[PATCH v4 0/2] virtio-blk: support polling I/O and mq_ops->queue_rqs()

[PATCH v4 0/2] virtio-blk: support polling I/O and mq_ops->queue_rqs()

[Suwan Kim]

This patch serise adds support for polling I/O and mq_ops->queue_rqs()
to virtio-blk driver.

Changes

v3 -> v4
    - patch1 : virtblk_poll
        - Add print the number of default/read/poll queues in init_vq()
        - Add blk_mq_start_stopped_hw_queues() to virtblk_poll()
              virtblk_poll()
                  ...
                  if (req_done)
		                   blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
                  ...

    - patch2 : virtio_queue_rqs
        - Modify virtio_queue_rqs() to hold lock only once when it adds
          requests to virtqueue just before virtqueue notify.
	  It will guarantee that virtio_queue_rqs() will not use
	  previous req again.

v2 -> v3
        - Fix warning by kernel test robot
          
            static int virtblk_poll()
                ...
                if (!blk_mq_add_to_batch(req, iob, virtblk_result(vbr),
                                                   -> vbr->status,

v1 -> v2
        - To receive the number of poll queues from user,
          use module parameter instead of QEMU uapi change.

        - Add the comment about virtblk_map_queues().

        - Add support for mq_ops->queue_rqs() to implement submit side
          batch.

Suwan Kim (2):
  virtio-blk: support polling I/O
  virtio-blk: support mq_ops->queue_rqs()

 drivers/block/virtio_blk.c | 222 ++++++++++++++++++++++++++++++++++---
 1 file changed, 207 insertions(+), 15 deletions(-)

-- 
2.26.3

[PATCH v4 1/2] virtio-blk: support polling I/O

[Suwan Kim]

This patch supports polling I/O via virtio-blk driver. Polling
feature is enabled by module parameter "num_poll_queues" and it
sets dedicated polling queues for virtio-blk. This patch improves
the polling I/O throughput and latency.

The virtio-blk driver doesn't not have a poll function and a poll
queue and it has been operating in interrupt driven method even if
the polling function is called in the upper layer.

virtio-blk polling is implemented upon 'batched completion' of block
layer. virtblk_poll() queues completed request to io_comp_batch->req_list
and later, virtblk_complete_batch() calls unmap function and ends
the requests in batch.

virtio-blk reads the number of poll queues from module parameter
"num_poll_queues". If VM sets queue parameter as below,
("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
queues, the poll queues have no callback function.

Regarding HW-SW queue mapping, the default queue mapping uses the
existing method that condsiders MSI irq vector. But the poll queue
doesn't have an irq, so it uses the regular blk-mq cpu mapping.

For verifying the improvement, I did Fio polling I/O performance test
with io_uring engine with the options below.
(io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
queues for VM.

As a result, IOPS and average latency improved about 10%.

Test result:

- Fio io_uring poll without virtio-blk poll support
	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us

- Fio io_uring poll with virtio-blk poll support
	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
---
 drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
 1 file changed, 108 insertions(+), 4 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 8c415be86732..c2d955da0006 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
 		 "0 for no limit. "
 		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
 
+static unsigned int num_poll_queues;
+module_param(num_poll_queues, uint, 0644);
+MODULE_PARM_DESC(num_poll_queues, "The number of dedicated virtqueues for polling I/O");
+
 static int major;
 static DEFINE_IDA(vd_index_ida);
 
@@ -81,6 +85,7 @@ struct virtio_blk {
 
 	/* num of vqs */
 	int num_vqs;
+	int io_queues[HCTX_MAX_TYPES];
 	struct virtio_blk_vq *vqs;
 };
 
@@ -548,6 +553,7 @@ static int init_vq(struct virtio_blk *vblk)
 	const char **names;
 	struct virtqueue **vqs;
 	unsigned short num_vqs;
+	unsigned int num_poll_vqs;
 	struct virtio_device *vdev = vblk->vdev;
 	struct irq_affinity desc = { 0, };
 
@@ -556,6 +562,7 @@ static int init_vq(struct virtio_blk *vblk)
 				   &num_vqs);
 	if (err)
 		num_vqs = 1;
+
 	if (!err && !num_vqs) {
 		dev_err(&vdev->dev, "MQ advertised but zero queues reported\n");
 		return -EINVAL;
@@ -565,6 +572,18 @@ static int init_vq(struct virtio_blk *vblk)
 			min_not_zero(num_request_queues, nr_cpu_ids),
 			num_vqs);
 
+	num_poll_vqs = min_t(unsigned int, num_poll_queues, num_vqs - 1);
+
+	memset(vblk->io_queues, 0, sizeof(int) * HCTX_MAX_TYPES);
+	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
+	vblk->io_queues[HCTX_TYPE_READ] = 0;
+	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
+
+	dev_info(&vdev->dev, "%d/%d/%d default/read/poll queues\n",
+				vblk->io_queues[HCTX_TYPE_DEFAULT],
+				vblk->io_queues[HCTX_TYPE_READ],
+				vblk->io_queues[HCTX_TYPE_POLL]);
+
 	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
 	if (!vblk->vqs)
 		return -ENOMEM;
@@ -578,8 +597,13 @@ static int init_vq(struct virtio_blk *vblk)
 	}
 
 	for (i = 0; i < num_vqs; i++) {
-		callbacks[i] = virtblk_done;
-		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
+		if (i < num_vqs - num_poll_vqs) {
+			callbacks[i] = virtblk_done;
+			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
+		} else {
+			callbacks[i] = NULL;
+			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
+		}
 		names[i] = vblk->vqs[i].name;
 	}
 
@@ -728,16 +752,93 @@ static const struct attribute_group *virtblk_attr_groups[] = {
 static int virtblk_map_queues(struct blk_mq_tag_set *set)
 {
 	struct virtio_blk *vblk = set->driver_data;
+	int i, qoff;
+
+	for (i = 0, qoff = 0; i < set->nr_maps; i++) {
+		struct blk_mq_queue_map *map = &set->map[i];
+
+		map->nr_queues = vblk->io_queues[i];
+		map->queue_offset = qoff;
+		qoff += map->nr_queues;
+
+		if (map->nr_queues == 0)
+			continue;
+
+		/*
+		 * Regular queues have interrupts and hence CPU affinity is
+		 * defined by the core virtio code, but polling queues have
+		 * no interrupts so we let the block layer assign CPU affinity.
+		 */
+		if (i == HCTX_TYPE_DEFAULT)
+			blk_mq_virtio_map_queues(&set->map[i], vblk->vdev, 0);
+		else
+			blk_mq_map_queues(&set->map[i]);
+	}
+
+	return 0;
+}
+
+static void virtblk_complete_batch(struct io_comp_batch *iob)
+{
+	struct request *req;
+	struct virtblk_req *vbr;
+
+	rq_list_for_each(&iob->req_list, req) {
+		vbr = blk_mq_rq_to_pdu(req);
+		virtblk_unmap_data(req, vbr);
+		virtblk_cleanup_cmd(req);
+	}
+	blk_mq_end_request_batch(iob);
+}
+
+static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
+{
+	struct virtio_blk *vblk = hctx->queue->queuedata;
+	struct virtio_blk_vq *vq = hctx->driver_data;
+	struct virtblk_req *vbr;
+	bool req_done = false;
+	unsigned long flags;
+	unsigned int len;
+	int found = 0;
+
+	spin_lock_irqsave(&vq->lock, flags);
+
+	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
+		struct request *req = blk_mq_rq_from_pdu(vbr);
 
