Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1696862 > unrolled thread

[PATCH net] Revert "vhost: cache used event for better performance"

Back to article view | Back to linux.kernel

Contents

  [PATCH net] Revert "vhost: cache used event for better performance" Jason Wang <jasowang@redhat.com> - 2017-07-26 10:10 +0200
    Re: [PATCH net] Revert "vhost: cache used event for better  performance" Christian Borntraeger <borntraeger@de.ibm.com> - 2017-07-26 13:00 +0200
      Re: [PATCH net] Revert "vhost: cache used event for better  performance" Jason Wang <jasowang@redhat.com> - 2017-07-26 14:00 +0200
    Re: [PATCH net] Revert "vhost: cache used event for better  performance" "Michael S. Tsirkin" <mst@redhat.com> - 2017-07-26 15:00 +0200
      Re: [PATCH net] Revert "vhost: cache used event for better  performance" Jason Wang <jasowang@redhat.com> - 2017-07-26 15:20 +0200
        Re: [PATCH net] Revert "vhost: cache used event for better  performance" Jason Wang <jasowang@redhat.com> - 2017-07-26 15:40 +0200
          Re: [PATCH net] Revert "vhost: cache used event for better  performance" "Michael S. Tsirkin" <mst@redhat.com> - 2017-07-26 18:10 +0200
            Re: [PATCH net] Revert "vhost: cache used event for better  performance" "K. Den" <den@klaipeden.com> - 2017-07-30 08:30 +0200
        Re: [PATCH net] Revert "vhost: cache used event for better  performance" "Michael S. Tsirkin" <mst@redhat.com> - 2017-07-26 17:30 +0200

#1696862 — [PATCH net] Revert "vhost: cache used event for better performance"

This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
was reported to break vhost_net. We want to cache used event and use
it to check for notification. We try to valid cached used event by
checking whether or not it was ahead of new, but this is not correct
all the time, it could be stale and there's no way to know about this.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/vhost/vhost.c | 28 ++++++----------------------
 drivers/vhost/vhost.h |  3 ---
 2 files changed, 6 insertions(+), 25 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index e4613a3..9cb3f72 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -308,7 +308,6 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	vq->avail = NULL;
 	vq->used = NULL;
 	vq->last_avail_idx = 0;
-	vq->last_used_event = 0;
 	vq->avail_idx = 0;
 	vq->last_used_idx = 0;
 	vq->signalled_used = 0;
@@ -1402,7 +1401,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
 			r = -EINVAL;
 			break;
 		}
-		vq->last_avail_idx = vq->last_used_event = s.num;
+		vq->last_avail_idx = s.num;
 		/* Forget the cached index value. */
 		vq->avail_idx = vq->last_avail_idx;
 		break;
@@ -2241,6 +2240,10 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	__u16 old, new;
 	__virtio16 event;
 	bool v;
+	/* Flush out used index updates. This is paired
+	 * with the barrier that the Guest executes when enabling
+	 * interrupts. */
+	smp_mb();
 
 	if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
 	    unlikely(vq->avail_idx == vq->last_avail_idx))
@@ -2248,10 +2251,6 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 
 	if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
 		__virtio16 flags;
-		/* Flush out used index updates. This is paired
-		 * with the barrier that the Guest executes when enabling
-		 * interrupts. */
-		smp_mb();
 		if (vhost_get_avail(vq, flags, &vq->avail->flags)) {
 			vq_err(vq, "Failed to get flags");
 			return true;
@@ -2266,26 +2265,11 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	if (unlikely(!v))
 		return true;
 
-	/* We're sure if the following conditions are met, there's no
-	 * need to notify guest:
-	 * 1) cached used event is ahead of new
-	 * 2) old to new updating does not cross cached used event. */
-	if (vring_need_event(vq->last_used_event, new + vq->num, new) &&
-	    !vring_need_event(vq->last_used_event, new, old))
-		return false;
-
-	/* Flush out used index updates. This is paired
-	 * with the barrier that the Guest executes when enabling
-	 * interrupts. */
-	smp_mb();
-
 	if (vhost_get_avail(vq, event, vhost_used_event(vq))) {
 		vq_err(vq, "Failed to get used event idx");
 		return true;
 	}
-	vq->last_used_event = vhost16_to_cpu(vq, event);
-
-	return vring_need_event(vq->last_used_event, new, old);
+	return vring_need_event(vhost16_to_cpu(vq, event), new, old);
 }
 
