Re: [dpdk-dev] vhost: batch used descriptors chains write-back with packed ring

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 12/5/18 5:01 PM, Ilya Maximets wrote:
> On 28.11.2018 12:47, Maxime Coquelin wrote:
>> Instead of writing back descriptors chains in order, let's
>> write the first chain flags last in order to improve batching.
> 
> I'm not sure if this fully compliant with virtio spec.
> It says that 'each side (driver and device) are only required to poll
> (or test) a single location in memory', but it does not forbid to
> test other descriptors. So, if the driver will try to check not only
> 'the next device descriptor after the one they processed previously,
> in circular order' but a few descriptors ahead, it could read an
> inconsistent memory because there are no more write barriers between
> updates for flags and id/len for them.
> 
> What do you think ?

Yes, that makes sense.
It should have no cost on x86 moreover.

I'll fix it in v2.
Thanks,
Maxime

>>
>> With Kernel's pktgen benchmark, ~3% performance gain is measured.
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>> Tested-by: Jens Freimann <jfreimann@redhat.com>
>> Reviewed-by: Jens Freimann <jfreimann@redhat.com>
>> ---
>>   lib/librte_vhost/virtio_net.c | 37 ++++++++++++++++++++++-------------
>>   1 file changed, 23 insertions(+), 14 deletions(-)
>>
>> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
>> index 5e1a1a727..f54642c2d 100644
>> --- a/lib/librte_vhost/virtio_net.c
>> +++ b/lib/librte_vhost/virtio_net.c
>> @@ -135,19 +135,10 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>>   			struct vhost_virtqueue *vq)
>>   {
>>   	int i;
>> -	uint16_t used_idx = vq->last_used_idx;
>> +	uint16_t head_flags, head_idx = vq->last_used_idx;
>>   
>> -	/* Split loop in two to save memory barriers */
>> -	for (i = 0; i < vq->shadow_used_idx; i++) {
>> -		vq->desc_packed[used_idx].id = vq->shadow_used_packed[i].id;
>> -		vq->desc_packed[used_idx].len = vq->shadow_used_packed[i].len;
>> -
>> -		used_idx += vq->shadow_used_packed[i].count;
>> -		if (used_idx >= vq->size)
>> -			used_idx -= vq->size;
>> -	}
>> -
>> -	rte_smp_wmb();
>> +	if (unlikely(vq->shadow_used_idx == 0))
>> +		return;
>>   
>>   	for (i = 0; i < vq->shadow_used_idx; i++) {
>>   		uint16_t flags;
>> @@ -165,12 +156,22 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>>   			flags &= ~VRING_DESC_F_AVAIL;
>>   		}
>>   
>> -		vq->desc_packed[vq->last_used_idx].flags = flags;
>> +		vq->desc_packed[vq->last_used_idx].id =
>> +			vq->shadow_used_packed[i].id;
>> +		vq->desc_packed[vq->last_used_idx].len =
>> +			vq->shadow_used_packed[i].len;
>> +
>> +		if (i > 0) {
>> +			vq->desc_packed[vq->last_used_idx].flags = flags;
>>   
>> -		vhost_log_cache_used_vring(dev, vq,
>> +			vhost_log_cache_used_vring(dev, vq,
>>   					vq->last_used_idx *
>>   					sizeof(struct vring_packed_desc),
>>   					sizeof(struct vring_packed_desc));
>> +		} else {
>> +			head_idx = vq->last_used_idx;
>> +			head_flags = flags;
>> +		}
>>   
>>   		vq->last_used_idx += vq->shadow_used_packed[i].count;
>>   		if (vq->last_used_idx >= vq->size) {
>> @@ -180,7 +181,15 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>>   	}
>>   
>>   	rte_smp_wmb();
>> +
>> +	vq->desc_packed[head_idx].flags = head_flags;
>>   	vq->shadow_used_idx = 0;
>> +
>> +	vhost_log_cache_used_vring(dev, vq,
>> +				head_idx *
>> +				sizeof(struct vring_packed_desc),
>> +				sizeof(struct vring_packed_desc));
>> +
>>   	vhost_log_cache_sync(dev, vq);
>>   }
>>   
>>