Re: [PATCH v2 net-next 2/4] vhost_net: Avoid tx vring kicks during busyloop

[Jason Wang <jasowang@redhat.com>]

On 2018年07月03日 15:31, Toshiaki Makita wrote:
> Under heavy load vhost busypoll may run without suppressing
> notification. For example tx zerocopy callback can push tx work while
> handle_tx() is running, then busyloop exits due to vhost_has_work()
> condition and enables notification but immediately reenters handle_tx()
> because the pushed work was tx. In this case handle_tx() tries to
> disable notification again, but when using event_idx it by design
> cannot. Then busyloop will run without suppressing notification.
> Another example is the case where handle_tx() tries to enable
> notification but avail idx is advanced so disables it again. This case
> also leads to the same situation with event_idx.
>
> The problem is that once we enter this situation busyloop does not work
> under heavy load for considerable amount of time, because notification
> is likely to happen during busyloop and handle_tx() immediately enables
> notification after notification happens. Specifically busyloop detects
> notification by vhost_has_work() and then handle_tx() calls
> vhost_enable_notify(). Because the detected work was the tx work, it
> enters handle_tx(), and enters busyloop without suppression again.
> This is likely to be repeated, so with event_idx we are almost not able
> to suppress notification in this case.
>
> To fix this, poll the work instead of enabling notification when
> busypoll is interrupted by something. IMHO vhost_has_work() is kind of
> interruption rather than a signal to completely cancel the busypoll, so
> let's run busypoll after the necessary work is done.
>
> Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
> ---
>   drivers/vhost/net.c | 35 ++++++++++++++++++++++-------------
>   1 file changed, 22 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 3939c50..811c0e5 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -396,13 +396,10 @@ static inline unsigned long busy_clock(void)
>   	return local_clock() >> 10;
>   }
>   
> -static bool vhost_can_busy_poll(struct vhost_dev *dev,
> -				unsigned long endtime)
> +static bool vhost_can_busy_poll(unsigned long endtime)
>   {
> -	return likely(!need_resched()) &&
> -	       likely(!time_after(busy_clock(), endtime)) &&
> -	       likely(!signal_pending(current)) &&
> -	       !vhost_has_work(dev);
> +	return likely(!need_resched() && !time_after(busy_clock(), endtime) &&
> +		      !signal_pending(current));
>   }
>   
>   static void vhost_net_disable_vq(struct vhost_net *n,
> @@ -434,7 +431,8 @@ static int vhost_net_enable_vq(struct vhost_net *n,
>   static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
>   				    struct vhost_virtqueue *vq,
>   				    struct iovec iov[], unsigned int iov_size,
> -				    unsigned int *out_num, unsigned int *in_num)
> +				    unsigned int *out_num, unsigned int *in_num,
> +				    bool *busyloop_intr)
>   {
>   	unsigned long uninitialized_var(endtime);
>   	int r = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
> @@ -443,9 +441,15 @@ static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
>   	if (r == vq->num && vq->busyloop_timeout) {
>   		preempt_disable();
>   		endtime = busy_clock() + vq->busyloop_timeout;
> -		while (vhost_can_busy_poll(vq->dev, endtime) &&
> -		       vhost_vq_avail_empty(vq->dev, vq))
> +		while (vhost_can_busy_poll(endtime)) {
> +			if (vhost_has_work(vq->dev)) {
> +				*busyloop_intr = true;
> +				break;
> +			}
> +			if (!vhost_vq_avail_empty(vq->dev, vq))
> +				break;
>   			cpu_relax();
> +		}
>   		preempt_enable();
>   		r = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
>   				      out_num, in_num, NULL, NULL);
> @@ -501,20 +505,24 @@ static void handle_tx(struct vhost_net *net)
>   	zcopy = nvq->ubufs;
>   
>   	for (;;) {
> +		bool busyloop_intr;
> +
>   		/* Release DMAs done buffers first */
>   		if (zcopy)
>   			vhost_zerocopy_signal_used(net, vq);
>   
> -
> +		busyloop_intr = false;
>   		head = vhost_net_tx_get_vq_desc(net, vq, vq->iov,
>   						ARRAY_SIZE(vq->iov),
> -						&out, &in);
> +						&out, &in, &busyloop_intr);
>   		/* On error, stop handling until the next kick. */
>   		if (unlikely(head < 0))
>   			break;
>   		/* Nothing new?  Wait for eventfd to tell us they refilled. */
>   		if (head == vq->num) {
> -			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> +			if (unlikely(busyloop_intr)) {
> +				vhost_poll_queue(&vq->poll);
> +			} else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>   				vhost_disable_notify(&net->dev, vq);
>   				continue;
>   			}
> @@ -663,7 +671,8 @@ 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(&net->dev, endtime) &&
> +		while (vhost_can_busy_poll(endtime) &&
> +		       !vhost_has_work(&net->dev) &&
>   		       !sk_has_rx_data(sk) &&
>   		       vhost_vq_avail_empty(&net->dev, tvq))
>   			cpu_relax();

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

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization