diff mbox series

vhost: avoid memory barriers when no descriptors dequeued

Message ID 20181019140058.4981-1-maxime.coquelin@redhat.com (mailing list archive)
State Superseded, archived
Delegated to: Maxime Coquelin
Headers
Series vhost: avoid memory barriers when no descriptors dequeued |

Checks

Context Check Description
ci/Intel-compilation success Compilation OK

Commit Message

Maxime Coquelin Oct. 19, 2018, 2 p.m. UTC 
  In both split and packed dequeue paths, flush_shadow_used_ring
and vhost_ring_call variants gets called even if not packets
have been dequeued, and so no descriptors updates happened.

It has an impact on CPU pipeline, as memory barriers are used
in these functions.

This patch don't call these functions if no descriptors have
been dequeued. The performance gain with split ring when
dequeue zero-copy is disabled should be null, but should be
noticeable with packed ring or dequeue zero-copy enabled.

Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring")
Fixes: 915cf9404225 ("vhost: use shadow used ring in dequeue path")
Cc: stable@dpdk.org

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/librte_vhost/virtio_net.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

Comments

Tiwei Bie Oct. 22, 2018, 7:15 a.m. UTC | #1 
On Fri, Oct 19, 2018 at 04:00:58PM +0200, Maxime Coquelin wrote:
> In both split and packed dequeue paths, flush_shadow_used_ring
> and vhost_ring_call variants gets called even if not packets
> have been dequeued, and so no descriptors updates happened.
> 
> It has an impact on CPU pipeline, as memory barriers are used
> in these functions.
> 
> This patch don't call these functions if no descriptors have
> been dequeued. The performance gain with split ring when
> dequeue zero-copy is disabled should be null, but should be
> noticeable with packed ring or dequeue zero-copy enabled.
> 
> Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring")
> Fixes: 915cf9404225 ("vhost: use shadow used ring in dequeue path")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  lib/librte_vhost/virtio_net.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
[...]
>  
> -	if (likely(dev->dequeue_zero_copy == 0)) {
> +	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {
>  		do_data_copy_dequeue(vq);
>  		if (unlikely(i < count))
>  			vq->shadow_used_idx = i;

When i is 0, we may need to update vq->shadow_used_idx to 0,
e.g. when error happens after update_shadow_used_ring_split()
in the first iteration of the loop.

> @@ -1475,8 +1477,10 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  			}
>  		}
>  
> -		flush_shadow_used_ring_packed(dev, vq);
> -		vhost_vring_call_packed(dev, vq);
> +		if (likely(vq->shadow_used_idx)) {
> +			flush_shadow_used_ring_packed(dev, vq);
> +			vhost_vring_call_packed(dev, vq);
> +		}
>  	}
>  
>  	VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
> @@ -1550,7 +1554,7 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  		}
>  	}
>  
> -	if (likely(dev->dequeue_zero_copy == 0)) {
> +	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {

Ditto

>  		do_data_copy_dequeue(vq);
>  		if (unlikely(i < count))
>  			vq->shadow_used_idx = i;
> -- 
> 2.17.1
>
Maxime Coquelin Oct. 22, 2018, 8:31 a.m. UTC | #2 
On 10/22/2018 09:15 AM, Tiwei Bie wrote:
> On Fri, Oct 19, 2018 at 04:00:58PM +0200, Maxime Coquelin wrote:
>> In both split and packed dequeue paths, flush_shadow_used_ring
>> and vhost_ring_call variants gets called even if not packets
>> have been dequeued, and so no descriptors updates happened.
>>
>> It has an impact on CPU pipeline, as memory barriers are used
>> in these functions.
>>
>> This patch don't call these functions if no descriptors have
>> been dequeued. The performance gain with split ring when
>> dequeue zero-copy is disabled should be null, but should be
>> noticeable with packed ring or dequeue zero-copy enabled.
>>
>> Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring")
>> Fixes: 915cf9404225 ("vhost: use shadow used ring in dequeue path")
>> Cc: stable@dpdk.org
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>> ---
>>   lib/librte_vhost/virtio_net.c | 16 ++++++++++------
>>   1 file changed, 10 insertions(+), 6 deletions(-)
> [...]
>>   
>> -	if (likely(dev->dequeue_zero_copy == 0)) {
>> +	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {
>>   		do_data_copy_dequeue(vq);
>>   		if (unlikely(i < count))
>>   			vq->shadow_used_idx = i;
> 
> When i is 0, we may need to update vq->shadow_used_idx to 0,
> e.g. when error happens after update_shadow_used_ring_split()
> in the first iteration of the loop.

I totally agree, it is broken when error happens.
I will fix that in next revision.

Thanks,
Maxime

> 
>> @@ -1475,8 +1477,10 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>>   			}
>>   		}
>>   
>> -		flush_shadow_used_ring_packed(dev, vq);
>> -		vhost_vring_call_packed(dev, vq);
>> +		if (likely(vq->shadow_used_idx)) {
>> +			flush_shadow_used_ring_packed(dev, vq);
>> +			vhost_vring_call_packed(dev, vq);
>> +		}
>>   	}
>>   
>>   	VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
>> @@ -1550,7 +1554,7 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>>   		}
>>   	}
>>   
>> -	if (likely(dev->dequeue_zero_copy == 0)) {
>> +	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {
> 
> Ditto
> 
>>   		do_data_copy_dequeue(vq);
>>   		if (unlikely(i < count))
>>   			vq->shadow_used_idx = i;
>> -- 
>> 2.17.1
>>
diff mbox series

Patch

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index f8794ee19..48228f16b 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -1359,8 +1359,10 @@  virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			}
 		}
 
-		flush_shadow_used_ring_split(dev, vq);
-		vhost_vring_call_split(dev, vq);
+		if (likely(vq->shadow_used_idx)) {
+			flush_shadow_used_ring_split(dev, vq);
+			vhost_vring_call_split(dev, vq);
+		}
 	}
 
 	rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
@@ -1435,7 +1437,7 @@  virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	}
 	vq->last_avail_idx += i;
 
-	if (likely(dev->dequeue_zero_copy == 0)) {
+	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {
 		do_data_copy_dequeue(vq);
 		if (unlikely(i < count))
 			vq->shadow_used_idx = i;
@@ -1475,8 +1477,10 @@  virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			}
 		}
 
-		flush_shadow_used_ring_packed(dev, vq);
-		vhost_vring_call_packed(dev, vq);
+		if (likely(vq->shadow_used_idx)) {
+			flush_shadow_used_ring_packed(dev, vq);
+			vhost_vring_call_packed(dev, vq);
+		}
 	}
 
 	VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
@@ -1550,7 +1554,7 @@  virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		}
 	}
 
-	if (likely(dev->dequeue_zero_copy == 0)) {
+	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {
 		do_data_copy_dequeue(vq);
 		if (unlikely(i < count))
 			vq->shadow_used_idx = i;