On Thu, Aug 31, 2023 at 4:44 PM Maxime Coquelin
<maxime.coque...@redhat.com> wrote:
>
> Guest notifications offloading, which has been introduced
> in v23.07, aims at offloading syscalls out of the datapath.
>
> This patch optimizes the offloading by not offloading the
> guest notification for a given virtqueue if one is already
> being offloaded by the application.
>
> With a single VDUSE device, we can already see few
> notifications being suppressed when doing throughput
> testing with Iperf3. We can expect to see much more being
> suppressed when the offloading thread is under pressure.
>
> Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
> ---
> lib/vhost/vhost.c | 4 ++++
> lib/vhost/vhost.h | 27 +++++++++++++++++++++------
> 2 files changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/lib/vhost/vhost.c b/lib/vhost/vhost.c
> index eb6309b681..7794f29c18 100644
> --- a/lib/vhost/vhost.c
> +++ b/lib/vhost/vhost.c
> @@ -48,6 +48,8 @@ static const struct vhost_vq_stats_name_off
> vhost_vq_stat_strings[] = {
> stats.guest_notifications_offloaded)},
> {"guest_notifications_error", offsetof(struct vhost_virtqueue,
> stats.guest_notifications_error)},
> + {"guest_notifications_suppressed", offsetof(struct vhost_virtqueue,
> + stats.guest_notifications_suppressed)},
> {"iotlb_hits", offsetof(struct vhost_virtqueue,
> stats.iotlb_hits)},
> {"iotlb_misses", offsetof(struct vhost_virtqueue,
> stats.iotlb_misses)},
> {"inflight_submitted", offsetof(struct vhost_virtqueue,
> stats.inflight_submitted)},
> @@ -1516,6 +1518,8 @@ rte_vhost_notify_guest(int vid, uint16_t queue_id)
>
> rte_rwlock_read_lock(&vq->access_lock);
>
> + __atomic_store_n(&vq->irq_pending, false, __ATOMIC_RELEASE);
> +
> if (dev->backend_ops->inject_irq(dev, vq)) {
> if (dev->flags & VIRTIO_DEV_STATS_ENABLED)
>
> __atomic_fetch_add(&vq->stats.guest_notifications_error,
> diff --git a/lib/vhost/vhost.h b/lib/vhost/vhost.h
> index 9723429b1c..3e78379e48 100644
> --- a/lib/vhost/vhost.h
> +++ b/lib/vhost/vhost.h
> @@ -156,6 +156,7 @@ struct virtqueue_stats {
> uint64_t iotlb_misses;
> uint64_t inflight_submitted;
> uint64_t inflight_completed;
> + uint64_t guest_notifications_suppressed;
> /* Counters below are atomic, and should be incremented as such. */
> uint64_t guest_notifications;
> uint64_t guest_notifications_offloaded;
> @@ -346,6 +347,8 @@ struct vhost_virtqueue {
>
> struct vhost_vring_addr ring_addrs;
> struct virtqueue_stats stats;
> +
> + bool irq_pending;
> } __rte_cache_aligned;
>
> /* Virtio device status as per Virtio specification */
> @@ -908,12 +911,24 @@ vhost_need_event(uint16_t event_idx, uint16_t new_idx,
> uint16_t old)
> static __rte_always_inline void
> vhost_vring_inject_irq(struct virtio_net *dev, struct vhost_virtqueue *vq)
> {
> - if (dev->notify_ops->guest_notify &&
> - dev->notify_ops->guest_notify(dev->vid, vq->index)) {
> - if (dev->flags & VIRTIO_DEV_STATS_ENABLED)
> -
> __atomic_fetch_add(&vq->stats.guest_notifications_offloaded,
> - 1, __ATOMIC_RELAXED);
> - return;
> + bool expected = false;
> +
> + if (dev->notify_ops->guest_notify) {
> + if (__atomic_compare_exchange_n(&vq->irq_pending, &expected,
> true, 0,
> + __ATOMIC_RELEASE, __ATOMIC_RELAXED)) {
> + if (dev->notify_ops->guest_notify(dev->vid,
> vq->index)) {
> + if (dev->flags & VIRTIO_DEV_STATS_ENABLED)
> +
> __atomic_fetch_add(&vq->stats.guest_notifications_offloaded,
> + 1, __ATOMIC_RELAXED);
> + return;
> + }
> +
> + /* Offloading failed, fallback to direct IRQ
> injection */
> + __atomic_store_n(&vq->irq_pending, 0,
> __ATOMIC_RELEASE);
Nit: s/0/false/
> + } else {
> + vq->stats.guest_notifications_suppressed++;
> + return;
> + }
> }
>
> if (dev->backend_ops->inject_irq(dev, vq)) {
> --
> 2.41.0
>
--
David Marchand
