Michael S. Tsirkin
2021-Sep-01 14:50 UTC
[PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data
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 at nvidia.com> > Reviewed-by: Israel Rukshin <israelr at nvidia.com>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