 /* This actually signals the guest, using eventfd. */
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index f720958..bb7c29b 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -115,9 +115,6 @@ struct vhost_virtqueue {
 	/* Last index we used. */
 	u16 last_used_idx;
 
-	/* Last used evet we've seen */
-	u16 last_used_event;
-
 	/* Used flags */
 	u16 used_flags;
 
-- 
2.7.4

[toc] | [next] | [standalone]

#1696986 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On 07/26/2017 10:03 AM, Jason Wang wrote:
> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
> was reported to break vhost_net. We want to cache used event and use
> it to check for notification. We try to valid cached used event by
> checking whether or not it was ahead of new, but this is not correct
> all the time, it could be stale and there's no way to know about this.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>

This would then qualify for stable ?

[toc] | [prev] | [next] | [standalone]

#1697017 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On 2017年07月26日 18:53, Christian Borntraeger wrote:
> On 07/26/2017 10:03 AM, Jason Wang wrote:
>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
>> was reported to break vhost_net. We want to cache used event and use
>> it to check for notification. We try to valid cached used event by
>> checking whether or not it was ahead of new, but this is not correct
>> all the time, it could be stale and there's no way to know about this.
>>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> This would then qualify for stable ?
>

Yes it is.

Thanks

[toc] | [prev] | [next] | [standalone]

#1697065 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
> was reported to break vhost_net. We want to cache used event and use
> it to check for notification. We try to valid cached used event by
> checking whether or not it was ahead of new, but this is not correct
> all the time, it could be stale and there's no way to know about this.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>


Could you supply a bit more data here please?  How does it get stale?
What does guest need to do to make it stale?  This will be helpful if
anyone wants to bring it back, or if we want to extend the protocol.

> ---
>  drivers/vhost/vhost.c | 28 ++++++----------------------
>  drivers/vhost/vhost.h |  3 ---
>  2 files changed, 6 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index e4613a3..9cb3f72 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -308,7 +308,6 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->avail = NULL;
>  	vq->used = NULL;
>  	vq->last_avail_idx = 0;
> -	vq->last_used_event = 0;
>  	vq->avail_idx = 0;
>  	vq->last_used_idx = 0;
>  	vq->signalled_used = 0;
> @@ -1402,7 +1401,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>  			r = -EINVAL;
>  			break;
>  		}
> -		vq->last_avail_idx = vq->last_used_event = s.num;
> +		vq->last_avail_idx = s.num;
>  		/* Forget the cached index value. */
>  		vq->avail_idx = vq->last_avail_idx;
>  		break;
> @@ -2241,6 +2240,10 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	__u16 old, new;
>  	__virtio16 event;
>  	bool v;
> +	/* Flush out used index updates. This is paired
> +	 * with the barrier that the Guest executes when enabling
> +	 * interrupts. */
> +	smp_mb();
>  
>  	if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
>  	    unlikely(vq->avail_idx == vq->last_avail_idx))
> @@ -2248,10 +2251,6 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  
>  	if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
>  		__virtio16 flags;
> -		/* Flush out used index updates. This is paired
> -		 * with the barrier that the Guest executes when enabling
> -		 * interrupts. */
> -		smp_mb();
>  		if (vhost_get_avail(vq, flags, &vq->avail->flags)) {
>  			vq_err(vq, "Failed to get flags");
>  			return true;
> @@ -2266,26 +2265,11 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	if (unlikely(!v))
>  		return true;
>  
> -	/* We're sure if the following conditions are met, there's no
> -	 * need to notify guest:
> -	 * 1) cached used event is ahead of new
> -	 * 2) old to new updating does not cross cached used event. */
> -	if (vring_need_event(vq->last_used_event, new + vq->num, new) &&
> -	    !vring_need_event(vq->last_used_event, new, old))
> -		return false;
> -
> -	/* Flush out used index updates. This is paired
> -	 * with the barrier that the Guest executes when enabling
> -	 * interrupts. */
> -	smp_mb();
> -
>  	if (vhost_get_avail(vq, event, vhost_used_event(vq))) {
>  		vq_err(vq, "Failed to get used event idx");
>  		return true;
>  	}
> -	vq->last_used_event = vhost16_to_cpu(vq, event);
> -
> -	return vring_need_event(vq->last_used_event, new, old);
> +	return vring_need_event(vhost16_to_cpu(vq, event), new, old);
>  }
>  
>  /* This actually signals the guest, using eventfd. */
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index f720958..bb7c29b 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -115,9 +115,6 @@ struct vhost_virtqueue {
>  	/* Last index we used. */
>  	u16 last_used_idx;
>  
> -	/* Last used evet we've seen */
> -	u16 last_used_event;
> -
>  	/* Used flags */
>  	u16 used_flags;
>  
> -- 
> 2.7.4

