[PATCH V4 0/3] Improve virtio-blk performance

Hi, Jens & Rusty

This version is rebased against linux-next which resolves the conflict with
Paolo Bonzini's 'virtio-blk: allow toggling host cache between writeback and
writethrough' patch.

Patch 1/3 and 2/3 applies on linus's master as well. Since Rusty will pick up
patch 3/3 so the changes to block core (adding blk_bio_map_sg()) will have a
user.

Jens, could you please consider picking up the dependencies 1/3 and 2/3 in your
tree. Thanks!

This patchset implements bio-based IO path for virito-blk to improve
performance.

Fio test shows bio-based IO path gives the following performance improvement:

1) Ramdisk device
     With bio-based IO path, sequential read/write, random read/write
     IOPS boost         : 28%, 24%, 21%, 16%
     Latency improvement: 32%, 17%, 21%, 16%
2) Fusion IO device
     With bio-based IO path, sequential read/write, random read/write
     IOPS boost         : 11%, 11%, 13%, 10%
     Latency improvement: 10%, 10%, 12%, 10%

Asias He (3):
  block: Introduce __blk_segment_map_sg() helper
  block: Add blk_bio_map_sg() helper
  virtio-blk: Add bio-based IO path for virtio-blk

 block/blk-merge.c          |  117 +++++++++++++++++--------
 drivers/block/virtio_blk.c |  203 +++++++++++++++++++++++++++++++++++---------
 include/linux/blkdev.h     |    2 +
 3 files changed, 247 insertions(+), 75 deletions(-)

-- 
1.7.10.4

This patch introduces bio-based IO path for virtio-blk.

Compared to request-based IO path, bio-based IO path uses driver
provided ->make_request_fn() method to bypasses the IO scheduler. It
handles the bio to device directly without allocating a request in block
layer. This reduces the IO path in guest kernel to achieve high IOPS
and lower latency. The downside is that guest can not use the IO
scheduler to merge and sort requests. However, this is not a big problem
if the backend disk in host side uses faster disk device.

When the bio-based IO path is not enabled, virtio-blk still uses the
original request-based IO path, no performance difference is observed.

Performance evaluation:
-----------------------------
1) Fio test is performed in a 8 vcpu guest with ramdisk based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 28%, 24%, 21%, 16%
 Latency improvement: 32%, 17%, 21%, 16%

Long version:
 With bio-based IO path:
  seq-read  : io=2048.0MB, bw=116996KB/s, iops=233991 , runt= 17925msec
  seq-write : io=2048.0MB, bw=100829KB/s, iops=201658 , runt= 20799msec
  rand-read : io=3095.7MB, bw=112134KB/s, iops=224268 , runt= 28269msec
  rand-write: io=3095.7MB, bw=96198KB/s,  iops=192396 , runt= 32952msec
    clat (usec): min=0 , max=2631.6K, avg=58716.99, stdev=191377.30
    clat (usec): min=0 , max=1753.2K, avg=66423.25, stdev=81774.35
    clat (usec): min=0 , max=2915.5K, avg=61685.70, stdev=120598.39
    clat (usec): min=0 , max=1933.4K, avg=76935.12, stdev=96603.45
  cpu : usr=74.08%, sys=703.84%, ctx=29661403, majf=21354, minf=22460954
  cpu : usr=70.92%, sys=702.81%, ctx=77219828, majf=13980, minf=27713137
  cpu : usr=72.23%, sys=695.37%, ctx=88081059, majf=18475, minf=28177648
  cpu : usr=69.69%, sys=654.13%, ctx=145476035, majf=15867, minf=26176375
 With request-based IO path:
  seq-read  : io=2048.0MB, bw=91074KB/s, iops=182147 , runt= 23027msec
  seq-write : io=2048.0MB, bw=80725KB/s, iops=161449 , runt= 25979msec
  rand-read : io=3095.7MB, bw=92106KB/s, iops=184211 , runt= 34416msec
  rand-write: io=3095.7MB, bw=82815KB/s, iops=165630 , runt= 38277msec
    clat (usec): min=0 , max=1932.4K, avg=77824.17, stdev=170339.49
    clat (usec): min=0 , max=2510.2K, avg=78023.96, stdev=146949.15
    clat (usec): min=0 , max=3037.2K, avg=74746.53, stdev=128498.27
    clat (usec): min=0 , max=1363.4K, avg=89830.75, stdev=114279.68
  cpu : usr=53.28%, sys=724.19%, ctx=37988895, majf=17531, minf=23577622
  cpu : usr=49.03%, sys=633.20%, ctx=205935380, majf=18197, minf=27288959
  cpu : usr=55.78%, sys=722.40%, ctx=101525058, majf=19273, minf=28067082
  cpu : usr=56.55%, sys=690.83%, ctx=228205022, majf=18039, minf=26551985