-	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
-					vblk->vdev, 0);
+		found++;
+		if (!blk_mq_add_to_batch(req, iob, vbr->status,
+						virtblk_complete_batch))
+			blk_mq_complete_request(req);
+		req_done = true;
+	}
+
+	if (req_done)
+		blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
+
+	spin_unlock_irqrestore(&vq->lock, flags);
+
+	return found;
+}
+
+static int virtblk_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
+			  unsigned int hctx_idx)
+{
+	struct virtio_blk *vblk = data;
+	struct virtio_blk_vq *vq = &vblk->vqs[hctx_idx];
+
+	WARN_ON(vblk->tag_set.tags[hctx_idx] != hctx->tags);
+	hctx->driver_data = vq;
+	return 0;
 }
 
 static const struct blk_mq_ops virtio_mq_ops = {
 	.queue_rq	= virtio_queue_rq,
 	.commit_rqs	= virtio_commit_rqs,
+	.init_hctx	= virtblk_init_hctx,
 	.complete	= virtblk_request_done,
 	.map_queues	= virtblk_map_queues,
+	.poll		= virtblk_poll,
 };
 
 static unsigned int virtblk_queue_depth;
@@ -816,6 +917,9 @@ static int virtblk_probe(struct virtio_device *vdev)
 		sizeof(struct scatterlist) * VIRTIO_BLK_INLINE_SG_CNT;
 	vblk->tag_set.driver_data = vblk;
 	vblk->tag_set.nr_hw_queues = vblk->num_vqs;
+	vblk->tag_set.nr_maps = 1;
+	if (vblk->io_queues[HCTX_TYPE_POLL])
+		vblk->tag_set.nr_maps = 3;
 
 	err = blk_mq_alloc_tag_set(&vblk->tag_set);
 	if (err)
-- 
2.26.3

Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Suwan Kim]

On Mon, Apr 04, 2022 at 06:28:04PM +0900, Suwan Kim wrote:
> This patch supports polling I/O via virtio-blk driver. Polling
> feature is enabled by module parameter "num_poll_queues" and it
> sets dedicated polling queues for virtio-blk. This patch improves
> the polling I/O throughput and latency.
> 
> The virtio-blk driver doesn't not have a poll function and a poll
> queue and it has been operating in interrupt driven method even if
> the polling function is called in the upper layer.
> 
> virtio-blk polling is implemented upon 'batched completion' of block
> layer. virtblk_poll() queues completed request to io_comp_batch->req_list
> and later, virtblk_complete_batch() calls unmap function and ends
> the requests in batch.
> 
> virtio-blk reads the number of poll queues from module parameter
> "num_poll_queues". If VM sets queue parameter as below,
> ("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
> It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
> as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
> queues, the poll queues have no callback function.
> 
> Regarding HW-SW queue mapping, the default queue mapping uses the
> existing method that condsiders MSI irq vector. But the poll queue
> doesn't have an irq, so it uses the regular blk-mq cpu mapping.
> 
> For verifying the improvement, I did Fio polling I/O performance test
> with io_uring engine with the options below.
> (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
> I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
> queues for VM.
> 
> As a result, IOPS and average latency improved about 10%.
> 
> Test result:
> 
> - Fio io_uring poll without virtio-blk poll support
> 	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
> 	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
> 	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us
> 
> - Fio io_uring poll with virtio-blk poll support
> 	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
> 	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
> 	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us
> 
> Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> ---
>  drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 108 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 8c415be86732..c2d955da0006 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
>  		 "0 for no limit. "
>  		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
>  
> +static unsigned int num_poll_queues;
> +module_param(num_poll_queues, uint, 0644);
> +MODULE_PARM_DESC(num_poll_queues, "The number of dedicated virtqueues for polling I/O");
 
Sorry. I forgot to fix the parameter name.
I resended this path.

Regards,
Suwan Kim

[PATCH v4 2/2] virtio-blk: support mq_ops->queue_rqs()

[Suwan Kim]

This patch supports mq_ops->queue_rqs() hook. It has an advantage of
batch submission to virtio-blk driver. It also helps polling I/O because
polling uses batched completion of block layer. Batch submission in
queue_rqs() can boost polling performance.

In queue_rqs(), it iterates plug->mq_list, collects requests that
belong to same HW queue until it encounters a request from other
HW queue or sees the end of the list.
Then, virtio-blk adds requests into virtqueue and kicks virtqueue
to submit requests.

If there is an error, it inserts error request to requeue_list and
passes it to ordinary block layer path.

For verification, I did fio test.
(io_uring, randread, direct=1, bs=4K, iodepth=64 numjobs=N)
I set 4 vcpu and 2 virtio-blk queues for VM and run fio test 5 times.
It shows about 2% improvement.

                                 |   numjobs=2   |   numjobs=4
      -----------------------------------------------------------
        fio without queue_rqs()  |   291K IOPS   |   238K IOPS
      -----------------------------------------------------------
        fio with queue_rqs()     |   295K IOPS   |   243K IOPS

For polling I/O performance, I also did fio test as below.
(io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=4)
I set 4 vcpu and 2 poll queues for VM.
It shows about 2% improvement in polling I/O.

