Re: [PATCH v3 3/4] virtio: fix up virtio_disable_cb

[Jason Wang]

在 2021/5/26 下午4:24, Michael S. Tsirkin 写道:
> virtio_disable_cb is currently a nop for split ring with event index.
> This is because it used to be always called from a callback when we know
> device won't trigger more events until we update the index.  However,
> now that we run with interrupts enabled a lot we also poll without a
> callback so that is different: disabling callbacks will help reduce the
> number of spurious interrupts.
> Further, if using event index with a packed ring, and if being called
> from a callback, we actually do disable interrupts which is unnecessary.
>
> Fix both issues by tracking whenever we get a callback. If that is
> the case disabling interrupts with event index can be a nop.


This seems unnecessary:

1) we check avail_flags_shadow before touching touching the index
2) The nop is not good at least for split, if we choose a suitable event 
index, it can help to reduce the chance of 1/N interrupt, (see below).


> If not the case disable interrupts. Note: with a split ring
> there's no explicit "no interrupts" value. For now we write
> a fixed value so our chance of triggering an interupt
> is 1/ring size.


1/65535 actually? If yes, do we still need this trick?


>   It's probably better to write something
> related to the last used index there to reduce the chance
> even further. For now I'm keeping it simple.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>   drivers/virtio/virtio_ring.c | 26 +++++++++++++++++++++++++-
>   1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 71e16b53e9c1..88f0b16b11b8 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -113,6 +113,9 @@ struct vring_virtqueue {
>   	/* Last used index we've seen. */
>   	u16 last_used_idx;
>   
> +	/* Hint for event idx: already triggered no need to disable. */
> +	bool event_triggered;
> +
>   	union {
>   		/* Available for split ring */
>   		struct {
> @@ -739,7 +742,10 @@ static void virtqueue_disable_cb_split(struct virtqueue *_vq)
>   
>   	if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
>   		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> +		if (vq->event)
> +			/* TODO: this is a hack. Figure out a cleaner value to write. */
> +			vring_used_event(&vq->split.vring) = 0x0;


used_idx or last_used_idx seems better here.


> +		else
>   			vq->split.vring.avail->flags =
>   				cpu_to_virtio16(_vq->vdev,
>   						vq->split.avail_flags_shadow);
> @@ -1605,6 +1611,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
>   	vq->weak_barriers = weak_barriers;
>   	vq->broken = false;
>   	vq->last_used_idx = 0;
> +	vq->event_triggered = false;
>   	vq->num_added = 0;
>   	vq->packed_ring = true;
>   	vq->use_dma_api = vring_use_dma_api(vdev);
> @@ -1919,6 +1926,12 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> +	/* If device triggered an event already it won't trigger one again:
> +	 * no need to disable.
> +	 */
> +	if (vq->event_triggered)
> +		return;
> +
>   	if (vq->packed_ring)
>   		virtqueue_disable_cb_packed(_vq);
>   	else
> @@ -1942,6 +1955,9 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> +	if (vq->event_triggered)
> +		vq->event_triggered = false;
> +
>   	return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(_vq) :
>   				 virtqueue_enable_cb_prepare_split(_vq);
>   }
> @@ -2005,6 +2021,9 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> +	if (vq->event_triggered)
> +		vq->event_triggered = false;
> +
>   	return vq->packed_ring ? virtqueue_enable_cb_delayed_packed(_vq) :
>   				 virtqueue_enable_cb_delayed_split(_vq);
>   }


Miss the case of virtqueue_enable_cb()?

Thanks


> @@ -2044,6 +2063,10 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
>   	if (unlikely(vq->broken))
>   		return IRQ_HANDLED;
>   
> +	/* Just a hint for performance: so it's ok that this can be racy! */
> +	if (vq->event)
> +		vq->event_triggered = true;
> +
>   	pr_debug("virtqueue callback for %p (%p)\n", vq, vq->vq.callback);
>   	if (vq->vq.callback)
>   		vq->vq.callback(&vq->vq);
> @@ -2083,6 +2106,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>   	vq->weak_barriers = weak_barriers;
>   	vq->broken = false;
>   	vq->last_used_idx = 0;
> +	vq->event_triggered = false;
>   	vq->num_added = 0;
>   	vq->use_dma_api = vring_use_dma_api(vdev);
>   #ifdef DEBUG