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Date: Fri, 29 Jun 2018 19:38:21 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc: Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH vhost] vhost_net: Fix too many vring kick on busypoll
On Fri, Jun 29, 2018 at 05:09:50PM +0900, 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 lead 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().
I'd like to understand the problem a bit better.
Why does this happen?
Doesn't this only happen if ring is empty?
> 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 signal_pending() and
> vhost_has_work() are kind of interruptions rather than signals to
> completely cancel the busypoll, so let's run busypoll after the
> necessary work is done. In order to avoid too long busyloop due to
> interruption, save the endtime in vq field and use it when reentering
> the work function.
>
> I observed this problem makes tx performance poor but does not rx. I
> guess it is because rx notification from socket is not that heavy so
> did not impact the performance even when we failed to mask the
> notification. Anyway for consistency I fixed rx routine as well as tx.
>
> Performance numbers:
>
> - Bulk transfer from guest to external physical server.
> [Guest]->vhost_net->tap--(XDP_REDIRECT)-->i40e --(wire)--> [Server]
> - Set 10us busypoll.
> - Guest disables checksum and TSO because of host XDP.
> - Measured single flow Mbps by netperf, and kicks by perf kvm stat
> (EPT_MISCONFIG event).
>
> Before After
> Mbps kicks/s Mbps kicks/s
> UDP_STREAM 1472byte 247758 27
> Send 3645.37 6958.10
> Recv 3588.56 6958.10
> 1byte 9865 37
> Send 4.34 5.43
> Recv 4.17 5.26
> TCP_STREAM 8801.03 45794 9592.77 2884
>
> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Is this with busy poll enabled?
Are there CPU utilization #s?
> ---
> drivers/vhost/net.c | 94 +++++++++++++++++++++++++++++++++++----------------
> drivers/vhost/vhost.c | 1 +
> drivers/vhost/vhost.h | 1 +
> 3 files changed, 66 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index eeaf6739215f..0e85f628b965 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -391,13 +391,14 @@ 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));
> +}
> +
> +static bool vhost_busy_poll_interrupted(struct vhost_dev *dev)
> +{
> + return unlikely(signal_pending(current)) || vhost_has_work(dev);
> }
>
> static void vhost_net_disable_vq(struct vhost_net *n,
> @@ -437,10 +438,21 @@ 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))
> + if (vq->busyloop_endtime) {
> + endtime = vq->busyloop_endtime;
> + vq->busyloop_endtime = 0;
> + } else {
> + endtime = busy_clock() + vq->busyloop_timeout;
> + }
> + while (vhost_can_busy_poll(endtime)) {
> + if (vhost_busy_poll_interrupted(vq->dev)) {
> + vq->busyloop_endtime = endtime;
> + 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);
> @@ -509,12 +521,16 @@ static void handle_tx(struct vhost_net *net)
> 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(vq->busyloop_endtime)) {
> + /* Busy poll is interrupted. */
> + vhost_poll_queue(&vq->poll);
> + } else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> vhost_disable_notify(&net->dev, vq);
> continue;
> }
> break;
> }
> + vq->busyloop_endtime = 0;
> if (in) {
> vq_err(vq, "Unexpected descriptor format for TX: "
> "out %d, int %d\n", out, in);
> @@ -642,39 +658,51 @@ static void vhost_rx_signal_used(struct vhost_net_virtqueue *nvq)
>
> 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];
> - struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
> - struct vhost_virtqueue *vq = &nvq->vq;
> + struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
> + struct vhost_net_virtqueue *tnvq = &net->vqs[VHOST_NET_VQ_TX];
> + struct vhost_virtqueue *rvq = &rnvq->vq;
> + struct vhost_virtqueue *tvq = &tnvq->vq;
> unsigned long uninitialized_var(endtime);
> - int len = peek_head_len(rvq, sk);
> + int len = peek_head_len(rnvq, sk);
>
> - if (!len && vq->busyloop_timeout) {
> + if (!len && tvq->busyloop_timeout) {
> /* Flush batched heads first */
> - vhost_rx_signal_used(rvq);
> + vhost_rx_signal_used(rnvq);
> /* Both tx vq and rx socket were polled here */
> - mutex_lock_nested(&vq->mutex, 1);
> - vhost_disable_notify(&net->dev, vq);
> + mutex_lock_nested(&tvq->mutex, 1);
> + vhost_disable_notify(&net->dev, tvq);
>
> preempt_disable();
> - endtime = busy_clock() + vq->busyloop_timeout;
> + if (rvq->busyloop_endtime) {
> + endtime = rvq->busyloop_endtime;
> + rvq->busyloop_endtime = 0;
> + } else {
> + endtime = busy_clock() + tvq->busyloop_timeout;
> + }
>
> - while (vhost_can_busy_poll(&net->dev, endtime) &&
> - !sk_has_rx_data(sk) &&
> - vhost_vq_avail_empty(&net->dev, vq))
> + while (vhost_can_busy_poll(endtime)) {
> + if (vhost_busy_poll_interrupted(&net->dev)) {
> + rvq->busyloop_endtime = endtime;
> + break;
> + }
> + if (sk_has_rx_data(sk) ||
> + !vhost_vq_avail_empty(&net->dev, tvq))
> + break;
> cpu_relax();
> + }
>
> preempt_enable();
>
> - if (!vhost_vq_avail_empty(&net->dev, vq))
> - vhost_poll_queue(&vq->poll);
> - else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> - vhost_disable_notify(&net->dev, vq);
> - vhost_poll_queue(&vq->poll);
> + if (!vhost_vq_avail_empty(&net->dev, tvq)) {
> + vhost_poll_queue(&tvq->poll);
> + } else if (unlikely(vhost_enable_notify(&net->dev, tvq))) {
> + vhost_disable_notify(&net->dev, tvq);
> + vhost_poll_queue(&tvq->poll);
> }
>
> - mutex_unlock(&vq->mutex);
> + mutex_unlock(&tvq->mutex);
>
> - len = peek_head_len(rvq, sk);
> + len = peek_head_len(rnvq, sk);
> }
>
> return len;
> @@ -804,6 +832,7 @@ static void handle_rx(struct vhost_net *net)
> mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
>
> while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk))) {
> + vq->busyloop_endtime = 0;
> sock_len += sock_hlen;
> vhost_len = sock_len + vhost_hlen;
> headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
> @@ -889,7 +918,12 @@ static void handle_rx(struct vhost_net *net)
> goto out;
> }
> }
> - vhost_net_enable_vq(net, vq);
> + if (unlikely(vq->busyloop_endtime)) {
> + /* Busy poll is interrupted. */
> + vhost_poll_queue(&vq->poll);
> + } else {
> + vhost_net_enable_vq(net, vq);
> + }
> out:
> vhost_rx_signal_used(nvq);
> mutex_unlock(&vq->mutex);
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 9beefa6ed1ce..fe83578fe336 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -323,6 +323,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> vhost_reset_is_le(vq);
> vhost_disable_cross_endian(vq);
> vq->busyloop_timeout = 0;
> + vq->busyloop_endtime = 0;
> vq->umem = NULL;
> vq->iotlb = NULL;
> __vhost_vq_meta_reset(vq);
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 6c844b90a168..7e9cf80ccae9 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -144,6 +144,7 @@ struct vhost_virtqueue {
> bool user_be;
> #endif
> u32 busyloop_timeout;
> + unsigned long busyloop_endtime;
> };
>
> struct vhost_msg_node {
> --
> 2.14.2
>
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