                                      |   IOPS   |  avg latency
      -----------------------------------------------------------
        fio poll without queue_rqs()  |   424K   |   613.05 usec
      -----------------------------------------------------------
        fio poll with queue_rqs()     |   435K   |   601.01 usec

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
---
 drivers/block/virtio_blk.c | 110 +++++++++++++++++++++++++++++++++----
 1 file changed, 99 insertions(+), 11 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c2d955da0006..03e4455c23e3 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -92,6 +92,7 @@ struct virtio_blk {
 struct virtblk_req {
 	struct virtio_blk_outhdr out_hdr;
 	u8 status;
+	int sg_num;
 	struct sg_table sg_table;
 	struct scatterlist sg[];
 };
@@ -311,18 +312,13 @@ static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
 		virtqueue_notify(vq->vq);
 }
 
-static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
-			   const struct blk_mq_queue_data *bd)
+static blk_status_t virtblk_prep_rq(struct blk_mq_hw_ctx *hctx,
+					struct virtio_blk *vblk,
+					struct request *req,
+					struct virtblk_req *vbr)
 {
-	struct virtio_blk *vblk = hctx->queue->queuedata;
-	struct request *req = bd->rq;
-	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
-	unsigned long flags;
-	int num;
-	int qid = hctx->queue_num;
-	bool notify = false;
 	blk_status_t status;
-	int err;
+	int num;
 
 	status = virtblk_setup_cmd(vblk->vdev, req, vbr);
 	if (unlikely(status))
@@ -335,9 +331,30 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 		virtblk_cleanup_cmd(req);
 		return BLK_STS_RESOURCE;
 	}
+	vbr->sg_num = num;
+
+	return BLK_STS_OK;
+}
+
+static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
+			   const struct blk_mq_queue_data *bd)
+{
+	struct virtio_blk *vblk = hctx->queue->queuedata;
+	struct request *req = bd->rq;
+	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
+	unsigned long flags;
+	int qid = hctx->queue_num;
+	bool notify = false;
+	blk_status_t status;
+	int err;
+
+	status = virtblk_prep_rq(hctx, vblk, req, vbr);
+	if (unlikely(status))
+		return status;
 
 	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
-	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg_table.sgl, num);
+	err = virtblk_add_req(vblk->vqs[qid].vq, vbr,
+				vbr->sg_table.sgl, vbr->sg_num);
 	if (err) {
 		virtqueue_kick(vblk->vqs[qid].vq);
 		/* Don't stop the queue if -ENOMEM: we may have failed to
@@ -367,6 +384,76 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 	return BLK_STS_OK;
 }
 
+static bool virtblk_prep_rq_batch(struct request *req)
+{
+	struct virtio_blk *vblk = req->mq_hctx->queue->queuedata;
+	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
+
+	req->mq_hctx->tags->rqs[req->tag] = req;
+
+	return virtblk_prep_rq(req->mq_hctx, vblk, req, vbr) == BLK_STS_OK;
+}
+
+static bool virtblk_add_req_batch(struct virtio_blk_vq *vq,
+					struct request **rqlist,
+					struct request **requeue_list)
+{
+	unsigned long flags;
+	int err;
+	bool kick;
+
+	spin_lock_irqsave(&vq->lock, flags);
+
+	while (!rq_list_empty(*rqlist)) {
+		struct request *req = rq_list_pop(rqlist);
+		struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
+
+		err = virtblk_add_req(vq->vq, vbr,
+					vbr->sg_table.sgl, vbr->sg_num);
+		if (err) {
+			virtblk_unmap_data(req, vbr);
+			virtblk_cleanup_cmd(req);
+			rq_list_add(requeue_list, req);
+		}
+	}
+
+	kick = virtqueue_kick_prepare(vq->vq);
+	spin_unlock_irqrestore(&vq->lock, flags);
+
+	return kick;
+}
+
+static void virtio_queue_rqs(struct request **rqlist)
+{
+	struct request *req, *next, *prev = NULL;
+	struct request *requeue_list = NULL;
+
+	rq_list_for_each_safe(rqlist, req, next) {
+		struct virtio_blk_vq *vq = req->mq_hctx->driver_data;
+		bool kick;
+
+		if (!virtblk_prep_rq_batch(req)) {
+			rq_list_move(rqlist, &requeue_list, req, prev);
+			req = prev;
+			if (!req)
+				continue;
+		}
+
+		if (!next || req->mq_hctx != next->mq_hctx) {
+			req->rq_next = NULL;
+			kick = virtblk_add_req_batch(vq, rqlist, &requeue_list);
+			if (kick)
+				virtqueue_notify(vq->vq);
+
+			*rqlist = next;
+			prev = NULL;
+		} else
+			prev = req;
+	}
+
+	*rqlist = requeue_list;
+}
+
 /* return id (s/n) string for *disk to *id_str
  */
 static int virtblk_get_id(struct gendisk *disk, char *id_str)
@@ -834,6 +921,7 @@ static int virtblk_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
 
 static const struct blk_mq_ops virtio_mq_ops = {
 	.queue_rq	= virtio_queue_rq,
+	.queue_rqs	= virtio_queue_rqs,
 	.commit_rqs	= virtio_commit_rqs,
 	.init_hctx	= virtblk_init_hctx,
 	.complete	= virtblk_request_done,
-- 
2.26.3

[PATCH v4 0/2] virtio-blk: support polling I/O and mq_ops->queue_rqs()

[Suwan Kim]

Sorry. I forgot to fix module parameter name in patch1.
So I resend it.

This patch serise adds support for polling I/O and mq_ops->queue_rqs()
to virtio-blk driver.

Changes

v3 -> v4
    - patch1 : virtblk_poll
        - Add print the number of default/read/poll queues in init_vq()
        - Add blk_mq_start_stopped_hw_queues() to virtblk_poll()
              virtblk_poll()
                  ...
                  if (req_done)
                                   blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
                  ...

    - patch2 : virtio_queue_rqs
        - Modify virtio_queue_rqs() to hold lock only once when it adds
          requests to virtqueue just before virtqueue notify.
          It will guarantee that virtio_queue_rqs() will not use
          previous req again.

v2 -> v3
        - Fix warning by kernel test robot

            static int virtblk_poll()
                ...
                if (!blk_mq_add_to_batch(req, iob, virtblk_result(vbr),
                                                   -> vbr->status,

v1 -> v2
        - To receive the number of poll queues from user,
          use module parameter instead of QEMU uapi change.

        - Add the comment about virtblk_map_queues().

        - Add support for mq_ops->queue_rqs() to implement submit side
          batch.


Suwan Kim (2):
  virtio-blk: support polling I/O
  virtio-blk: support mq_ops->queue_rqs()

 drivers/block/virtio_blk.c | 222 ++++++++++++++++++++++++++++++++++---
 1 file changed, 207 insertions(+), 15 deletions(-)

-- 
2.26.3

[PATCH v4 1/2] virtio-blk: support polling I/O

[Suwan Kim]

This patch supports polling I/O via virtio-blk driver. Polling
feature is enabled by module parameter "num_poll_queues" and it
sets dedicated polling queues for virtio-blk. This patch improves
the polling I/O throughput and latency.

The virtio-blk driver doesn't not have a poll function and a poll
queue and it has been operating in interrupt driven method even if
the polling function is called in the upper layer.

virtio-blk polling is implemented upon 'batched completion' of block
layer. virtblk_poll() queues completed request to io_comp_batch->req_list
and later, virtblk_complete_batch() calls unmap function and ends
the requests in batch.

virtio-blk reads the number of poll queues from module parameter
"num_poll_queues". If VM sets queue parameter as below,
("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
queues, the poll queues have no callback function.

