Virtualization - Apr 2021 - [PATCH RFC v2 1/4] virtio: fix up virtio_disable_cb

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.
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. 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 at redhat.com>
---
 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;
+		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);
 }
@@ -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
-- 
MST

? 2021/4/13 ??1:47, Michael S. Tsirkin ??:
> 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.
> 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. 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.

So I wonder whether last_used_idx is better, it looks to me the device 
will never move used index "across" that:

https://lore.kernel.org/patchwork/patch/946475/

And it looks to me it's better to move the optimization 
(event_triggered) into a separated patch.

>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>   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;
> +		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;

I guess we nee to check vq->event as well.

Thanks

> +
>   	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);
>   }
> @@ -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

On Tue, Apr 13, 2021 at 1:47 AM Michael S. Tsirkin <mst at redhat.com>
wrote:
>
> 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.
The device may poll for transmit completions as a result of an interrupt
from virtnet_poll_tx.

As well as asynchronously to this transmit interrupt, from start_xmit or
from virtnet_poll_cleantx as a result of a receive interrupt.

As of napi-tx, transmit interrupts are left enabled to operate in standard
napi mode. While previously they would be left disabled for most of the
time, enabling only when the queue as low on descriptors.

(in practice, for the at the time common case of split ring with event index,
little changed, as that mode does not actually enable/disable the interrupt,
but looks at the consumer index in the ring to decide whether to interrupt)

Combined, this may cause the following:

1. device sends a packet and fires transmit interrupt
2. driver cleans interrupts using virtnet_poll_cleantx
3. driver handles transmit interrupt using vring_interrupt,
    detects that the vring is empty: !more_used(vq),
    and records a spurious interrupt.

I don't quite follow how suppressing interrupt suppression, i.e.,
skipping disable_cb, helps avoid this.

I'm probably missing something. Is this solving a subtly different
problem from the one as I understand it?
> 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.
> 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. 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 at redhat.com>