2) Fio test is performed in a 8 vcpu guest with Fusion-IO based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 11%, 11%, 13%, 10%
 Latency improvement: 10%, 10%, 12%, 10%
Long Version:
 With bio-based IO path:
  read : io=2048.0MB, bw=58920KB/s, iops=117840 , runt= 35593msec
  write: io=2048.0MB, bw=64308KB/s, iops=128616 , runt= 32611msec
  read : io=3095.7MB, bw=59633KB/s, iops=119266 , runt= 53157msec
  write: io=3095.7MB, bw=62993KB/s, iops=125985 , runt= 50322msec
    clat (usec): min=0 , max=1284.3K, avg=128109.01, stdev=71513.29
    clat (usec): min=94 , max=962339 , avg=116832.95, stdev=65836.80
    clat (usec): min=0 , max=1846.6K, avg=128509.99, stdev=89575.07
    clat (usec): min=0 , max=2256.4K, avg=121361.84, stdev=82747.25
  cpu : usr=56.79%, sys=421.70%, ctx=147335118, majf=21080, minf=19852517
  cpu : usr=61.81%, sys=455.53%, ctx=143269950, majf=16027, minf=24800604
  cpu : usr=63.10%, sys=455.38%, ctx=178373538, majf=16958, minf=24822612
  cpu : usr=62.04%, sys=453.58%, ctx=226902362, majf=16089, minf=23278105
 With request-based IO path:
  read : io=2048.0MB, bw=52896KB/s, iops=105791 , runt= 39647msec
  write: io=2048.0MB, bw=57856KB/s, iops=115711 , runt= 36248msec
  read : io=3095.7MB, bw=52387KB/s, iops=104773 , runt= 60510msec
  write: io=3095.7MB, bw=57310KB/s, iops=114619 , runt= 55312msec
    clat (usec): min=0 , max=1532.6K, avg=142085.62, stdev=109196.84
    clat (usec): min=0 , max=1487.4K, avg=129110.71, stdev=114973.64
    clat (usec): min=0 , max=1388.6K, avg=145049.22, stdev=107232.55
    clat (usec): min=0 , max=1465.9K, avg=133585.67, stdev=110322.95
  cpu : usr=44.08%, sys=590.71%, ctx=451812322, majf=14841, minf=17648641
  cpu : usr=48.73%, sys=610.78%, ctx=418953997, majf=22164, minf=26850689
  cpu : usr=45.58%, sys=581.16%, ctx=714079216, majf=21497, minf=22558223
  cpu : usr=48.40%, sys=599.65%, ctx=656089423, majf=16393, minf=23824409

How to use:
-----------------------------
Add 'virtio_blk.use_bio=1' to kernel cmdline or 'modprobe virtio_blk
use_bio=1' to enable ->make_request_fn() based I/O path.

Cc: Rusty Russell <redacted>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Rusty Russell <redacted>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Minchan Kim <redacted>
Signed-off-by: Asias He <redacted>
---
 drivers/block/virtio_blk.c |  203 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 163 insertions(+), 40 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c0bbeb4..95cfeed 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c

@@ -14,6 +14,9 @@
 
 #define PART_BITS 4
 
+static bool use_bio;
+module_param(use_bio, bool, S_IRUGO);
+
 static int major;
 static DEFINE_IDA(vd_index_ida);
 

@@ -23,6 +26,7 @@ struct virtio_blk
 {
 	struct virtio_device *vdev;
 	struct virtqueue *vq;
+	wait_queue_head_t queue_wait;
 
 	/* The disk structure for the kernel. */
 	struct gendisk *disk;

@@ -51,53 +55,87 @@ struct virtio_blk
 struct virtblk_req
 {
 	struct request *req;
+	struct bio *bio;
 	struct virtio_blk_outhdr out_hdr;
 	struct virtio_scsi_inhdr in_hdr;
 	u8 status;
+	struct scatterlist sg[];
 };
 