Regarding HW-SW queue mapping, the default queue mapping uses the
existing method that condsiders MSI irq vector. But the poll queue
doesn't have an irq, so it uses the regular blk-mq cpu mapping.

For verifying the improvement, I did Fio polling I/O performance test
with io_uring engine with the options below.
(io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
queues for VM.

As a result, IOPS and average latency improved about 10%.

Test result:

- Fio io_uring poll without virtio-blk poll support
	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us

- Fio io_uring poll with virtio-blk poll support
	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
---
 drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
 1 file changed, 108 insertions(+), 4 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 8c415be86732..712579dcd3cc 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
 		 "0 for no limit. "
 		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
 
+static unsigned int poll_queues;
+module_param(poll_queues, uint, 0644);
+MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
+
 static int major;
 static DEFINE_IDA(vd_index_ida);
 
@@ -81,6 +85,7 @@ struct virtio_blk {
 
 	/* num of vqs */
 	int num_vqs;
+	int io_queues[HCTX_MAX_TYPES];
 	struct virtio_blk_vq *vqs;
 };
 
@@ -548,6 +553,7 @@ static int init_vq(struct virtio_blk *vblk)
 	const char **names;
 	struct virtqueue **vqs;
 	unsigned short num_vqs;
+	unsigned int num_poll_vqs;
 	struct virtio_device *vdev = vblk->vdev;
 	struct irq_affinity desc = { 0, };
 
@@ -556,6 +562,7 @@ static int init_vq(struct virtio_blk *vblk)
 				   &num_vqs);
 	if (err)
 		num_vqs = 1;
+
 	if (!err && !num_vqs) {
 		dev_err(&vdev->dev, "MQ advertised but zero queues reported\n");
 		return -EINVAL;
@@ -565,6 +572,18 @@ static int init_vq(struct virtio_blk *vblk)
 			min_not_zero(num_request_queues, nr_cpu_ids),
 			num_vqs);
 
+	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
+
+	memset(vblk->io_queues, 0, sizeof(int) * HCTX_MAX_TYPES);
+	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
+	vblk->io_queues[HCTX_TYPE_READ] = 0;
+	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
+
+	dev_info(&vdev->dev, "%d/%d/%d default/read/poll queues\n",
+				vblk->io_queues[HCTX_TYPE_DEFAULT],
+				vblk->io_queues[HCTX_TYPE_READ],
+				vblk->io_queues[HCTX_TYPE_POLL]);
+
 	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
 	if (!vblk->vqs)
 		return -ENOMEM;
@@ -578,8 +597,13 @@ static int init_vq(struct virtio_blk *vblk)
 	}
 
 	for (i = 0; i < num_vqs; i++) {
-		callbacks[i] = virtblk_done;
-		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
+		if (i < num_vqs - num_poll_vqs) {
+			callbacks[i] = virtblk_done;
+			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
+		} else {
+			callbacks[i] = NULL;
+			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
+		}
 		names[i] = vblk->vqs[i].name;
 	}
 
@@ -728,16 +752,93 @@ static const struct attribute_group *virtblk_attr_groups[] = {
 static int virtblk_map_queues(struct blk_mq_tag_set *set)
 {
 	struct virtio_blk *vblk = set->driver_data;
+	int i, qoff;
+
+	for (i = 0, qoff = 0; i < set->nr_maps; i++) {
+		struct blk_mq_queue_map *map = &set->map[i];
+
+		map->nr_queues = vblk->io_queues[i];
+		map->queue_offset = qoff;
+		qoff += map->nr_queues;
+
+		if (map->nr_queues == 0)
+			continue;
+
+		/*
+		 * Regular queues have interrupts and hence CPU affinity is
+		 * defined by the core virtio code, but polling queues have
+		 * no interrupts so we let the block layer assign CPU affinity.
+		 */
+		if (i == HCTX_TYPE_DEFAULT)
+			blk_mq_virtio_map_queues(&set->map[i], vblk->vdev, 0);
+		else
+			blk_mq_map_queues(&set->map[i]);
+	}
+
+	return 0;
+}
+
+static void virtblk_complete_batch(struct io_comp_batch *iob)
+{
+	struct request *req;
+	struct virtblk_req *vbr;
+
+	rq_list_for_each(&iob->req_list, req) {
+		vbr = blk_mq_rq_to_pdu(req);
+		virtblk_unmap_data(req, vbr);
+		virtblk_cleanup_cmd(req);
+	}
+	blk_mq_end_request_batch(iob);
+}
+
+static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
+{
+	struct virtio_blk *vblk = hctx->queue->queuedata;
+	struct virtio_blk_vq *vq = hctx->driver_data;
+	struct virtblk_req *vbr;
+	bool req_done = false;
+	unsigned long flags;
+	unsigned int len;
+	int found = 0;
+
+	spin_lock_irqsave(&vq->lock, flags);
+
+	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
+		struct request *req = blk_mq_rq_from_pdu(vbr);
 
-	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
-					vblk->vdev, 0);
+		found++;
+		if (!blk_mq_add_to_batch(req, iob, vbr->status,
+						virtblk_complete_batch))
+			blk_mq_complete_request(req);
+		req_done = true;
+	}
+
+	if (req_done)
+		blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
+
+	spin_unlock_irqrestore(&vq->lock, flags);
+
+	return found;
+}
+
+static int virtblk_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
+			  unsigned int hctx_idx)
+{
+	struct virtio_blk *vblk = data;
+	struct virtio_blk_vq *vq = &vblk->vqs[hctx_idx];
+
+	WARN_ON(vblk->tag_set.tags[hctx_idx] != hctx->tags);
+	hctx->driver_data = vq;
+	return 0;
 }
 
 static const struct blk_mq_ops virtio_mq_ops = {
 	.queue_rq	= virtio_queue_rq,
 	.commit_rqs	= virtio_commit_rqs,
+	.init_hctx	= virtblk_init_hctx,
 	.complete	= virtblk_request_done,
 	.map_queues	= virtblk_map_queues,
+	.poll		= virtblk_poll,
 };
 
 static unsigned int virtblk_queue_depth;
@@ -816,6 +917,9 @@ static int virtblk_probe(struct virtio_device *vdev)
 		sizeof(struct scatterlist) * VIRTIO_BLK_INLINE_SG_CNT;
 	vblk->tag_set.driver_data = vblk;
 	vblk->tag_set.nr_hw_queues = vblk->num_vqs;
+	vblk->tag_set.nr_maps = 1;
+	if (vblk->io_queues[HCTX_TYPE_POLL])
+		vblk->tag_set.nr_maps = 3;
 
 	err = blk_mq_alloc_tag_set(&vblk->tag_set);
 	if (err)
-- 
2.26.3

Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Stefan Hajnoczi]

[-- Attachment #1.1: Type: text/plain, Size: 1044 bytes --]

On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> +static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
> +{
> +	struct virtio_blk *vblk = hctx->queue->queuedata;
> +	struct virtio_blk_vq *vq = hctx->driver_data;
> +	struct virtblk_req *vbr;
> +	bool req_done = false;
> +	unsigned long flags;
> +	unsigned int len;
> +	int found = 0;
> +
> +	spin_lock_irqsave(&vq->lock, flags);
> +
> +	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
> +		struct request *req = blk_mq_rq_from_pdu(vbr);
>  
> -	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
> -					vblk->vdev, 0);
> +		found++;
> +		if (!blk_mq_add_to_batch(req, iob, vbr->status,
> +						virtblk_complete_batch))
> +			blk_mq_complete_request(req);
> +		req_done = true;
> +	}
> +
> +	if (req_done)

Minor nit: req_done can be replaced with found > 0.

