On Wed, Apr 27, 2022 at 5:35 PM Stefan Hajnoczi <stefa...@redhat.com> wrote:
>
> The virtio-scsi event virtqueue is not emptied by its handler function.
> This is typical for rx virtqueues where the device uses buffers when
> some event occurs (e.g. a packet is received, an error condition
> happens, etc).
>
> Polling non-empty virtqueues wastes CPU cycles. We are not waiting for
> new buffers to become available, we are waiting for an event to occur,
> so it's a misuse of CPU resources to poll for buffers.
>
> Introduce the new virtio_queue_aio_attach_host_notifier_no_poll() API,
> which is identical to virtio_queue_aio_attach_host_notifier() except
> that it does not poll the virtqueue.
>
> Before this patch the following command-line consumed 100% CPU in the
> IOThread polling and calling virtio_scsi_handle_event():
>
> $ qemu-system-x86_64 -M accel=kvm -m 1G -cpu host \
> --object iothread,id=iothread0 \
> --device virtio-scsi-pci,iothread=iothread0 \
> --blockdev
> file,filename=test.img,aio=native,cache.direct=on,node-name=drive0 \
> --device scsi-hd,drive=drive0
>
> After this patch CPU is no longer wasted.
>
> Reported-by: Nir Soffer <nsof...@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
> ---
> include/hw/virtio/virtio.h | 1 +
> hw/scsi/virtio-scsi-dataplane.c | 2 +-
> hw/virtio/virtio.c | 13 +++++++++++++
> 3 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index b31c4507f5..b62a35fdca 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -317,6 +317,7 @@ EventNotifier *virtio_queue_get_host_notifier(VirtQueue
> *vq);
> void virtio_queue_set_host_notifier_enabled(VirtQueue *vq, bool enabled);
> void virtio_queue_host_notifier_read(EventNotifier *n);
> void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx);
> +void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext
> *ctx);
> void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx);
> VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);
> VirtQueue *virtio_vector_next_queue(VirtQueue *vq);
> diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
> index 29575cbaf6..8bb6e6acfc 100644
> --- a/hw/scsi/virtio-scsi-dataplane.c
> +++ b/hw/scsi/virtio-scsi-dataplane.c
> @@ -138,7 +138,7 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
>
> aio_context_acquire(s->ctx);
> virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);
> - virtio_queue_aio_attach_host_notifier(vs->event_vq, s->ctx);
> + virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx);
>
> for (i = 0; i < vs->conf.num_queues; i++) {
> virtio_queue_aio_attach_host_notifier(vs->cmd_vqs[i], s->ctx);
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 9d637e043e..67a873f54a 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -3534,6 +3534,19 @@ void virtio_queue_aio_attach_host_notifier(VirtQueue
> *vq, AioContext *ctx)
> virtio_queue_host_notifier_aio_poll_end);
> }
>
> +/*
> + * Same as virtio_queue_aio_attach_host_notifier() but without polling. Use
> + * this for rx virtqueues and similar cases where the virtqueue handler
> + * function does not pop all elements. When the virtqueue is left non-empty
> + * polling consumes CPU cycles and should not be used.
> + */
> +void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext
> *ctx)
> +{
> + aio_set_event_notifier(ctx, &vq->host_notifier, true,
> + virtio_queue_host_notifier_read,
> + NULL, NULL);
> +}
> +
> void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx)
> {
> aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL, NULL);
> --
> 2.35.1
>
I tested patches 1 and 2 on top of 34723f59371f3fd02ea59b94674314b875504426
and it solved the issue.
Tested-by: Nir Soffer <nsof...@redhat.com>
Nir