-static void blk_done(struct virtqueue *vq)
+static inline int virtblk_result(struct virtblk_req *vbr)
+{
+	switch (vbr->status) {
+	case VIRTIO_BLK_S_OK:
+		return 0;
+	case VIRTIO_BLK_S_UNSUPP:
+		return -ENOTTY;
+	default:
+		return -EIO;
+	}
+}
+
+static inline void virtblk_request_done(struct virtio_blk *vblk,
+					struct virtblk_req *vbr)
+{
+	struct request *req = vbr->req;
+	int error = virtblk_result(vbr);
+
+	if (req->cmd_type == REQ_TYPE_BLOCK_PC) {
+		req->resid_len = vbr->in_hdr.residual;
+		req->sense_len = vbr->in_hdr.sense_len;
+		req->errors = vbr->in_hdr.errors;
+	} else if (req->cmd_type == REQ_TYPE_SPECIAL) {
+		req->errors = (error != 0);
+	}
+
+	__blk_end_request_all(req, error);
+	mempool_free(vbr, vblk->pool);
+}
+
+static inline void virtblk_bio_done(struct virtio_blk *vblk,
+				    struct virtblk_req *vbr)
+{
+	bio_endio(vbr->bio, virtblk_result(vbr));
+	mempool_free(vbr, vblk->pool);
+}
+
+static void virtblk_done(struct virtqueue *vq)
 {
 	struct virtio_blk *vblk = vq->vdev->priv;
+	unsigned long bio_done = 0, req_done = 0;
 	struct virtblk_req *vbr;
-	unsigned int len;
 	unsigned long flags;
+	unsigned int len;
 
 	spin_lock_irqsave(vblk->disk->queue->queue_lock, flags);
 	while ((vbr = virtqueue_get_buf(vblk->vq, &len)) != NULL) {
-		int error;
-
-		switch (vbr->status) {
-		case VIRTIO_BLK_S_OK:
-			error = 0;
-			break;
-		case VIRTIO_BLK_S_UNSUPP:
-			error = -ENOTTY;
-			break;
-		default:
-			error = -EIO;
-			break;
-		}
-
-		switch (vbr->req->cmd_type) {
-		case REQ_TYPE_BLOCK_PC:
-			vbr->req->resid_len = vbr->in_hdr.residual;
-			vbr->req->sense_len = vbr->in_hdr.sense_len;
-			vbr->req->errors = vbr->in_hdr.errors;
-			break;
-		case REQ_TYPE_SPECIAL:
-			vbr->req->errors = (error != 0);
-			break;
-		default:
-			break;
+		if (vbr->bio) {
+			virtblk_bio_done(vblk, vbr);
+			bio_done++;
+		} else {
+			virtblk_request_done(vblk, vbr);
+			req_done++;
 		}
-
-		__blk_end_request_all(vbr->req, error);
-		mempool_free(vbr, vblk->pool);
 	}
 	/* In case queue is stopped waiting for more buffers. */
-	blk_start_queue(vblk->disk->queue);
+	if (req_done)
+		blk_start_queue(vblk->disk->queue);
 	spin_unlock_irqrestore(vblk->disk->queue->queue_lock, flags);
+
+	if (bio_done)
+		wake_up(&vblk->queue_wait);
+}
+
+static inline struct virtblk_req *virtblk_alloc_req(struct virtio_blk *vblk,
+						    gfp_t gfp_mask)
+{
+	struct virtblk_req *vbr;
+
+	vbr = mempool_alloc(vblk->pool, gfp_mask);
+	if (vbr && use_bio)
+		sg_init_table(vbr->sg, vblk->sg_elems);
+
+	return vbr;
 }
 
 static bool do_req(struct request_queue *q, struct virtio_blk *vblk,

@@ -106,13 +144,13 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
 	unsigned long num, out = 0, in = 0;
 	struct virtblk_req *vbr;
 
-	vbr = mempool_alloc(vblk->pool, GFP_ATOMIC);
+	vbr = virtblk_alloc_req(vblk, GFP_ATOMIC);
 	if (!vbr)
 		/* When another request finishes we'll try again. */
 		return false;
 
 	vbr->req = req;
-
+	vbr->bio = NULL;
 	if (req->cmd_flags & REQ_FLUSH) {
 		vbr->out_hdr.type = VIRTIO_BLK_T_FLUSH;
 		vbr->out_hdr.sector = 0;

@@ -172,7 +210,8 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
 		}
 	}
 
-	if (virtqueue_add_buf(vblk->vq, vblk->sg, out, in, vbr, GFP_ATOMIC)<0) {
+	if (virtqueue_add_buf(vblk->vq, vblk->sg, out, in, vbr,
+			      GFP_ATOMIC) < 0) {
 		mempool_free(vbr, vblk->pool);
 		return false;
 	}

@@ -180,7 +219,7 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
 	return true;
 }
 