> +		blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
> +
> +	spin_unlock_irqrestore(&vq->lock, flags);
> +
> +	return found;
> +}

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Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1044 bytes --]

On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> +static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
> +{
> +	struct virtio_blk *vblk = hctx->queue->queuedata;
> +	struct virtio_blk_vq *vq = hctx->driver_data;
> +	struct virtblk_req *vbr;
> +	bool req_done = false;
> +	unsigned long flags;
> +	unsigned int len;
> +	int found = 0;
> +
> +	spin_lock_irqsave(&vq->lock, flags);
> +
> +	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
> +		struct request *req = blk_mq_rq_from_pdu(vbr);
>  
> -	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
> -					vblk->vdev, 0);
> +		found++;
> +		if (!blk_mq_add_to_batch(req, iob, vbr->status,
> +						virtblk_complete_batch))
> +			blk_mq_complete_request(req);
> +		req_done = true;
> +	}
> +
> +	if (req_done)

Minor nit: req_done can be replaced with found > 0.

> +		blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
> +
> +	spin_unlock_irqrestore(&vq->lock, flags);
> +
> +	return found;
> +}

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Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Suwan Kim]

On Tue, Apr 05, 2022 at 08:26:34AM +0100, Stefan Hajnoczi wrote:
> On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> > +static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
> > +{
> > +	struct virtio_blk *vblk = hctx->queue->queuedata;
> > +	struct virtio_blk_vq *vq = hctx->driver_data;
> > +	struct virtblk_req *vbr;
> > +	bool req_done = false;
> > +	unsigned long flags;
> > +	unsigned int len;
> > +	int found = 0;
> > +
> > +	spin_lock_irqsave(&vq->lock, flags);
> > +
> > +	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
> > +		struct request *req = blk_mq_rq_from_pdu(vbr);
> >  
> > -	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
> > -					vblk->vdev, 0);
> > +		found++;
> > +		if (!blk_mq_add_to_batch(req, iob, vbr->status,
> > +						virtblk_complete_batch))
> > +			blk_mq_complete_request(req);
> > +		req_done = true;
> > +	}
> > +
> > +	if (req_done)
> 
> Minor nit: req_done can be replaced with found > 0.

It looks better. I will fix it.
Thanks for the feedback!

Regards,
Suwan Kim

Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Christoph Hellwig]

On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> This patch supports polling I/O via virtio-blk driver. Polling
> feature is enabled by module parameter "num_poll_queues" and it
> sets dedicated polling queues for virtio-blk. This patch improves
> the polling I/O throughput and latency.
> 
> The virtio-blk driver doesn't not have a poll function and a poll
> queue and it has been operating in interrupt driven method even if
> the polling function is called in the upper layer.
> 
> virtio-blk polling is implemented upon 'batched completion' of block
> layer. virtblk_poll() queues completed request to io_comp_batch->req_list
> and later, virtblk_complete_batch() calls unmap function and ends
> the requests in batch.
> 
> virtio-blk reads the number of poll queues from module parameter
> "num_poll_queues". If VM sets queue parameter as below,
> ("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
> It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
> as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
> queues, the poll queues have no callback function.
> 
> Regarding HW-SW queue mapping, the default queue mapping uses the
> existing method that condsiders MSI irq vector. But the poll queue
> doesn't have an irq, so it uses the regular blk-mq cpu mapping.
> 
> For verifying the improvement, I did Fio polling I/O performance test
> with io_uring engine with the options below.
> (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
> I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
> queues for VM.
> 
> As a result, IOPS and average latency improved about 10%.
> 
> Test result:
> 
> - Fio io_uring poll without virtio-blk poll support
> 	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
> 	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
> 	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us
> 
> - Fio io_uring poll with virtio-blk poll support
> 	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
> 	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
> 	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us
> 
> Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> ---
>  drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 108 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 8c415be86732..712579dcd3cc 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
>  		 "0 for no limit. "
>  		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
>  
> +static unsigned int poll_queues;
> +module_param(poll_queues, uint, 0644);
> +MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
> +
>  static int major;
>  static DEFINE_IDA(vd_index_ida);
>  
> @@ -81,6 +85,7 @@ struct virtio_blk {
>  
>  	/* num of vqs */
>  	int num_vqs;
> +	int io_queues[HCTX_MAX_TYPES];
>  	struct virtio_blk_vq *vqs;
>  };
>  
> @@ -548,6 +553,7 @@ static int init_vq(struct virtio_blk *vblk)
>  	const char **names;
>  	struct virtqueue **vqs;
>  	unsigned short num_vqs;
> +	unsigned int num_poll_vqs;
>  	struct virtio_device *vdev = vblk->vdev;
>  	struct irq_affinity desc = { 0, };
>  
> @@ -556,6 +562,7 @@ static int init_vq(struct virtio_blk *vblk)
>  				   &num_vqs);
>  	if (err)
>  		num_vqs = 1;
> +
>  	if (!err && !num_vqs) {
>  		dev_err(&vdev->dev, "MQ advertised but zero queues reported\n");
>  		return -EINVAL;
> @@ -565,6 +572,18 @@ static int init_vq(struct virtio_blk *vblk)
>  			min_not_zero(num_request_queues, nr_cpu_ids),
>  			num_vqs);
>  
> +	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
> +
> +	memset(vblk->io_queues, 0, sizeof(int) * HCTX_MAX_TYPES);
> +	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
> +	vblk->io_queues[HCTX_TYPE_READ] = 0;
> +	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
> +
> +	dev_info(&vdev->dev, "%d/%d/%d default/read/poll queues\n",
> +				vblk->io_queues[HCTX_TYPE_DEFAULT],
> +				vblk->io_queues[HCTX_TYPE_READ],
> +				vblk->io_queues[HCTX_TYPE_POLL]);
> +
>  	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>  	if (!vblk->vqs)
>  		return -ENOMEM;
> @@ -578,8 +597,13 @@ static int init_vq(struct virtio_blk *vblk)
>  	}
>  
>  	for (i = 0; i < num_vqs; i++) {
> +		if (i < num_vqs - num_poll_vqs) {
> +			callbacks[i] = virtblk_done;
> +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
> +		} else {
> +			callbacks[i] = NULL;
> +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> +		}
>  		names[i] = vblk->vqs[i].name;

