netdev - Re: [PATCH v2 net-next 3/4] vhost_net: Avoid rx queue wake-ups during busypoll

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Date:   Wed, 4 Jul 2018 11:26:35 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        "David S. Miller" <davem@...emloft.net>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Tonghao Zhang <xiangxia.m.yue@...il.com>
Subject: Re: [PATCH v2 net-next 3/4] vhost_net: Avoid rx queue wake-ups during
 busypoll



On 2018年07月03日 15:31, Toshiaki Makita wrote:
> We may run handle_rx() while rx work is queued. For example a packet can
> push the rx work during the window before handle_rx calls
> vhost_net_disable_vq().
> In that case busypoll immediately exits due to vhost_has_work()
> condition and enables vq again. This can lead to another unnecessary rx
> wake-ups, so poll rx work instead of enabling the vq.
>
> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
> ---
>   drivers/vhost/net.c | 26 +++++++++++++++++++-------
>   1 file changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 811c0e5..791bc8b 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -653,7 +653,8 @@ static void vhost_rx_signal_used(struct vhost_net_virtqueue *nvq)
>   	nvq->done_idx = 0;
>   }
>   
> -static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
> +static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
> +				      bool *busyloop_intr)
>   {
>   	struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
>   	struct vhost_net_virtqueue *tnvq = &net->vqs[VHOST_NET_VQ_TX];
> @@ -671,11 +672,16 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
>   		preempt_disable();
>   		endtime = busy_clock() + tvq->busyloop_timeout;
>   
> -		while (vhost_can_busy_poll(endtime) &&
> -		       !vhost_has_work(&net->dev) &&
> -		       !sk_has_rx_data(sk) &&
> -		       vhost_vq_avail_empty(&net->dev, tvq))
> +		while (vhost_can_busy_poll(endtime)) {
> +			if (vhost_has_work(&net->dev)) {
> +				*busyloop_intr = true;
> +				break;
> +			}
> +			if (sk_has_rx_data(sk) ||
> +			    !vhost_vq_avail_empty(&net->dev, tvq))
> +				break;
>   			cpu_relax();
> +		}
>   
>   		preempt_enable();
>   
> @@ -795,6 +801,7 @@ static void handle_rx(struct vhost_net *net)
>   	s16 headcount;
>   	size_t vhost_hlen, sock_hlen;
>   	size_t vhost_len, sock_len;
> +	bool busyloop_intr = false;
>   	struct socket *sock;
>   	struct iov_iter fixup;
>   	__virtio16 num_buffers;
> @@ -818,7 +825,9 @@ static void handle_rx(struct vhost_net *net)
>   		vq->log : NULL;
>   	mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
>   
> -	while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk))) {
> +	while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
> +						      &busyloop_intr))) {
> +		busyloop_intr = false;
>   		sock_len += sock_hlen;
>   		vhost_len = sock_len + vhost_hlen;
>   		headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
> @@ -905,7 +914,10 @@ static void handle_rx(struct vhost_net *net)
>   			goto out;
>   		}
>   	}
> -	vhost_net_enable_vq(net, vq);
> +	if (unlikely(busyloop_intr))
> +		vhost_poll_queue(&vq->poll);
> +	else
> +		vhost_net_enable_vq(net, vq);
>   out:
>   	vhost_rx_signal_used(nvq);
>   	mutex_unlock(&vq->mutex);

Acked-by: Jason Wang <jasowang@...hat.com>