-static void do_virtblk_request(struct request_queue *q)
+static void virtblk_request(struct request_queue *q)
 {
 	struct virtio_blk *vblk = q->queuedata;
 	struct request *req;

@@ -203,6 +242,82 @@ static void do_virtblk_request(struct request_queue *q)
 		virtqueue_kick(vblk->vq);
 }
 
+static void virtblk_add_buf_wait(struct virtio_blk *vblk,
+				 struct virtblk_req *vbr,
+				 unsigned long out,
+				 unsigned long in)
+{
+	DEFINE_WAIT(wait);
+
+	for (;;) {
+		prepare_to_wait_exclusive(&vblk->queue_wait, &wait,
+					  TASK_UNINTERRUPTIBLE);
+
+		spin_lock_irq(vblk->disk->queue->queue_lock);
+		if (virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
+				      GFP_ATOMIC) < 0) {
+			spin_unlock_irq(vblk->disk->queue->queue_lock);
+			io_schedule();
+		} else {
+			virtqueue_kick(vblk->vq);
+			spin_unlock_irq(vblk->disk->queue->queue_lock);
+			break;
+		}
+
+	}
+
+	finish_wait(&vblk->queue_wait, &wait);
+}
+
+static void virtblk_make_request(struct request_queue *q, struct bio *bio)
+{
+	struct virtio_blk *vblk = q->queuedata;
+	unsigned int num, out = 0, in = 0;
+	struct virtblk_req *vbr;
+
+	BUG_ON(bio->bi_phys_segments + 2 > vblk->sg_elems);
+	BUG_ON(bio->bi_rw & (REQ_FLUSH | REQ_FUA));
+
+	vbr = virtblk_alloc_req(vblk, GFP_NOIO);
+	if (!vbr) {
+		bio_endio(bio, -ENOMEM);
+		return;
+	}
+
+	vbr->bio = bio;
+	vbr->req = NULL;
+	vbr->out_hdr.type = 0;
+	vbr->out_hdr.sector = bio->bi_sector;
+	vbr->out_hdr.ioprio = bio_prio(bio);
+
+	sg_set_buf(&vbr->sg[out++], &vbr->out_hdr, sizeof(vbr->out_hdr));
+
+	num = blk_bio_map_sg(q, bio, vbr->sg + out);
+
+	sg_set_buf(&vbr->sg[num + out + in++], &vbr->status,
+		   sizeof(vbr->status));
+
+	if (num) {
+		if (bio->bi_rw & REQ_WRITE) {
+			vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
+			out += num;
+		} else {
+			vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
+			in += num;
+		}
+	}
+
+	spin_lock_irq(vblk->disk->queue->queue_lock);
+	if (unlikely(virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
+				       GFP_ATOMIC) < 0)) {
+		spin_unlock_irq(vblk->disk->queue->queue_lock);
+		virtblk_add_buf_wait(vblk, vbr, out, in);
+		return;
+	}
+	virtqueue_kick(vblk->vq);
+	spin_unlock_irq(vblk->disk->queue->queue_lock);
+}
+
 /* return id (s/n) string for *disk to *id_str
  */
 static int virtblk_get_id(struct gendisk *disk, char *id_str)

@@ -360,7 +475,7 @@ static int init_vq(struct virtio_blk *vblk)
 	int err = 0;
 
 	/* We expect one virtqueue, for output. */
-	vblk->vq = virtio_find_single_vq(vblk->vdev, blk_done, "requests");
+	vblk->vq = virtio_find_single_vq(vblk->vdev, virtblk_done, "requests");
 	if (IS_ERR(vblk->vq))
 		err = PTR_ERR(vblk->vq);
 

@@ -414,7 +529,7 @@ static void virtblk_update_cache_mode(struct virtio_device *vdev)
 	u8 writeback = virtblk_get_cache_mode(vdev);
 	struct virtio_blk *vblk = vdev->priv;
 
-	if (writeback)
+	if (writeback && !use_bio)
 		blk_queue_flush(vblk->disk->queue, REQ_FLUSH);
 	else
 		blk_queue_flush(vblk->disk->queue, 0);

@@ -477,6 +592,8 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 	struct virtio_blk *vblk;
 	struct request_queue *q;
 	int err, index;
+	int pool_size;
+
 	u64 cap;
 	u32 v, blk_size, sg_elems, opt_io_size;
 	u16 min_io_size;

@@ -506,10 +623,12 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 		goto out_free_index;
 	}
 
