Re: [PATCH 02/16] virtio_ring: virtqueue_add_sgs, to add multiplesgs.

[Michael S. Tsirkin]

On Tue, Feb 19, 2013 at 06:26:20PM +1030, Rusty Russell wrote:
> virtio_scsi can really use this, to avoid the current hack of copying
> the whole sg array.  Some other things get slightly neater, too.
> 
> Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

Hmm, this makes add_buf a bit slower. virtio_test results
(I'll send a patch to update the test shortly):

Before:
0.09user 0.01system 0:00.12elapsed 91%CPU (0avgtext+0avgdata 480maxresident)k
0inputs+0outputs (0major+145minor)pagefaults 0swaps

After:
0.11user 0.01system 0:00.13elapsed 90%CPU (0avgtext+0avgdata 480maxresident)k
0inputs+0outputs (0major+145minor)pagefaults 0swaps



> ---
>  drivers/virtio/virtio_ring.c |  144 ++++++++++++++++++++++++++++++------------
>  include/linux/virtio.h       |    7 ++
>  2 files changed, 109 insertions(+), 42 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 245177c..27e31d3 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -100,14 +100,16 @@ struct vring_virtqueue
>  
>  /* Set up an indirect table of descriptors and add it to the queue. */
>  static int vring_add_indirect(struct vring_virtqueue *vq,
> -			      struct scatterlist sg[],
> -			      unsigned int out,
> -			      unsigned int in,
> +			      struct scatterlist *sgs[],
> +			      unsigned int total_sg,
> +			      unsigned int out_sgs,
> +			      unsigned int in_sgs,
>  			      gfp_t gfp)
>  {
>  	struct vring_desc *desc;
>  	unsigned head;
> -	int i;
> +	struct scatterlist *sg;
> +	int i, n;
>  
>  	/*
>  	 * We require lowmem mappings for the descriptors because
> @@ -116,25 +118,31 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
>  	 */
>  	gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH);
>  
> -	desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
> +	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
>  	if (!desc)
>  		return -ENOMEM;
>  
> -	/* Transfer entries from the sg list into the indirect page */
> -	for (i = 0; i < out; i++) {
> -		desc[i].flags = VRING_DESC_F_NEXT;
> -		desc[i].addr = sg_phys(sg);
> -		desc[i].len = sg->length;
> -		desc[i].next = i+1;
> -		sg++;
> +	/* Transfer entries from the sg lists into the indirect page */
> +	i = 0;
> +	for (n = 0; n < out_sgs; n++) {
> +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> +			desc[i].flags = VRING_DESC_F_NEXT;
> +			desc[i].addr = sg_phys(sg);
> +			desc[i].len = sg->length;
> +			desc[i].next = i+1;
> +			i++;
> +		}
>  	}
> -	for (; i < (out + in); i++) {
> -		desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
> -		desc[i].addr = sg_phys(sg);
> -		desc[i].len = sg->length;
> -		desc[i].next = i+1;
> -		sg++;
> +	for (; n < (out_sgs + in_sgs); n++) {
> +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> +			desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
> +			desc[i].addr = sg_phys(sg);
> +			desc[i].len = sg->length;
> +			desc[i].next = i+1;
> +			i++;
> +		}
>  	}
> +	BUG_ON(i != total_sg);
>  
>  	/* Last one doesn't continue. */
>  	desc[i-1].flags &= ~VRING_DESC_F_NEXT;
> @@ -176,8 +184,48 @@ int virtqueue_add_buf(struct virtqueue *_vq,
>  		      void *data,
>  		      gfp_t gfp)
>  {
> +	struct scatterlist *sgs[2];
> +	unsigned int i;
> +
> +	sgs[0] = sg;
> +	sgs[1] = sg + out;
> +
> +	/* Workaround until callers pass well-formed sgs. */
> +	for (i = 0; i < out + in; i++)
> +		sg_unmark_end(sg + i);
> +
> +	sg_mark_end(sg + out + in - 1);
> +	if (out && in)
> +		sg_mark_end(sg + out - 1);
> +
> +	return virtqueue_add_sgs(_vq, sgs, out ? 1 : 0, in ? 1 : 0, data, gfp);
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_add_buf);
> +
> +/**
> + * virtqueue_add_sgs - expose buffers to other end
> + * @vq: the struct virtqueue we're talking about.
> + * @sgs: array of terminated scatterlists.
> + * @out_num: the number of scatterlists readable by other side
> + * @in_num: the number of scatterlists which are writable (after readable ones)
> + * @data: the token identifying the buffer.
> + * @gfp: how to do memory allocations (if necessary).
> + *