[toc] | [prev] | [next] | [standalone]

#1697086 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On 2017年07月26日 20:57, Michael S. Tsirkin wrote:
> On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
>> was reported to break vhost_net. We want to cache used event and use
>> it to check for notification. We try to valid cached used event by
>> checking whether or not it was ahead of new, but this is not correct
>> all the time, it could be stale and there's no way to know about this.
>>
>> Signed-off-by: Jason Wang<jasowang@redhat.com>
> Could you supply a bit more data here please?  How does it get stale?
> What does guest need to do to make it stale?  This will be helpful if
> anyone wants to bring it back, or if we want to extend the protocol.
>

The problem we don't know whether or not guest has published a new used 
event. The check vring_need_event(vq->last_used_event, new + vq->num, 
new) is not sufficient to check for this.

Thanks

[toc] | [prev] | [next] | [standalone]

#1697092 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On 2017年07月26日 21:18, Jason Wang wrote:
>
>
> On 2017年07月26日 20:57, Michael S. Tsirkin wrote:
>> On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
>>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
>>> was reported to break vhost_net. We want to cache used event and use
>>> it to check for notification. We try to valid cached used event by
>>> checking whether or not it was ahead of new, but this is not correct
>>> all the time, it could be stale and there's no way to know about this.
>>>
>>> Signed-off-by: Jason Wang<jasowang@redhat.com>
>> Could you supply a bit more data here please?  How does it get stale?
>> What does guest need to do to make it stale?  This will be helpful if
>> anyone wants to bring it back, or if we want to extend the protocol.
>>
>
> The problem we don't know whether or not guest has published a new 
> used event. The check vring_need_event(vq->last_used_event, new + 
> vq->num, new) is not sufficient to check for this.
>
> Thanks

More notes, the previous assumption is that we don't move used event 
back, but this could happen in fact if idx is wrapper around. Will 
repost and add this into commit log.

Thanks

[toc] | [prev] | [next] | [standalone]

#1697324 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On Wed, Jul 26, 2017 at 09:37:15PM +0800, Jason Wang wrote:
> 
> 
> On 2017年07月26日 21:18, Jason Wang wrote:
> > 
> > 
> > On 2017年07月26日 20:57, Michael S. Tsirkin wrote:
> > > On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
> > > > This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
> > > > was reported to break vhost_net. We want to cache used event and use
> > > > it to check for notification. We try to valid cached used event by
> > > > checking whether or not it was ahead of new, but this is not correct
> > > > all the time, it could be stale and there's no way to know about this.
> > > > 
> > > > Signed-off-by: Jason Wang<jasowang@redhat.com>
> > > Could you supply a bit more data here please?  How does it get stale?
> > > What does guest need to do to make it stale?  This will be helpful if
> > > anyone wants to bring it back, or if we want to extend the protocol.
> > > 
> > 
> > The problem we don't know whether or not guest has published a new used
> > event. The check vring_need_event(vq->last_used_event, new + vq->num,
> > new) is not sufficient to check for this.
> > 
> > Thanks
> 
> More notes, the previous assumption is that we don't move used event back,
> but this could happen in fact if idx is wrapper around.