+	init_waitqueue_head(&vblk->queue_wait);
 	vblk->vdev = vdev;
 	vblk->sg_elems = sg_elems;
 	sg_init_table(vblk->sg, vblk->sg_elems);
 	mutex_init(&vblk->config_lock);
+
 	INIT_WORK(&vblk->config_work, virtblk_config_changed_work);
 	vblk->config_enable = true;
 

@@ -517,7 +636,10 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 	if (err)
 		goto out_free_vblk;
 
-	vblk->pool = mempool_create_kmalloc_pool(1,sizeof(struct virtblk_req));
+	pool_size = sizeof(struct virtblk_req);
+	if (use_bio)
+		pool_size += sizeof(struct scatterlist) * sg_elems;
+	vblk->pool = mempool_create_kmalloc_pool(1, pool_size);
 	if (!vblk->pool) {
 		err = -ENOMEM;
 		goto out_free_vq;

@@ -530,12 +652,14 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 		goto out_mempool;
 	}
 
-	q = vblk->disk->queue = blk_init_queue(do_virtblk_request, NULL);
+	q = vblk->disk->queue = blk_init_queue(virtblk_request, NULL);
 	if (!q) {
 		err = -ENOMEM;
 		goto out_put_disk;
 	}
 
+	if (use_bio)
+		blk_queue_make_request(q, virtblk_make_request);
 	q->queuedata = vblk;
 
 	virtblk_name_format("vd", index, vblk->disk->disk_name, DISK_NAME_LEN);

@@ -620,7 +744,6 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 	if (!err && opt_io_size)
 		blk_queue_io_opt(q, blk_size * opt_io_size);
 
-
 	add_disk(vblk->disk);
 	err = device_create_file(disk_to_dev(vblk->disk), &dev_attr_serial);
 	if (err)

-- 
1.7.10.4

On 07/28/2012 04:21 AM, Asias He wrote:

This patch introduces bio-based IO path for virtio-blk.

Compared to request-based IO path, bio-based IO path uses driver
provided ->make_request_fn() method to bypasses the IO scheduler. It
handles the bio to device directly without allocating a request in block
layer. This reduces the IO path in guest kernel to achieve high IOPS
and lower latency. The downside is that guest can not use the IO
scheduler to merge and sort requests. However, this is not a big problem
if the backend disk in host side uses faster disk device.

When the bio-based IO path is not enabled, virtio-blk still uses the
original request-based IO path, no performance difference is observed.

Performance evaluation:
-----------------------------
1) Fio test is performed in a 8 vcpu guest with ramdisk based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 28%, 24%, 21%, 16%
 Latency improvement: 32%, 17%, 21%, 16%

Long version:
 With bio-based IO path:
  seq-read  : io=2048.0MB, bw=116996KB/s, iops=233991 , runt= 17925msec
  seq-write : io=2048.0MB, bw=100829KB/s, iops=201658 , runt= 20799msec
  rand-read : io=3095.7MB, bw=112134KB/s, iops=224268 , runt= 28269msec
  rand-write: io=3095.7MB, bw=96198KB/s,  iops=192396 , runt= 32952msec
    clat (usec): min=0 , max=2631.6K, avg=58716.99, stdev=191377.30
    clat (usec): min=0 , max=1753.2K, avg=66423.25, stdev=81774.35
    clat (usec): min=0 , max=2915.5K, avg=61685.70, stdev=120598.39
    clat (usec): min=0 , max=1933.4K, avg=76935.12, stdev=96603.45
  cpu : usr=74.08%, sys=703.84%, ctx=29661403, majf=21354, minf=22460954
  cpu : usr=70.92%, sys=702.81%, ctx=77219828, majf=13980, minf=27713137
  cpu : usr=72.23%, sys=695.37%, ctx=88081059, majf=18475, minf=28177648
  cpu : usr=69.69%, sys=654.13%, ctx=145476035, majf=15867, minf=26176375
 With request-based IO path:
  seq-read  : io=2048.0MB, bw=91074KB/s, iops=182147 , runt= 23027msec
  seq-write : io=2048.0MB, bw=80725KB/s, iops=161449 , runt= 25979msec
  rand-read : io=3095.7MB, bw=92106KB/s, iops=184211 , runt= 34416msec
  rand-write: io=3095.7MB, bw=82815KB/s, iops=165630 , runt= 38277msec
    clat (usec): min=0 , max=1932.4K, avg=77824.17, stdev=170339.49
    clat (usec): min=0 , max=2510.2K, avg=78023.96, stdev=146949.15
    clat (usec): min=0 , max=3037.2K, avg=74746.53, stdev=128498.27
    clat (usec): min=0 , max=1363.4K, avg=89830.75, stdev=114279.68
  cpu : usr=53.28%, sys=724.19%, ctx=37988895, majf=17531, minf=23577622
  cpu : usr=49.03%, sys=633.20%, ctx=205935380, majf=18197, minf=27288959
  cpu : usr=55.78%, sys=722.40%, ctx=101525058, majf=19273, minf=28067082
  cpu : usr=56.55%, sys=690.83%, ctx=228205022, majf=18039, minf=26551985