> + * Caller must ensure we don't call this with other virtqueue operations
> + * at the same time (except where noted).
> + *
> + * Returns zero or a negative error (ie. ENOSPC, ENOMEM).
> + */
> +int virtqueue_add_sgs(struct virtqueue *_vq,
> +		      struct scatterlist *sgs[],
> +		      unsigned int out_sgs,
> +		      unsigned int in_sgs,
> +		      void *data,
> +		      gfp_t gfp)
> +{
>  	struct vring_virtqueue *vq = to_vvq(_vq);
> -	unsigned int i, avail, uninitialized_var(prev);
> +	struct scatterlist *sg;
> +	unsigned int i, n, avail, uninitialized_var(prev), total_sg;
>  	int head;
>  
>  	START_USE(vq);
> @@ -197,46 +245,58 @@ int virtqueue_add_buf(struct virtqueue *_vq,
>  	}
>  #endif
>  
> +	/* Count them first. */
> +	for (i = total_sg = 0; i < out_sgs + in_sgs; i++) {
> +		struct scatterlist *sg;
> +		for (sg = sgs[i]; sg; sg = sg_next(sg))
> +			total_sg++;
> +	}
> +
>  	/* If the host supports indirect descriptor tables, and we have multiple
>  	 * buffers, then go indirect. FIXME: tune this threshold */
> -	if (vq->indirect && (out + in) > 1 && vq->vq.num_free) {
> -		head = vring_add_indirect(vq, sg, out, in, gfp);
> +	if (vq->indirect && total_sg > 1 && vq->vq.num_free) {
> +		head = vring_add_indirect(vq, sgs, total_sg, out_sgs, in_sgs,
> +					  gfp);
>  		if (likely(head >= 0))
>  			goto add_head;
>  	}
>  
> -	BUG_ON(out + in > vq->vring.num);
> -	BUG_ON(out + in == 0);
> +	BUG_ON(total_sg > vq->vring.num);
> +	BUG_ON(total_sg == 0);
>  
> -	if (vq->vq.num_free < out + in) {
> +	if (vq->vq.num_free < total_sg) {
>  		pr_debug("Can't add buf len %i - avail = %i\n",
> -			 out + in, vq->vq.num_free);
> +			 total_sg, vq->vq.num_free);
>  		/* FIXME: for historical reasons, we force a notify here if
>  		 * there are outgoing parts to the buffer.  Presumably the
>  		 * host should service the ring ASAP. */
> -		if (out)
> +		if (out_sgs)
>  			vq->notify(&vq->vq);
>  		END_USE(vq);
>  		return -ENOSPC;
>  	}
>  
>  	/* We're about to use some buffers from the free list. */
> -	vq->vq.num_free -= out + in;
> -
> -	head = vq->free_head;
> -	for (i = vq->free_head; out; i = vq->vring.desc[i].next, out--) {
> -		vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
> -		vq->vring.desc[i].addr = sg_phys(sg);
> -		vq->vring.desc[i].len = sg->length;
> -		prev = i;
> -		sg++;
> +	vq->vq.num_free -= total_sg;
> +
> +	head = i = vq->free_head;
> +	for (n = 0; n < out_sgs; n++) {
> +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> +			vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
> +			vq->vring.desc[i].addr = sg_phys(sg);
> +			vq->vring.desc[i].len = sg->length;
> +			prev = i;
> +			i = vq->vring.desc[i].next;
> +		}
>  	}
> -	for (; in; i = vq->vring.desc[i].next, in--) {
> -		vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
> -		vq->vring.desc[i].addr = sg_phys(sg);
> -		vq->vring.desc[i].len = sg->length;
> -		prev = i;
> -		sg++;
> +	for (; n < (out_sgs + in_sgs); n++) {
> +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> +			vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
> +			vq->vring.desc[i].addr = sg_phys(sg);
> +			vq->vring.desc[i].len = sg->length;
> +			prev = i;
> +			i = vq->vring.desc[i].next;
> +		}
>  	}
>  	/* Last one doesn't continue. */
>  	vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
> @@ -269,7 +329,7 @@ add_head:
>  
>  	return 0;
>  }
> -EXPORT_SYMBOL_GPL(virtqueue_add_buf);
> +EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
>  
>  /**
>   * virtqueue_kick_prepare - first half of split virtqueue_kick call.
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index ff6714e..6eff15b 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -40,6 +40,13 @@ int virtqueue_add_buf(struct virtqueue *vq,
>  		      void *data,
>  		      gfp_t gfp);
>  
> +int virtqueue_add_sgs(struct virtqueue *vq,
> +		      struct scatterlist *sgs[],
> +		      unsigned int out_sgs,
> +		      unsigned int in_sgs,
> +		      void *data,
> +		      gfp_t gfp);
> +
>  void virtqueue_kick(struct virtqueue *vq);
>  
>  bool virtqueue_kick_prepare(struct virtqueue *vq);
> -- 
> 1.7.10.4
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