
> >  	for (i = 0; i < vi->max_queue_pairs; i++) {
> > -		struct virtqueue *vq = vi->rq[i].vq;
> > -		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
> > -			virtnet_rq_free_unused_buf(vq, buf);
> > +		struct receive_queue *rq = &vi->rq[i];
> > +
> > +		while ((buf = virtnet_rq_detach_unused_buf(rq)) != NULL)
> > +			virtnet_rq_free_unused_buf(rq->vq, buf);
> >  		cond_resched();
> >  	}
> >  }
> > @@ -3767,6 +4014,10 @@ static int init_vqs(struct virtnet_info *vi)
> >  	if (ret)
> >  		goto err_free;
> >
> > +	ret = virtnet_rq_merge_map_init(vi);
> > +	if (ret)
> > +		goto err_free;
> > +
> >  	cpus_read_lock();
> >  	virtnet_set_affinity(vi);
> >  	cpus_read_unlock();
> > --
> > 2.32.0.3.g01195cf9f
>

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