Re: [PATCH 1/1] virtio-blk: avoid preallocating big SGL for data

[Feng Li <lifeng1519@gmail.com>]

Does this hurt the performance of virtio-blk?
I think a fio result is needed here.

On Tue, Aug 31, 2021 at 7:36 AM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>
> No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> has lots of deep queues, preallocation for the sg list can consume
> substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> can be 64 or 128 and each queue's depth might be 128. This means the
> resulting preallocation for the data SGLs is big.
>
> Switch to runtime allocation for SGL for lists longer than 2 entries.
> This is the approach used by NVMe drivers so it should be reasonable for
> virtio block as well. Runtime SGL allocation has always been the case
> for the legacy I/O path so this is nothing new.
>
> The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> support SG_CHAIN, use only runtime allocation for the SGL.
>
> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> Reviewed-by: Israel Rukshin <israelr@nvidia.com>
> ---
>  drivers/block/virtio_blk.c | 37 ++++++++++++++++++++++---------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 77e8468e8593..9a4c5d428b58 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -24,6 +24,12 @@
>  /* The maximum number of sg elements that fit into a virtqueue */
>  #define VIRTIO_BLK_MAX_SG_ELEMS 32768
>
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
> +#define VIRTIO_BLK_INLINE_SG_CNT       0
> +#else
> +#define VIRTIO_BLK_INLINE_SG_CNT       2
> +#endif
> +
>  static int virtblk_queue_count_set(const char *val,
>                 const struct kernel_param *kp)
>  {
> @@ -99,7 +105,7 @@ struct virtio_blk {
>  struct virtblk_req {
>         struct virtio_blk_outhdr out_hdr;
>         u8 status;
> -       struct scatterlist sg[];
> +       struct sg_table sg_table;
>  };
>
>  static inline blk_status_t virtblk_result(struct virtblk_req *vbr)
> @@ -188,6 +194,8 @@ static inline void virtblk_request_done(struct request *req)
>  {
>         struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
>
> +       sg_free_table_chained(&vbr->sg_table, VIRTIO_BLK_INLINE_SG_CNT);
> +
>         if (req->rq_flags & RQF_SPECIAL_PAYLOAD) {
>                 kfree(page_address(req->special_vec.bv_page) +
>                       req->special_vec.bv_offset);
> @@ -291,7 +299,15 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>                         return BLK_STS_RESOURCE;
>         }
>
> -       num = blk_rq_map_sg(hctx->queue, req, vbr->sg);
> +       vbr->sg_table.sgl = (struct scatterlist *)(vbr + 1);
> +       err = sg_alloc_table_chained(&vbr->sg_table,
> +                                    blk_rq_nr_phys_segments(req),
> +                                    vbr->sg_table.sgl,
> +                                    VIRTIO_BLK_INLINE_SG_CNT);
> +       if (err)
> +               return BLK_STS_RESOURCE;
> +
> +       num = blk_rq_map_sg(hctx->queue, req, vbr->sg_table.sgl);
>         if (num) {
>                 if (rq_data_dir(req) == WRITE)
>                         vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_OUT);
> @@ -300,7 +316,7 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>         }
>
>         spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
> -       err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg, num);
> +       err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg_table.sgl, num);
>         if (err) {
>                 virtqueue_kick(vblk->vqs[qid].vq);
>                 /* Don't stop the queue if -ENOMEM: we may have failed to
> @@ -309,6 +325,8 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>                 if (err == -ENOSPC)
>                         blk_mq_stop_hw_queue(hctx);
>                 spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
> +               sg_free_table_chained(&vbr->sg_table,
> +                                     VIRTIO_BLK_INLINE_SG_CNT);
>                 switch (err) {
>                 case -ENOSPC:
>                         return BLK_STS_DEV_RESOURCE;
> @@ -687,16 +705,6 @@ static const struct attribute_group *virtblk_attr_groups[] = {
>         NULL,
>  };
>
> -static int virtblk_init_request(struct blk_mq_tag_set *set, struct request *rq,
> -               unsigned int hctx_idx, unsigned int numa_node)
> -{
> -       struct virtio_blk *vblk = set->driver_data;
> -       struct virtblk_req *vbr = blk_mq_rq_to_pdu(rq);
> -
> -       sg_init_table(vbr->sg, vblk->sg_elems);
> -       return 0;
> -}
> -
>  static int virtblk_map_queues(struct blk_mq_tag_set *set)
>  {
>         struct virtio_blk *vblk = set->driver_data;
> @@ -709,7 +717,6 @@ static const struct blk_mq_ops virtio_mq_ops = {
>         .queue_rq       = virtio_queue_rq,
>         .commit_rqs     = virtio_commit_rqs,
>         .complete       = virtblk_request_done,
> -       .init_request   = virtblk_init_request,
>         .map_queues     = virtblk_map_queues,
>  };
>
> @@ -805,7 +812,7 @@ static int virtblk_probe(struct virtio_device *vdev)
>         vblk->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
>         vblk->tag_set.cmd_size =
>                 sizeof(struct virtblk_req) +
> -               sizeof(struct scatterlist) * sg_elems;
> +               sizeof(struct scatterlist) * VIRTIO_BLK_INLINE_SG_CNT;
>         vblk->tag_set.driver_data = vblk;
>         vblk->tag_set.nr_hw_queues = vblk->num_vqs;
>
> --
> 2.18.1
>