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Commit e4dab1e6 authored by Jason Wang's avatar Jason Wang Committed by David S. Miller
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vhost_net: mitigate page reference counting during page frag refill 



We do a get_page() which involves a atomic operation. This patch tries
to mitigate a per packet atomic operation by maintaining a reference
bias which is initially USHRT_MAX. Each time a page is got, instead of
calling get_page() we decrease the bias and when we find it's time to
use a new page we will decrease the bias at one time through
__page_cache_drain_cache().

Testpmd(virtio_user + vhost_net) + XDP_DROP on TAP shows about 1.6%
improvement.

Before: 4.63Mpps
After:  4.71Mpps

Signed-off-by: default avatarJason Wang <jasowang@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent b8b9618a

drivers/vhost/net.c

+51 −3
Original line number Diff line number Diff line
@@ -141,6 +141,10 @@ struct vhost_net { 
	unsigned tx_zcopy_err;

	/* Flush in progress. Protected by tx vq lock. */

	bool tx_flush;

	/* Private page frag */

	struct page_frag page_frag;

	/* Refcount bias of page frag */

	int refcnt_bias;

};



static unsigned vhost_net_zcopy_mask __read_mostly;
@@ -637,14 +641,53 @@ static bool tx_can_batch(struct vhost_virtqueue *vq, size_t total_len) 
	       !vhost_vq_avail_empty(vq->dev, vq);

}



#define SKB_FRAG_PAGE_ORDER     get_order(32768)



static bool vhost_net_page_frag_refill(struct vhost_net *net, unsigned int sz,

				       struct page_frag *pfrag, gfp_t gfp)

{

	if (pfrag->page) {

		if (pfrag->offset + sz <= pfrag->size)

			return true;

		__page_frag_cache_drain(pfrag->page, net->refcnt_bias);

	}



	pfrag->offset = 0;

	net->refcnt_bias = 0;

	if (SKB_FRAG_PAGE_ORDER) {

		/* Avoid direct reclaim but allow kswapd to wake */

		pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |

					  __GFP_COMP | __GFP_NOWARN |

					  __GFP_NORETRY,

					  SKB_FRAG_PAGE_ORDER);

		if (likely(pfrag->page)) {

			pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;

			goto done;

		}

	}

	pfrag->page = alloc_page(gfp);

	if (likely(pfrag->page)) {

		pfrag->size = PAGE_SIZE;

		goto done;

	}

	return false;



done:

	net->refcnt_bias = USHRT_MAX;

	page_ref_add(pfrag->page, USHRT_MAX - 1);

	return true;

}



#define VHOST_NET_RX_PAD (NET_IP_ALIGN + NET_SKB_PAD)



static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,

			       struct iov_iter *from)

{

	struct vhost_virtqueue *vq = &nvq->vq;

	struct vhost_net *net = container_of(vq->dev, struct vhost_net,

					     dev);

	struct socket *sock = vq->private_data;

	struct page_frag *alloc_frag = &current->task_frag;

	struct page_frag *alloc_frag = &net->page_frag;

	struct virtio_net_hdr *gso;

	struct xdp_buff *xdp = &nvq->xdp[nvq->batched_xdp];

	struct tun_xdp_hdr *hdr;
@@ -665,7 +708,8 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq, 


	buflen += SKB_DATA_ALIGN(len + pad);

	alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);

	if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))

	if (unlikely(!vhost_net_page_frag_refill(net, buflen,

						 alloc_frag, GFP_KERNEL)))

		return -ENOMEM;



	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
@@ -703,7 +747,7 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq, 
	xdp->data_end = xdp->data + len;

	hdr->buflen = buflen;



	get_page(alloc_frag->page);

	--net->refcnt_bias;

	alloc_frag->offset += buflen;



	++nvq->batched_xdp;
@@ -1292,6 +1336,8 @@ static int vhost_net_open(struct inode *inode, struct file *f) 
	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, EPOLLIN, dev);



	f->private_data = n;

	n->page_frag.page = NULL;

	n->refcnt_bias = 0;



	return 0;

}
@@ -1366,6 +1412,8 @@ static int vhost_net_release(struct inode *inode, struct file *f) 
	kfree(n->vqs[VHOST_NET_VQ_RX].rxq.queue);

	kfree(n->vqs[VHOST_NET_VQ_TX].xdp);

	kfree(n->dev.vqs);

	if (n->page_frag.page)

		__page_frag_cache_drain(n->page_frag.page, n->refcnt_bias);

	kvfree(n);

	return 0;

}