This would look a little cleaner with two loops:

 	for (i = 0; i < num_vqs - num_poll_vqs; i++) {
		callbacks[i] = virtblk_done;
		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
 		names[i] = vblk->vqs[i].name;
	}
 	for (; i < num_vqs; i++) {
		callbacks[i] = NULL;
		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
		names[i] = vblk->vqs[i].name;
	}

> +
> +		if (map->nr_queues == 0)
> +			continue;
> +
> +		/*
> +		 * Regular queues have interrupts and hence CPU affinity is
> +		 * defined by the core virtio code, but polling queues have
> +		 * no interrupts so we let the block layer assign CPU affinity.
> +		 */
> +		if (i == HCTX_TYPE_DEFAULT)

I'd check for
		i != HCTX_TYPE_POLL

here instead to make the check a little more explicit and future proof
for the potential addition of read queues (which would be a Linux only
change without hypervisor or spec changes).  In fact you might as well
add that support now as doing it is completely trivial once a driver
supports multiple map types.

> +static void virtblk_complete_batch(struct io_comp_batch *iob)
> +{
> +	struct request *req;
> +	struct virtblk_req *vbr;
> +
> +	rq_list_for_each(&iob->req_list, req) {
> +		vbr = blk_mq_rq_to_pdu(req);
> +		virtblk_unmap_data(req, vbr);
> +		virtblk_cleanup_cmd(req);

vbr is only used ones, so why not just:

		virtblk_unmap_data(req, blk_mq_rq_to_pdu);
?

Or even better add a cleanup patch to just remove the vbr argument to
virtblk_unmap_data as it is not needed at all.
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Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Christoph Hellwig]

On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> This patch supports polling I/O via virtio-blk driver. Polling
> feature is enabled by module parameter "num_poll_queues" and it
> sets dedicated polling queues for virtio-blk. This patch improves
> the polling I/O throughput and latency.
> 
> The virtio-blk driver doesn't not have a poll function and a poll
> queue and it has been operating in interrupt driven method even if
> the polling function is called in the upper layer.
> 
> virtio-blk polling is implemented upon 'batched completion' of block
> layer. virtblk_poll() queues completed request to io_comp_batch->req_list
> and later, virtblk_complete_batch() calls unmap function and ends
> the requests in batch.
> 
> virtio-blk reads the number of poll queues from module parameter
> "num_poll_queues". If VM sets queue parameter as below,
> ("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
> It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
> as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
> queues, the poll queues have no callback function.
> 
> Regarding HW-SW queue mapping, the default queue mapping uses the
> existing method that condsiders MSI irq vector. But the poll queue
> doesn't have an irq, so it uses the regular blk-mq cpu mapping.
> 
> For verifying the improvement, I did Fio polling I/O performance test
> with io_uring engine with the options below.
> (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
> I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
> queues for VM.
> 
> As a result, IOPS and average latency improved about 10%.
> 
> Test result:
> 
> - Fio io_uring poll without virtio-blk poll support
> 	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
> 	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
> 	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us
> 
> - Fio io_uring poll with virtio-blk poll support
> 	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
> 	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
> 	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us
> 
> Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> ---
>  drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 108 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 8c415be86732..712579dcd3cc 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
>  		 "0 for no limit. "
>  		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
>  
> +static unsigned int poll_queues;
> +module_param(poll_queues, uint, 0644);
> +MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
> +
>  static int major;
>  static DEFINE_IDA(vd_index_ida);
>  
> @@ -81,6 +85,7 @@ struct virtio_blk {
>  
>  	/* num of vqs */
>  	int num_vqs;
> +	int io_queues[HCTX_MAX_TYPES];
>  	struct virtio_blk_vq *vqs;
>  };
>  
> @@ -548,6 +553,7 @@ static int init_vq(struct virtio_blk *vblk)
>  	const char **names;
>  	struct virtqueue **vqs;
>  	unsigned short num_vqs;
> +	unsigned int num_poll_vqs;
>  	struct virtio_device *vdev = vblk->vdev;
>  	struct irq_affinity desc = { 0, };
>  
> @@ -556,6 +562,7 @@ static int init_vq(struct virtio_blk *vblk)
>  				   &num_vqs);
>  	if (err)
>  		num_vqs = 1;
> +
>  	if (!err && !num_vqs) {
>  		dev_err(&vdev->dev, "MQ advertised but zero queues reported\n");
>  		return -EINVAL;
> @@ -565,6 +572,18 @@ static int init_vq(struct virtio_blk *vblk)
>  			min_not_zero(num_request_queues, nr_cpu_ids),
>  			num_vqs);
>  
> +	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
> +
> +	memset(vblk->io_queues, 0, sizeof(int) * HCTX_MAX_TYPES);
> +	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
> +	vblk->io_queues[HCTX_TYPE_READ] = 0;
> +	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
> +
> +	dev_info(&vdev->dev, "%d/%d/%d default/read/poll queues\n",
> +				vblk->io_queues[HCTX_TYPE_DEFAULT],
> +				vblk->io_queues[HCTX_TYPE_READ],
> +				vblk->io_queues[HCTX_TYPE_POLL]);
> +
>  	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>  	if (!vblk->vqs)
>  		return -ENOMEM;
> @@ -578,8 +597,13 @@ static int init_vq(struct virtio_blk *vblk)
>  	}
>  
>  	for (i = 0; i < num_vqs; i++) {
> +		if (i < num_vqs - num_poll_vqs) {
> +			callbacks[i] = virtblk_done;
> +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
> +		} else {
> +			callbacks[i] = NULL;
> +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> +		}
>  		names[i] = vblk->vqs[i].name;

This would look a little cleaner with two loops:

 	for (i = 0; i < num_vqs - num_poll_vqs; i++) {
		callbacks[i] = virtblk_done;
		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
 		names[i] = vblk->vqs[i].name;
	}
 	for (; i < num_vqs; i++) {
		callbacks[i] = NULL;
		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
		names[i] = vblk->vqs[i].name;
	}

> +
> +		if (map->nr_queues == 0)
> +			continue;
> +
> +		/*
> +		 * Regular queues have interrupts and hence CPU affinity is
> +		 * defined by the core virtio code, but polling queues have
> +		 * no interrupts so we let the block layer assign CPU affinity.
> +		 */
> +		if (i == HCTX_TYPE_DEFAULT)

I'd check for
		i != HCTX_TYPE_POLL

here instead to make the check a little more explicit and future proof
for the potential addition of read queues (which would be a Linux only
change without hypervisor or spec changes).  In fact you might as well
add that support now as doing it is completely trivial once a driver
supports multiple map types.