2) Fio test is performed in a 8 vcpu guest with Fusion-IO based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 11%, 11%, 13%, 10%
 Latency improvement: 10%, 10%, 12%, 10%
Long Version:
 With bio-based IO path:
  read : io=2048.0MB, bw=58920KB/s, iops=117840 , runt= 35593msec
  write: io=2048.0MB, bw=64308KB/s, iops=128616 , runt= 32611msec
  read : io=3095.7MB, bw=59633KB/s, iops=119266 , runt= 53157msec
  write: io=3095.7MB, bw=62993KB/s, iops=125985 , runt= 50322msec
    clat (usec): min=0 , max=1284.3K, avg=128109.01, stdev=71513.29
    clat (usec): min=94 , max=962339 , avg=116832.95, stdev=65836.80
    clat (usec): min=0 , max=1846.6K, avg=128509.99, stdev=89575.07
    clat (usec): min=0 , max=2256.4K, avg=121361.84, stdev=82747.25
  cpu : usr=56.79%, sys=421.70%, ctx=147335118, majf=21080, minf=19852517
  cpu : usr=61.81%, sys=455.53%, ctx=143269950, majf=16027, minf=24800604
  cpu : usr=63.10%, sys=455.38%, ctx=178373538, majf=16958, minf=24822612
  cpu : usr=62.04%, sys=453.58%, ctx=226902362, majf=16089, minf=23278105
 With request-based IO path:
  read : io=2048.0MB, bw=52896KB/s, iops=105791 , runt= 39647msec
  write: io=2048.0MB, bw=57856KB/s, iops=115711 , runt= 36248msec
  read : io=3095.7MB, bw=52387KB/s, iops=104773 , runt= 60510msec
  write: io=3095.7MB, bw=57310KB/s, iops=114619 , runt= 55312msec
    clat (usec): min=0 , max=1532.6K, avg=142085.62, stdev=109196.84
    clat (usec): min=0 , max=1487.4K, avg=129110.71, stdev=114973.64
    clat (usec): min=0 , max=1388.6K, avg=145049.22, stdev=107232.55
    clat (usec): min=0 , max=1465.9K, avg=133585.67, stdev=110322.95
  cpu : usr=44.08%, sys=590.71%, ctx=451812322, majf=14841, minf=17648641
  cpu : usr=48.73%, sys=610.78%, ctx=418953997, majf=22164, minf=26850689
  cpu : usr=45.58%, sys=581.16%, ctx=714079216, majf=21497, minf=22558223
  cpu : usr=48.40%, sys=599.65%, ctx=656089423, majf=16393, minf=23824409

What are the cases where we'll see a performance degradation with using the bio path? Could we measure performance for those as well?

How to use:
-----------------------------
Add 'virtio_blk.use_bio=1' to kernel cmdline or 'modprobe virtio_blk
use_bio=1' to enable ->make_request_fn() based I/O path.

If there are, in fact, no cases where performance is degraded, can use_bio=1 be the default?

On Sat, Jul 28, 2012 at 10:21:05AM +0800, Asias He wrote:

This patch introduces bio-based IO path for virtio-blk.

Compared to request-based IO path, bio-based IO path uses driver
provided ->make_request_fn() method to bypasses the IO scheduler. It
handles the bio to device directly without allocating a request in block
layer. This reduces the IO path in guest kernel to achieve high IOPS
and lower latency. The downside is that guest can not use the IO
scheduler to merge and sort requests. However, this is not a big problem
if the backend disk in host side uses faster disk device.

If this optimization depends on the host, then it
should be reported to the guest using a feature bit,
as opposed to being guest driven.

When the bio-based IO path is not enabled, virtio-blk still uses the
original request-based IO path, no performance difference is observed.

