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

Jason Wang
2018-07-03T09:04:07Z

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>

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