> +static void virtblk_complete_batch(struct io_comp_batch *iob)
> +{
> +	struct request *req;
> +	struct virtblk_req *vbr;
> +
> +	rq_list_for_each(&iob->req_list, req) {
> +		vbr = blk_mq_rq_to_pdu(req);
> +		virtblk_unmap_data(req, vbr);
> +		virtblk_cleanup_cmd(req);

vbr is only used ones, so why not just:

		virtblk_unmap_data(req, blk_mq_rq_to_pdu);
?

Or even better add a cleanup patch to just remove the vbr argument to
virtblk_unmap_data as it is not needed at all.

Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Suwan Kim]

On Tue, Apr 05, 2022 at 01:51:40AM -0700, Christoph Hellwig wrote:
> On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> > This patch supports polling I/O via virtio-blk driver. Polling
> > feature is enabled by module parameter "num_poll_queues" and it
> > sets dedicated polling queues for virtio-blk. This patch improves
> > the polling I/O throughput and latency.
> > 
> > The virtio-blk driver doesn't not have a poll function and a poll
> > queue and it has been operating in interrupt driven method even if
> > the polling function is called in the upper layer.
> > 
> > virtio-blk polling is implemented upon 'batched completion' of block
> > layer. virtblk_poll() queues completed request to io_comp_batch->req_list
> > and later, virtblk_complete_batch() calls unmap function and ends
> > the requests in batch.
> > 
> > virtio-blk reads the number of poll queues from module parameter
> > "num_poll_queues". If VM sets queue parameter as below,
> > ("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
> > It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
> > as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
> > queues, the poll queues have no callback function.
> > 
> > Regarding HW-SW queue mapping, the default queue mapping uses the
> > existing method that condsiders MSI irq vector. But the poll queue
> > doesn't have an irq, so it uses the regular blk-mq cpu mapping.
> > 
> > For verifying the improvement, I did Fio polling I/O performance test
> > with io_uring engine with the options below.
> > (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
> > I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
> > queues for VM.
> > 
> > As a result, IOPS and average latency improved about 10%.
> > 
> > Test result:
> > 
> > - Fio io_uring poll without virtio-blk poll support
> > 	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
> > 	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
> > 	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us
> > 
> > - Fio io_uring poll with virtio-blk poll support
> > 	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
> > 	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
> > 	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us
> > 
> > Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> > ---
> >  drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
> >  1 file changed, 108 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 8c415be86732..712579dcd3cc 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
> >  		 "0 for no limit. "
> >  		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
> >  
> > +static unsigned int poll_queues;
> > +module_param(poll_queues, uint, 0644);
> > +MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
> > +
> >  static int major;
> >  static DEFINE_IDA(vd_index_ida);
> >  
> > @@ -81,6 +85,7 @@ struct virtio_blk {
> >  
> >  	/* num of vqs */
> >  	int num_vqs;
> > +	int io_queues[HCTX_MAX_TYPES];
> >  	struct virtio_blk_vq *vqs;
> >  };
> >  
> > @@ -548,6 +553,7 @@ static int init_vq(struct virtio_blk *vblk)
> >  	const char **names;
> >  	struct virtqueue **vqs;
> >  	unsigned short num_vqs;
> > +	unsigned int num_poll_vqs;
> >  	struct virtio_device *vdev = vblk->vdev;
> >  	struct irq_affinity desc = { 0, };
> >  
> > @@ -556,6 +562,7 @@ static int init_vq(struct virtio_blk *vblk)
> >  				   &num_vqs);
> >  	if (err)
> >  		num_vqs = 1;
> > +
> >  	if (!err && !num_vqs) {
> >  		dev_err(&vdev->dev, "MQ advertised but zero queues reported\n");
> >  		return -EINVAL;
> > @@ -565,6 +572,18 @@ static int init_vq(struct virtio_blk *vblk)
> >  			min_not_zero(num_request_queues, nr_cpu_ids),
> >  			num_vqs);
> >  
> > +	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
> > +
> > +	memset(vblk->io_queues, 0, sizeof(int) * HCTX_MAX_TYPES);
> > +	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
> > +	vblk->io_queues[HCTX_TYPE_READ] = 0;
> > +	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
> > +
> > +	dev_info(&vdev->dev, "%d/%d/%d default/read/poll queues\n",
> > +				vblk->io_queues[HCTX_TYPE_DEFAULT],
> > +				vblk->io_queues[HCTX_TYPE_READ],
> > +				vblk->io_queues[HCTX_TYPE_POLL]);
> > +
> >  	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> >  	if (!vblk->vqs)
> >  		return -ENOMEM;
> > @@ -578,8 +597,13 @@ static int init_vq(struct virtio_blk *vblk)
> >  	}
> >  
> >  	for (i = 0; i < num_vqs; i++) {
> > +		if (i < num_vqs - num_poll_vqs) {
> > +			callbacks[i] = virtblk_done;
> > +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
> > +		} else {
> > +			callbacks[i] = NULL;
> > +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> > +		}
> >  		names[i] = vblk->vqs[i].name;
> 
> This would look a little cleaner with two loops:
> 
>  	for (i = 0; i < num_vqs - num_poll_vqs; i++) {
> 		callbacks[i] = virtblk_done;
> 		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
>  		names[i] = vblk->vqs[i].name;
> 	}
>  	for (; i < num_vqs; i++) {
> 		callbacks[i] = NULL;
> 		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> 		names[i] = vblk->vqs[i].name;
> 	}
> 

It looks better. I will fix it.

> > +
> > +		if (map->nr_queues == 0)
> > +			continue;
> > +
> > +		/*
> > +		 * Regular queues have interrupts and hence CPU affinity is
> > +		 * defined by the core virtio code, but polling queues have
> > +		 * no interrupts so we let the block layer assign CPU affinity.
> > +		 */
> > +		if (i == HCTX_TYPE_DEFAULT)
> 
> I'd check for
> 		i != HCTX_TYPE_POLL
> 
> here instead to make the check a little more explicit and future proof
> for the potential addition of read queues (which would be a Linux only
> change without hypervisor or spec changes).  In fact you might as well
> add that support now as doing it is completely trivial once a driver
> supports multiple map types.

Your if condition is better choice when we consider future use of read
queue. I will fix it too.

> > +static void virtblk_complete_batch(struct io_comp_batch *iob)
> > +{
> > +	struct request *req;
> > +	struct virtblk_req *vbr;
> > +
> > +	rq_list_for_each(&iob->req_list, req) {
> > +		vbr = blk_mq_rq_to_pdu(req);
> > +		virtblk_unmap_data(req, vbr);
> > +		virtblk_cleanup_cmd(req);
> 
> vbr is only used ones, so why not just:
> 
> 		virtblk_unmap_data(req, blk_mq_rq_to_pdu);
> ?
> Or even better add a cleanup patch to just remove the vbr argument to
> virtblk_unmap_data as it is not needed at all.

