Subject: Re: [PATCH RFC v5 net-next 2/6] virtio_ring: try to disable event index callbacks in virtqueue_disable_cb()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 10 Feb 2015 11:24:17 +0100

On Mon, Feb 09, 2015 at 03:39:21AM -0500, Jason Wang wrote:
> Currently, we do nothing to prevent the callbacks in
> virtqueue_disable_cb() when event index is used. This may cause
> spurious interrupts which may damage the performance. This patch tries
> to publish avail event as the used even to prevent the callbacks.
> 
> Signed-off-by: Jason Wang <[email-masked]>

I'm surprised that this ever happens though.
Normally we call this after getting an interrupt, and
interrupts won't trigger again until the rings wraps around.

When I tested this, touching an extra cache line was more
expensive.

Does this really reduce number of interrupts?
Could you pls share some numbers with and without this patch?

> ---
>  drivers/virtio/virtio_ring.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 545fed5..e9ffbfb 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -539,6 +539,8 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
>  	vq->vring.avail->flags |= cpu_to_virtio16(_vq->vdev, VRING_AVAIL_F_NO_INTERRUPT);
> +	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
> +						       vq->vring.avail->idx);

Hmm in fact, can't this actually cause an extra interrupt
when avail->idx is completed?

I think that if you really can show disabling interrupts like this helps, you should
set some invalid value like 0xfffff, or move it back to vq->vring.avail->idx - 1.
No?

>  }
>  EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>  
> -- 
> 1.8.3.1