Re: [PATCH RFC 03/13] vhost: batching fetches

[Jason Wang <jasowang@redhat.com>]

On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
> With this patch applied, new and old code perform identically.
>
> Lots of extra optimizations are now possible, e.g.
> we can fetch multiple heads with copy_from/to_user now.
> We can get rid of maintaining the log array.  Etc etc.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> Link: https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>   drivers/vhost/test.c  |  2 +-
>   drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
>   drivers/vhost/vhost.h |  5 ++++-
>   3 files changed, 47 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> index 9a3a09005e03..02806d6f84ef 100644
> --- a/drivers/vhost/test.c
> +++ b/drivers/vhost/test.c
> @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
>   	dev = &n->dev;
>   	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
>   	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
>   		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
>   
>   	f->private_data = n;
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8f9a07282625..aca2a5b0d078 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>   {
>   	vq->num = 1;
>   	vq->ndescs = 0;
> +	vq->first_desc = 0;
>   	vq->desc = NULL;
>   	vq->avail = NULL;
>   	vq->used = NULL;
> @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
>   	return 0;
>   }
>   
> +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> +{
> +	return vq->max_descs - UIO_MAXIOV;
> +}


1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte 
length memory regions for us to translate.


> +
>   static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
>   {
>   	kfree(vq->descs);
> @@ -389,6 +395,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
>   	for (i = 0; i < dev->nvqs; ++i) {
>   		vq = dev->vqs[i];
>   		vq->max_descs = dev->iov_limit;
> +		if (vhost_vq_num_batch_descs(vq) < 0) {
> +			return -EINVAL;
> +		}
>   		vq->descs = kmalloc_array(vq->max_descs,
>   					  sizeof(*vq->descs),
>   					  GFP_KERNEL);
> @@ -1570,6 +1579,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
>   		vq->last_avail_idx = s.num;
>   		/* Forget the cached index value. */
>   		vq->avail_idx = vq->last_avail_idx;
> +		vq->ndescs = vq->first_desc = 0;
>   		break;
>   	case VHOST_GET_VRING_BASE:
>   		s.index = idx;
> @@ -2136,7 +2146,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
>   	return 0;
>   }
>   
> -static int fetch_descs(struct vhost_virtqueue *vq)
> +static int fetch_buf(struct vhost_virtqueue *vq)
>   {
>   	unsigned int i, head, found = 0;
>   	struct vhost_desc *last;
> @@ -2149,7 +2159,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>   	/* Check it isn't doing very strange things with descriptor numbers. */
>   	last_avail_idx = vq->last_avail_idx;
>   
> -	if (vq->avail_idx == vq->last_avail_idx) {
> +	if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
> +		/* If we already have work to do, don't bother re-checking. */
> +		if (likely(vq->ndescs))
> +			return vq->num;
> +
>   		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
>   			vq_err(vq, "Failed to access avail idx at %p\n",
>   				&vq->avail->idx);
> @@ -2240,6 +2254,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>   	return 0;
>   }
>   
> +static int fetch_descs(struct vhost_virtqueue *vq)
> +{
> +	int ret = 0;
> +
> +	if (unlikely(vq->first_desc >= vq->ndescs)) {
> +		vq->first_desc = 0;
> +		vq->ndescs = 0;
> +	}
> +
> +	if (vq->ndescs)
> +		return 0;
> +
> +	while (!ret && vq->ndescs <= vhost_vq_num_batch_descs(vq))
> +		ret = fetch_buf(vq);
> +
> +	return vq->ndescs ? 0 : ret;
> +}
> +
>   /* This looks in the virtqueue and for the first available buffer, and converts
>    * it to an iovec for convenient access.  Since descriptors consist of some
>    * number of output then some number of input descriptors, it's actually two
> @@ -2265,7 +2297,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>   	if (unlikely(log))
>   		*log_num = 0;
>   
> -	for (i = 0; i < vq->ndescs; ++i) {
> +	for (i = vq->first_desc; i < vq->ndescs; ++i) {
>   		unsigned iov_count = *in_num + *out_num;
>   		struct vhost_desc *desc = &vq->descs[i];
>   		int access;
> @@ -2311,14 +2343,19 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>   		}
>   
>   		ret = desc->id;
> +
> +		if (!(desc->flags & VRING_DESC_F_NEXT))
> +			break;
>   	}
>   
> -	vq->ndescs = 0;
> +	vq->first_desc = i + 1;
>   
>   	return ret;
>   
>   err:
> -	vhost_discard_vq_desc(vq, 1);
> +	for (i = vq->first_desc; i < vq->ndescs; ++i)
> +		if (!(vq->descs[i].flags & VRING_DESC_F_NEXT))
> +			vhost_discard_vq_desc(vq, 1);
>   	vq->ndescs = 0;
>   
>   	return ret;
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 76356edee8e5..a67bda9792ec 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -81,6 +81,7 @@ struct vhost_virtqueue {
>   
>   	struct vhost_desc *descs;
>   	int ndescs;
> +	int first_desc;
>   	int max_descs;
>   
>   	struct file *kick;
> @@ -229,7 +230,7 @@ void vhost_iotlb_map_free(struct vhost_iotlb *iotlb,
>   			  struct vhost_iotlb_map *map);
>   
>   #define vq_err(vq, fmt, ...) do {                                  \
> -		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
> +		pr_err(pr_fmt(fmt), ##__VA_ARGS__);       \


Need a separate patch for this?

Thanks


>   		if ((vq)->error_ctx)                               \
>   				eventfd_signal((vq)->error_ctx, 1);\
>   	} while (0)
> @@ -255,6 +256,8 @@ static inline void vhost_vq_set_backend(struct vhost_virtqueue *vq,
>   					void *private_data)
>   {
>   	vq->private_data = private_data;
> +	vq->ndescs = 0;
> +	vq->first_desc = 0;
>   }
>   
>   /**