Performance evaluation:
-----------------------------
1) Fio test is performed in a 8 vcpu guest with ramdisk based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 28%, 24%, 21%, 16%
 Latency improvement: 32%, 17%, 21%, 16%

Long version:
 With bio-based IO path:
  seq-read  : io=2048.0MB, bw=116996KB/s, iops=233991 , runt= 17925msec
  seq-write : io=2048.0MB, bw=100829KB/s, iops=201658 , runt= 20799msec
  rand-read : io=3095.7MB, bw=112134KB/s, iops=224268 , runt= 28269msec
  rand-write: io=3095.7MB, bw=96198KB/s,  iops=192396 , runt= 32952msec
    clat (usec): min=0 , max=2631.6K, avg=58716.99, stdev=191377.30
    clat (usec): min=0 , max=1753.2K, avg=66423.25, stdev=81774.35
    clat (usec): min=0 , max=2915.5K, avg=61685.70, stdev=120598.39
    clat (usec): min=0 , max=1933.4K, avg=76935.12, stdev=96603.45
  cpu : usr=74.08%, sys=703.84%, ctx=29661403, majf=21354, minf=22460954
  cpu : usr=70.92%, sys=702.81%, ctx=77219828, majf=13980, minf=27713137
  cpu : usr=72.23%, sys=695.37%, ctx=88081059, majf=18475, minf=28177648
  cpu : usr=69.69%, sys=654.13%, ctx=145476035, majf=15867, minf=26176375
 With request-based IO path:
  seq-read  : io=2048.0MB, bw=91074KB/s, iops=182147 , runt= 23027msec
  seq-write : io=2048.0MB, bw=80725KB/s, iops=161449 , runt= 25979msec
  rand-read : io=3095.7MB, bw=92106KB/s, iops=184211 , runt= 34416msec
  rand-write: io=3095.7MB, bw=82815KB/s, iops=165630 , runt= 38277msec
    clat (usec): min=0 , max=1932.4K, avg=77824.17, stdev=170339.49
    clat (usec): min=0 , max=2510.2K, avg=78023.96, stdev=146949.15
    clat (usec): min=0 , max=3037.2K, avg=74746.53, stdev=128498.27
    clat (usec): min=0 , max=1363.4K, avg=89830.75, stdev=114279.68
  cpu : usr=53.28%, sys=724.19%, ctx=37988895, majf=17531, minf=23577622
  cpu : usr=49.03%, sys=633.20%, ctx=205935380, majf=18197, minf=27288959
  cpu : usr=55.78%, sys=722.40%, ctx=101525058, majf=19273, minf=28067082
  cpu : usr=56.55%, sys=690.83%, ctx=228205022, majf=18039, minf=26551985

So bio based causes cpu to jump up by some 30%?
What happens if you divide IOPS/CPU?
Any improvement that comes from increasing the cpu share
of the given guest on the host will not scale well on
a typical overcommitted host.

2) Fio test is performed in a 8 vcpu guest with Fusion-IO based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 11%, 11%, 13%, 10%
 Latency improvement: 10%, 10%, 12%, 10%
Long Version:
 With bio-based IO path:
  read : io=2048.0MB, bw=58920KB/s, iops=117840 , runt= 35593msec
  write: io=2048.0MB, bw=64308KB/s, iops=128616 , runt= 32611msec
  read : io=3095.7MB, bw=59633KB/s, iops=119266 , runt= 53157msec
  write: io=3095.7MB, bw=62993KB/s, iops=125985 , runt= 50322msec
    clat (usec): min=0 , max=1284.3K, avg=128109.01, stdev=71513.29
    clat (usec): min=94 , max=962339 , avg=116832.95, stdev=65836.80
    clat (usec): min=0 , max=1846.6K, avg=128509.99, stdev=89575.07
    clat (usec): min=0 , max=2256.4K, avg=121361.84, stdev=82747.25
  cpu : usr=56.79%, sys=421.70%, ctx=147335118, majf=21080, minf=19852517
  cpu : usr=61.81%, sys=455.53%, ctx=143269950, majf=16027, minf=24800604
  cpu : usr=63.10%, sys=455.38%, ctx=178373538, majf=16958, minf=24822612
  cpu : usr=62.04%, sys=453.58%, ctx=226902362, majf=16089, minf=23278105
 With request-based IO path:
  read : io=2048.0MB, bw=52896KB/s, iops=105791 , runt= 39647msec
  write: io=2048.0MB, bw=57856KB/s, iops=115711 , runt= 36248msec
  read : io=3095.7MB, bw=52387KB/s, iops=104773 , runt= 60510msec
  write: io=3095.7MB, bw=57310KB/s, iops=114619 , runt= 55312msec
    clat (usec): min=0 , max=1532.6K, avg=142085.62, stdev=109196.84
    clat (usec): min=0 , max=1487.4K, avg=129110.71, stdev=114973.64
    clat (usec): min=0 , max=1388.6K, avg=145049.22, stdev=107232.55
    clat (usec): min=0 , max=1465.9K, avg=133585.67, stdev=110322.95
  cpu : usr=44.08%, sys=590.71%, ctx=451812322, majf=14841, minf=17648641
  cpu : usr=48.73%, sys=610.78%, ctx=418953997, majf=22164, minf=26850689
  cpu : usr=45.58%, sys=581.16%, ctx=714079216, majf=21497, minf=22558223
  cpu : usr=48.40%, sys=599.65%, ctx=656089423, majf=16393, minf=23824409

Is this host or guest cpu? We should probably measure host cpu
as this includes device overhead which could vary by load.

