Re: [dpdk-dev] [PATCH 1/3] vhost: pre update used ring for Tx and Rx

["Xie, Huawei" <huawei.xie@intel.com>]

On 5/3/2016 8:42 AM, Yuanhan Liu wrote:
> Pre update and update used ring in batch for Tx and Rx at the stage
> while fetching all avail desc idx. This would reduce some cache misses
> and hence, increase the performance a bit.
>
> Pre update would be feasible as guest driver will not start processing
> those entries as far as we don't update "used->idx". (I'm not 100%
> certain I don't miss anything, though).
>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> ---
>  lib/librte_vhost/vhost_rxtx.c | 58 +++++++++++++++++++++----------------------
>  1 file changed, 28 insertions(+), 30 deletions(-)
>
> diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
> index c9cd1c5..2c3b810 100644
> --- a/lib/librte_vhost/vhost_rxtx.c
> +++ b/lib/librte_vhost/vhost_rxtx.c
> @@ -137,7 +137,7 @@ copy_virtio_net_hdr(struct virtio_net *dev, uint64_t desc_addr,
>  
>  static inline int __attribute__((always_inline))
>  copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
> -		  struct rte_mbuf *m, uint16_t desc_idx, uint32_t *copied)
> +		  struct rte_mbuf *m, uint16_t desc_idx)
>  {
>  	uint32_t desc_avail, desc_offset;
>  	uint32_t mbuf_avail, mbuf_offset;
> @@ -161,7 +161,6 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  	desc_offset = dev->vhost_hlen;
>  	desc_avail  = desc->len - dev->vhost_hlen;
>  
> -	*copied = rte_pktmbuf_pkt_len(m);
>  	mbuf_avail  = rte_pktmbuf_data_len(m);
>  	mbuf_offset = 0;
>  	while (mbuf_avail != 0 || m->next != NULL) {
> @@ -262,6 +261,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
>  	struct vhost_virtqueue *vq;
>  	uint16_t res_start_idx, res_end_idx;
>  	uint16_t desc_indexes[MAX_PKT_BURST];
> +	uint16_t used_idx;
>  	uint32_t i;
>  
>  	LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
> @@ -285,27 +285,29 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
>  	/* Retrieve all of the desc indexes first to avoid caching issues. */
>  	rte_prefetch0(&vq->avail->ring[res_start_idx & (vq->size - 1)]);
>  	for (i = 0; i < count; i++) {
> -		desc_indexes[i] = vq->avail->ring[(res_start_idx + i) &
> -						  (vq->size - 1)];
> +		used_idx = (res_start_idx + i) & (vq->size - 1);
> +		desc_indexes[i] = vq->avail->ring[used_idx];
> +		vq->used->ring[used_idx].id = desc_indexes[i];
> +		vq->used->ring[used_idx].len = pkts[i]->pkt_len +
> +					       dev->vhost_hlen;
> +		vhost_log_used_vring(dev, vq,
> +			offsetof(struct vring_used, ring[used_idx]),
> +			sizeof(vq->used->ring[used_idx]));
>  	}
>  
>  	rte_prefetch0(&vq->desc[desc_indexes[0]]);
>  	for (i = 0; i < count; i++) {
>  		uint16_t desc_idx = desc_indexes[i];
> -		uint16_t used_idx = (res_start_idx + i) & (vq->size - 1);
> -		uint32_t copied;
>  		int err;
>  
> -		err = copy_mbuf_to_desc(dev, vq, pkts[i], desc_idx, &copied);
> -
> -		vq->used->ring[used_idx].id = desc_idx;
> -		if (unlikely(err))
> +		err = copy_mbuf_to_desc(dev, vq, pkts[i], desc_idx);
> +		if (unlikely(err)) {
> +			used_idx = (res_start_idx + i) & (vq->size - 1);
>  			vq->used->ring[used_idx].len = dev->vhost_hlen;
> -		else
> -			vq->used->ring[used_idx].len = copied + dev->vhost_hlen;
> -		vhost_log_used_vring(dev, vq,
> -			offsetof(struct vring_used, ring[used_idx]),
> -			sizeof(vq->used->ring[used_idx]));
> +			vhost_log_used_vring(dev, vq,
> +				offsetof(struct vring_used, ring[used_idx]),
> +				sizeof(vq->used->ring[used_idx]));
> +		}
>  
>  		if (i + 1 < count)
>  			rte_prefetch0(&vq->desc[desc_indexes[i+1]]);
> @@ -879,6 +881,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>  	/* Prefetch available ring to retrieve head indexes. */
>  	used_idx = vq->last_used_idx & (vq->size - 1);
>  	rte_prefetch0(&vq->avail->ring[used_idx]);
> +	rte_prefetch0(&vq->used->ring[used_idx]);
>  
>  	count = RTE_MIN(count, MAX_PKT_BURST);
>  	count = RTE_MIN(count, free_entries);
> @@ -887,22 +890,23 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>  
>  	/* Retrieve all of the head indexes first to avoid caching issues. */
>  	for (i = 0; i < count; i++) {
> -		desc_indexes[i] = vq->avail->ring[(vq->last_used_idx + i) &
> -					(vq->size - 1)];
> +		used_idx = (vq->last_used_idx + i) & (vq->size - 1);
> +		desc_indexes[i] = vq->avail->ring[used_idx];
> +
> +		vq->used->ring[used_idx].id  = desc_indexes[i];
> +		vq->used->ring[used_idx].len = 0;
> +		vhost_log_used_vring(dev, vq,
> +				offsetof(struct vring_used, ring[used_idx]),
> +				sizeof(vq->used->ring[used_idx]));
>  	}
>  
>  	/* Prefetch descriptor index. */
>  	rte_prefetch0(&vq->desc[desc_indexes[0]]);
> -	rte_prefetch0(&vq->used->ring[vq->last_used_idx & (vq->size - 1)]);
> -
>  	for (i = 0; i < count; i++) {
>  		int err;
>  
> -		if (likely(i + 1 < count)) {
> +		if (likely(i + 1 < count))
>  			rte_prefetch0(&vq->desc[desc_indexes[i + 1]]);
> -			rte_prefetch0(&vq->used->ring[(used_idx + 1) &
> -						      (vq->size - 1)]);
> -		}
>  
>  		pkts[i] = rte_pktmbuf_alloc(mbuf_pool);
>  		if (unlikely(pkts[i] == NULL)) {
> @@ -916,18 +920,12 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>  			rte_pktmbuf_free(pkts[i]);
>  			break;
>  		}
> -
> -		used_idx = vq->last_used_idx++ & (vq->size - 1);
> -		vq->used->ring[used_idx].id  = desc_indexes[i];
> -		vq->used->ring[used_idx].len = 0;
> -		vhost_log_used_vring(dev, vq,
> -				offsetof(struct vring_used, ring[used_idx]),
> -				sizeof(vq->used->ring[used_idx]));
>  	}

Had tried post-updating used ring in batch,  but forget the perf change.

One optimization would be on vhost_log_used_ring.
I have two ideas,
a) In QEMU side, we always assume use ring will be changed. so that we
don't need to log used ring in VHOST.

Michael: feasible in QEMU? comments on this?

b) We could always mark the total used ring modified rather than entry
by entry.


>  
>  	rte_smp_wmb();
>  	rte_smp_rmb();
>  	vq->used->idx += i;
> +	vq->last_used_idx += i;
>  	vhost_log_used_vring(dev, vq, offsetof(struct vring_used, idx),
>  			sizeof(vq->used->idx));
>