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SubjectRe: [PATCH net] vhost_net: flush batched heads before trying to busy polling
On Tue, May 29, 2018 at 02:18:19PM +0800, Jason Wang wrote:
> After commit e2b3b35eb989 ("vhost_net: batch used ring update in rx"),
> we tend to batch updating used heads. But it doesn't flush batched
> heads before trying to do busy polling, this will cause vhost to wait
> for guest TX which waits for the used RX. Fixing by flush batched
> heads before busy loop.
>
> 1 byte TCP_RR performance recovers from 13107.83 to 50402.65.
>
> Fixes: e2b3b35eb989 ("vhost_net: batch used ring update in rx")
> Signed-off-by: Jason Wang <jasowang@redhat.com>

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

> ---
> drivers/vhost/net.c | 37 ++++++++++++++++++++++++-------------
> 1 file changed, 24 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 986058a..eeaf673 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -105,7 +105,9 @@ struct vhost_net_virtqueue {
> /* vhost zerocopy support fields below: */
> /* last used idx for outstanding DMA zerocopy buffers */
> int upend_idx;
> - /* first used idx for DMA done zerocopy buffers */
> + /* For TX, first used idx for DMA done zerocopy buffers
> + * For RX, number of batched heads
> + */
> int done_idx;
> /* an array of userspace buffers info */
> struct ubuf_info *ubuf_info;
> @@ -626,6 +628,18 @@ static int sk_has_rx_data(struct sock *sk)
> return skb_queue_empty(&sk->sk_receive_queue);
> }
>
> +static void vhost_rx_signal_used(struct vhost_net_virtqueue *nvq)
> +{
> + struct vhost_virtqueue *vq = &nvq->vq;
> + struct vhost_dev *dev = vq->dev;
> +
> + if (!nvq->done_idx)
> + return;
> +
> + vhost_add_used_and_signal_n(dev, vq, vq->heads, nvq->done_idx);
> + nvq->done_idx = 0;
> +}
> +
> static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
> {
> struct vhost_net_virtqueue *rvq = &net->vqs[VHOST_NET_VQ_RX];
> @@ -635,6 +649,8 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
> int len = peek_head_len(rvq, sk);
>
> if (!len && vq->busyloop_timeout) {
> + /* Flush batched heads first */
> + vhost_rx_signal_used(rvq);
> /* Both tx vq and rx socket were polled here */
> mutex_lock_nested(&vq->mutex, 1);
> vhost_disable_notify(&net->dev, vq);
> @@ -762,7 +778,7 @@ static void handle_rx(struct vhost_net *net)
> };
> size_t total_len = 0;
> int err, mergeable;
> - s16 headcount, nheads = 0;
> + s16 headcount;
> size_t vhost_hlen, sock_hlen;
> size_t vhost_len, sock_len;
> struct socket *sock;
> @@ -790,8 +806,8 @@ static void handle_rx(struct vhost_net *net)
> while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk))) {
> sock_len += sock_hlen;
> vhost_len = sock_len + vhost_hlen;
> - headcount = get_rx_bufs(vq, vq->heads + nheads, vhost_len,
> - &in, vq_log, &log,
> + headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
> + vhost_len, &in, vq_log, &log,
> likely(mergeable) ? UIO_MAXIOV : 1);
> /* On error, stop handling until the next kick. */
> if (unlikely(headcount < 0))
> @@ -862,12 +878,9 @@ static void handle_rx(struct vhost_net *net)
> vhost_discard_vq_desc(vq, headcount);
> goto out;
> }
> - nheads += headcount;
> - if (nheads > VHOST_RX_BATCH) {
> - vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
> - nheads);
> - nheads = 0;
> - }
> + nvq->done_idx += headcount;
> + if (nvq->done_idx > VHOST_RX_BATCH)
> + vhost_rx_signal_used(nvq);
> if (unlikely(vq_log))
> vhost_log_write(vq, vq_log, log, vhost_len);
> total_len += vhost_len;
> @@ -878,9 +891,7 @@ static void handle_rx(struct vhost_net *net)
> }
> vhost_net_enable_vq(net, vq);
> out:
> - if (nheads)
> - vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
> - nheads);
> + vhost_rx_signal_used(nvq);
> mutex_unlock(&vq->mutex);
> }
>
> --
> 2.7.4

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