You mean if the 16 bit index wraps around after 64K entries.
Makes sense.

> Will repost and add
> this into commit log.
> 
> Thanks

[toc] | [prev] | [next] | [standalone]

#1699422 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On Wed, 2017-07-26 at 19:08 +0300, Michael S. Tsirkin wrote:
> On Wed, Jul 26, 2017 at 09:37:15PM +0800, Jason Wang wrote:
> > 
> > 
> > On 2017年07月26日 21:18, Jason Wang wrote:
> > > 
> > > 
> > > On 2017年07月26日 20:57, Michael S. Tsirkin wrote:
> > > > On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
> > > > > This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
> > > > > was reported to break vhost_net. We want to cache used event and use
> > > > > it to check for notification. We try to valid cached used event by
> > > > > checking whether or not it was ahead of new, but this is not correct
> > > > > all the time, it could be stale and there's no way to know about this.
> > > > > 
> > > > > Signed-off-by: Jason Wang<jasowang@redhat.com>
> > > > 
> > > > Could you supply a bit more data here please?  How does it get stale?
> > > > What does guest need to do to make it stale?  This will be helpful if
> > > > anyone wants to bring it back, or if we want to extend the protocol.
> > > > 
> > > 
> > > The problem we don't know whether or not guest has published a new used
> > > event. The check vring_need_event(vq->last_used_event, new + vq->num,
> > > new) is not sufficient to check for this.
> > > 
> > > Thanks
> > 
> > More notes, the previous assumption is that we don't move used event back,
> > but this could happen in fact if idx is wrapper around.
> 
> You mean if the 16 bit index wraps around after 64K entries.
> Makes sense.
> 
> > Will repost and add
> > this into commit log.
> > 
> > Thanks

Hi,

I am just curious but I have got a question:
AFAIU, if you wanted to keep the caching mechanism alive in the code base,
the following two changes could clear off the issue, or not?:
(1) Always fetch the latest event value from guest when signalled_used event is
invalid, which includes last_used_idx wraps-around case. Otherwise we might need
changes which would complicate too much the logic to properly decide whether or
not to skip signalling in the next vhost_notify round.
(2) On top of that, split the signal-postponing logic to three cases like:
* if the interval of vq.num is [2^16, UINT_MAX]:
any cached event is in should-postpone-signalling interval, so paradoxically
must always do signalling.
* else if the interval of vq.num is [2^15, 2^16):
the logic in the original patch (809ecb9bca6a9) suffices
* else (= less than 2^15) (optional):
checking only (vring_need_event(vq->last_used_event, new + vq->num, new)
would suffice.

Am I missing something, or is this irrelevant?
I would appreciate if you could elaborate a bit more how the situation where
event idx wraps around and moves back would make trouble.

Thanks.

[toc] | [prev] | [next] | [standalone]

#1697281 — Re: [PATCH net] Revert "vhost: cache used event for better performance"

On Wed, Jul 26, 2017 at 09:18:02PM +0800, Jason Wang wrote:
> 
> 
> On 2017年07月26日 20:57, Michael S. Tsirkin wrote:
> > On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
> > > This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
> > > was reported to break vhost_net. We want to cache used event and use
> > > it to check for notification. We try to valid cached used event by
> > > checking whether or not it was ahead of new, but this is not correct
> > > all the time, it could be stale and there's no way to know about this.
> > > 
> > > Signed-off-by: Jason Wang<jasowang@redhat.com>
> > Could you supply a bit more data here please?  How does it get stale?
> > What does guest need to do to make it stale?  This will be helpful if
> > anyone wants to bring it back, or if we want to extend the protocol.
> > 
> 
> The problem we don't know whether or not guest has published a new used
> event. The check vring_need_event(vq->last_used_event, new + vq->num, new)
> is not sufficient to check for this.
> 
> Thanks

Figured it out after an offline discussion.

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web