Thanks for the feedback. I will add cleanup patch in this series.

Ragards,
Suwan Kim

Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Suwan Kim]

On Tue, Apr 05, 2022 at 01:51:40AM -0700, Christoph Hellwig wrote:
> On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> > This patch supports polling I/O via virtio-blk driver. Polling
> > feature is enabled by module parameter "num_poll_queues" and it
> > sets dedicated polling queues for virtio-blk. This patch improves
> > the polling I/O throughput and latency.
> > 
> > The virtio-blk driver doesn't not have a poll function and a poll
> > queue and it has been operating in interrupt driven method even if
> > the polling function is called in the upper layer.
> > 
> > virtio-blk polling is implemented upon 'batched completion' of block
> > layer. virtblk_poll() queues completed request to io_comp_batch->req_list
> > and later, virtblk_complete_batch() calls unmap function and ends
> > the requests in batch.
> > 
> > virtio-blk reads the number of poll queues from module parameter
> > "num_poll_queues". If VM sets queue parameter as below,
> > ("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
> > It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
> > as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
> > queues, the poll queues have no callback function.
> > 
> > Regarding HW-SW queue mapping, the default queue mapping uses the
> > existing method that condsiders MSI irq vector. But the poll queue
> > doesn't have an irq, so it uses the regular blk-mq cpu mapping.
> > 
> > For verifying the improvement, I did Fio polling I/O performance test
> > with io_uring engine with the options below.
> > (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
> > I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
> > queues for VM.
> > 
> > As a result, IOPS and average latency improved about 10%.
> > 
> > Test result:
> > 
> > - Fio io_uring poll without virtio-blk poll support
> > 	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
> > 	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
> > 	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us
> > 
> > - Fio io_uring poll with virtio-blk poll support
> > 	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
> > 	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
> > 	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us
> > 
> > Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> > ---
> >  drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
> >  1 file changed, 108 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 8c415be86732..712579dcd3cc 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
> >  		 "0 for no limit. "
> >  		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
> >  
> > +static unsigned int poll_queues;
> > +module_param(poll_queues, uint, 0644);
> > +MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
> > +
> >  static int major;
> >  static DEFINE_IDA(vd_index_ida);
> >  
> > @@ -81,6 +85,7 @@ struct virtio_blk {
> >  
> >  	/* num of vqs */
> >  	int num_vqs;
> > +	int io_queues[HCTX_MAX_TYPES];
> >  	struct virtio_blk_vq *vqs;
> >  };
> >  
> > @@ -548,6 +553,7 @@ static int init_vq(struct virtio_blk *vblk)
> >  	const char **names;
> >  	struct virtqueue **vqs;
> >  	unsigned short num_vqs;
> > +	unsigned int num_poll_vqs;
> >  	struct virtio_device *vdev = vblk->vdev;
> >  	struct irq_affinity desc = { 0, };
> >  
> > @@ -556,6 +562,7 @@ static int init_vq(struct virtio_blk *vblk)
> >  				   &num_vqs);
> >  	if (err)
> >  		num_vqs = 1;
> > +
> >  	if (!err && !num_vqs) {
> >  		dev_err(&vdev->dev, "MQ advertised but zero queues reported\n");
> >  		return -EINVAL;
> > @@ -565,6 +572,18 @@ static int init_vq(struct virtio_blk *vblk)
> >  			min_not_zero(num_request_queues, nr_cpu_ids),
> >  			num_vqs);
> >  
> > +	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
> > +
> > +	memset(vblk->io_queues, 0, sizeof(int) * HCTX_MAX_TYPES);
> > +	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
> > +	vblk->io_queues[HCTX_TYPE_READ] = 0;
> > +	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
> > +
> > +	dev_info(&vdev->dev, "%d/%d/%d default/read/poll queues\n",
> > +				vblk->io_queues[HCTX_TYPE_DEFAULT],
> > +				vblk->io_queues[HCTX_TYPE_READ],
> > +				vblk->io_queues[HCTX_TYPE_POLL]);
> > +
> >  	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> >  	if (!vblk->vqs)
> >  		return -ENOMEM;
> > @@ -578,8 +597,13 @@ static int init_vq(struct virtio_blk *vblk)
> >  	}
> >  
> >  	for (i = 0; i < num_vqs; i++) {
> > +		if (i < num_vqs - num_poll_vqs) {
> > +			callbacks[i] = virtblk_done;
> > +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
> > +		} else {
> > +			callbacks[i] = NULL;
> > +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> > +		}
> >  		names[i] = vblk->vqs[i].name;
> 
> This would look a little cleaner with two loops:
> 
>  	for (i = 0; i < num_vqs - num_poll_vqs; i++) {
> 		callbacks[i] = virtblk_done;
> 		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
>  		names[i] = vblk->vqs[i].name;
> 	}
>  	for (; i < num_vqs; i++) {
> 		callbacks[i] = NULL;
> 		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> 		names[i] = vblk->vqs[i].name;
> 	}
> 
> > +
> > +		if (map->nr_queues == 0)
> > +			continue;
> > +
> > +		/*
> > +		 * Regular queues have interrupts and hence CPU affinity is
> > +		 * defined by the core virtio code, but polling queues have
> > +		 * no interrupts so we let the block layer assign CPU affinity.
> > +		 */
> > +		if (i == HCTX_TYPE_DEFAULT)
> 
> I'd check for
> 		i != HCTX_TYPE_POLL
> 
> here instead to make the check a little more explicit and future proof
> for the potential addition of read queues (which would be a Linux only
> change without hypervisor or spec changes).  In fact you might as well
> add that support now as doing it is completely trivial once a driver
> supports multiple map types.
> 
> > +static void virtblk_complete_batch(struct io_comp_batch *iob)
> > +{
> > +	struct request *req;
> > +	struct virtblk_req *vbr;
> > +
> > +	rq_list_for_each(&iob->req_list, req) {
> > +		vbr = blk_mq_rq_to_pdu(req);
> > +		virtblk_unmap_data(req, vbr);
> > +		virtblk_cleanup_cmd(req);
> 
> vbr is only used ones, so why not just:
> 
> 		virtblk_unmap_data(req, blk_mq_rq_to_pdu);
> ?
> 
> Or even better add a cleanup patch to just remove the vbr argument to
> virtblk_unmap_data as it is not needed at all.

I replied that I will add cleanup path but after thinking about it
again, it seems better not to remove the argument because
blk_mq_rq_to_pdu() needs type casting anyway.

So I will fix it to "virtblk_unmap_data(req, blk_mq_rq_to_pdu);"
as you mentioned.

Regards,
Suwan Kim