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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: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Jason Wang 2018-11-15 17:43:09 +08:00 committed by David S. Miller
parent b8b9618a4f
commit e4dab1e6ea
1 changed files with 51 additions and 3 deletions
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54
drivers/vhost/net.c
View File

@ -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;
}