On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy 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.
>
> Re-organize the setup of the IO request to fit the new sg chain
> mechanism.
>
> No performance degradation was seen (fio libaio engine with 16 jobs and
> 128 iodepth):
>
> IO size IOPs Rand Read (before/after) IOPs Rand Write (before/after)
> -------- --------------------------------- ----------------------------------
> 512B 318K/316K 329K/325K
>
> 4KB 323K/321K 353K/349K
>
> 16KB 199K/208K 250K/275K
>
> 128KB 36K/36.1K 39.2K/41.7K
>
> Signed-off-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>
> Reviewed-by: Israel Rukshin <israelr@xxxxxxxxxx>
Could you use something to give confidence intervals maybe?
As it is it looks like a 1-2% regression for 512B and 4KB.
> ---
>
> changes from V2:
> - initialize vbr->out_hdr.sector during virtblk_setup_cmd
>
> changes from V1:
> - Kconfig update (from Christoph)
> - Re-order cmd setup (from Christoph)
> - use flexible sg pointer in the cmd (from Christoph)
> - added perf numbers to commit msg (from Feng Li)
>
> ---
> drivers/block/Kconfig | 1 +
> drivers/block/virtio_blk.c | 155 +++++++++++++++++++++++--------------
> 2 files changed, 100 insertions(+), 56 deletions(-)
>
> diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
> index 63056cfd4b62..ca25a122b8ee 100644
> --- a/drivers/block/Kconfig
> +++ b/drivers/block/Kconfig
> @@ -395,6 +395,7 @@ config XEN_BLKDEV_BACKEND
> config VIRTIO_BLK
> tristate "Virtio block driver"
> depends on VIRTIO
> + select SG_POOL
> help
> This is the virtual block driver for virtio. It can be used with
> QEMU based VMMs (like KVM or Xen). Say Y or M.
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 9332fc4e9b31..bdd6d415bd20 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)
> {
> @@ -93,6 +99,7 @@ struct virtio_blk {
> struct virtblk_req {
> struct virtio_blk_outhdr out_hdr;
> u8 status;
> + struct sg_table sg_table;
> struct scatterlist sg[];
> };
>
> @@ -178,15 +185,94 @@ static int virtblk_setup_discard_write_zeroes(struct request *req, bool unmap)
> return 0;
> }
>
> -static inline void virtblk_request_done(struct request *req)
> +static void virtblk_unmap_data(struct request *req, struct virtblk_req *vbr)
> {
> - struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
> + if (blk_rq_nr_phys_segments(req))
> + sg_free_table_chained(&vbr->sg_table,
> + VIRTIO_BLK_INLINE_SG_CNT);
> +}
> +
> +static int virtblk_map_data(struct blk_mq_hw_ctx *hctx, struct request *req,
> + struct virtblk_req *vbr)
> +{
> + int err;
> +
> + if (!blk_rq_nr_phys_segments(req))
> + return 0;
> +
> + vbr->sg_table.sgl = vbr->sg;
> + err = sg_alloc_table_chained(&vbr->sg_table,
> + blk_rq_nr_phys_segments(req),
> + vbr->sg_table.sgl,
> + VIRTIO_BLK_INLINE_SG_CNT);
> + if (unlikely(err))
> + return -ENOMEM;
>
> + return blk_rq_map_sg(hctx->queue, req, vbr->sg_table.sgl);
> +}
> +
> +static void virtblk_cleanup_cmd(struct request *req)
> +{
> if (req->rq_flags & RQF_SPECIAL_PAYLOAD) {
> kfree(page_address(req->special_vec.bv_page) +
> req->special_vec.bv_offset);
> }
> +}
> +
> +static int virtblk_setup_cmd(struct virtio_device *vdev, struct request *req,
> + struct virtblk_req *vbr)
> +{
> + bool unmap = false;
> + u32 type;
> +
> + vbr->out_hdr.sector = 0;
> +
> + switch (req_op(req)) {
> + case REQ_OP_READ:
> + type = VIRTIO_BLK_T_IN;
> + vbr->out_hdr.sector = cpu_to_virtio64(vdev,
> + blk_rq_pos(req));
> + break;
> + case REQ_OP_WRITE:
> + type = VIRTIO_BLK_T_OUT;
> + vbr->out_hdr.sector = cpu_to_virtio64(vdev,
> + blk_rq_pos(req));
> + break;
> + case REQ_OP_FLUSH:
> + type = VIRTIO_BLK_T_FLUSH;
> + break;
> + case REQ_OP_DISCARD:
> + type = VIRTIO_BLK_T_DISCARD;
> + break;
> + case REQ_OP_WRITE_ZEROES:
> + type = VIRTIO_BLK_T_WRITE_ZEROES;
> + unmap = !(req->cmd_flags & REQ_NOUNMAP);
> + break;
> + case REQ_OP_DRV_IN:
> + type = VIRTIO_BLK_T_GET_ID;
> + break;
> + default:
> + WARN_ON_ONCE(1);
> + return BLK_STS_IOERR;
> + }
>
> + vbr->out_hdr.type = cpu_to_virtio32(vdev, type);
> + vbr->out_hdr.ioprio = cpu_to_virtio32(vdev, req_get_ioprio(req));
> +
> + if (type == VIRTIO_BLK_T_DISCARD || type == VIRTIO_BLK_T_WRITE_ZEROES) {
> + if (virtblk_setup_discard_write_zeroes(req, unmap))
> + return BLK_STS_RESOURCE;
> + }
> +
> + return 0;
> +}
> +
> +static inline void virtblk_request_done(struct request *req)
> +{
> + struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
> +
> + virtblk_unmap_data(req, vbr);
> + virtblk_cleanup_cmd(req);
> blk_mq_end_request(req, virtblk_result(vbr));
> }
>
> @@ -244,57 +330,23 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
> int qid = hctx->queue_num;
> int err;
> bool notify = false;
> - bool unmap = false;
> - u32 type;
>
> BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
>
> - switch (req_op(req)) {
> - case REQ_OP_READ:
> - case REQ_OP_WRITE:
> - type = 0;
> - break;
> - case REQ_OP_FLUSH:
> - type = VIRTIO_BLK_T_FLUSH;
> - break;
> - case REQ_OP_DISCARD:
> - type = VIRTIO_BLK_T_DISCARD;
> - break;
> - case REQ_OP_WRITE_ZEROES:
> - type = VIRTIO_BLK_T_WRITE_ZEROES;
> - unmap = !(req->cmd_flags & REQ_NOUNMAP);
> - break;
> - case REQ_OP_DRV_IN:
> - type = VIRTIO_BLK_T_GET_ID;
> - break;
> - default:
> - WARN_ON_ONCE(1);
> - return BLK_STS_IOERR;
> - }
> -
> - vbr->out_hdr.type = cpu_to_virtio32(vblk->vdev, type);
> - vbr->out_hdr.sector = type ?
> - 0 : cpu_to_virtio64(vblk->vdev, blk_rq_pos(req));
> - vbr->out_hdr.ioprio = cpu_to_virtio32(vblk->vdev, req_get_ioprio(req));
> + err = virtblk_setup_cmd(vblk->vdev, req, vbr);
> + if (unlikely(err))
> + return err;
>
> blk_mq_start_request(req);
>
> - if (type == VIRTIO_BLK_T_DISCARD || type == VIRTIO_BLK_T_WRITE_ZEROES) {
> - err = virtblk_setup_discard_write_zeroes(req, unmap);
> - if (err)
> - return BLK_STS_RESOURCE;
> - }
> -
> - num = blk_rq_map_sg(hctx->queue, req, vbr->sg);
> - if (num) {
> - if (rq_data_dir(req) == WRITE)
> - vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_OUT);
> - else
> - vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_IN);
> + num = virtblk_map_data(hctx, req, vbr);
> + if (unlikely(num < 0)) {
> + virtblk_cleanup_cmd(req);
> + return BLK_STS_RESOURCE;
> }
>
> 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
> @@ -303,6 +355,8 @@ 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);
> + virtblk_unmap_data(req, vbr);
> + virtblk_cleanup_cmd(req);
> switch (err) {
> case -ENOSPC:
> return BLK_STS_DEV_RESOURCE;
> @@ -681,16 +735,6 @@ 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;
> @@ -703,7 +747,6 @@ 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,
> };
>
> @@ -783,7 +826,7 @@ 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