On Sun, 29 Jul 2012 14:11:15 +0300, "Michael S. Tsirkin" [off-list ref] wrote:

On Sat, Jul 28, 2012 at 10:21:05AM +0800, Asias He wrote:

quoted

This patch introduces bio-based IO path for virtio-blk.

Compared to request-based IO path, bio-based IO path uses driver
provided ->make_request_fn() method to bypasses the IO scheduler. It
handles the bio to device directly without allocating a request in block
layer. This reduces the IO path in guest kernel to achieve high IOPS
and lower latency. The downside is that guest can not use the IO
scheduler to merge and sort requests. However, this is not a big problem
if the backend disk in host side uses faster disk device.

If this optimization depends on the host, then it
should be reported to the guest using a feature bit,
as opposed to being guest driven.

I consider this approach a half-way step.  Quick attempts on my laptop
and I couldn't find a case where the bio path was a loss, but in theory
if the host wasn't doing any reordering and it was a slow device, you'd
want the guest to do so.

I'm not sure if current qemu can be configured to do such a thing?

Cheers,
Rusty.

On 07/29/2012 07:11 PM, Michael S. Tsirkin wrote:

On Sat, Jul 28, 2012 at 10:21:05AM +0800, Asias He wrote:

quoted

This patch introduces bio-based IO path for virtio-blk.

Compared to request-based IO path, bio-based IO path uses driver
provided ->make_request_fn() method to bypasses the IO scheduler. It
handles the bio to device directly without allocating a request in block
layer. This reduces the IO path in guest kernel to achieve high IOPS
and lower latency. The downside is that guest can not use the IO
scheduler to merge and sort requests. However, this is not a big problem
if the backend disk in host side uses faster disk device.

If this optimization depends on the host, then it
should be reported to the guest using a feature bit,
as opposed to being guest driven.

quoted

When the bio-based IO path is not enabled, virtio-blk still uses the
original request-based IO path, no performance difference is observed.

Performance evaluation:
-----------------------------
1) Fio test is performed in a 8 vcpu guest with ramdisk based guest using
kvm tool.

Short version:
  With bio-based IO path, sequential read/write, random read/write
  IOPS boost         : 28%, 24%, 21%, 16%
  Latency improvement: 32%, 17%, 21%, 16%

Long version:
  With bio-based IO path:
   seq-read  : io=2048.0MB, bw=116996KB/s, iops=233991 , runt= 17925msec
   seq-write : io=2048.0MB, bw=100829KB/s, iops=201658 , runt= 20799msec
   rand-read : io=3095.7MB, bw=112134KB/s, iops=224268 , runt= 28269msec
   rand-write: io=3095.7MB, bw=96198KB/s,  iops=192396 , runt= 32952msec
     clat (usec): min=0 , max=2631.6K, avg=58716.99, stdev=191377.30
     clat (usec): min=0 , max=1753.2K, avg=66423.25, stdev=81774.35
     clat (usec): min=0 , max=2915.5K, avg=61685.70, stdev=120598.39
     clat (usec): min=0 , max=1933.4K, avg=76935.12, stdev=96603.45
   cpu : usr=74.08%, sys=703.84%, ctx=29661403, majf=21354, minf=22460954
   cpu : usr=70.92%, sys=702.81%, ctx=77219828, majf=13980, minf=27713137
   cpu : usr=72.23%, sys=695.37%, ctx=88081059, majf=18475, minf=28177648
   cpu : usr=69.69%, sys=654.13%, ctx=145476035, majf=15867, minf=26176375
  With request-based IO path:
   seq-read  : io=2048.0MB, bw=91074KB/s, iops=182147 , runt= 23027msec
   seq-write : io=2048.0MB, bw=80725KB/s, iops=161449 , runt= 25979msec
   rand-read : io=3095.7MB, bw=92106KB/s, iops=184211 , runt= 34416msec
   rand-write: io=3095.7MB, bw=82815KB/s, iops=165630 , runt= 38277msec
     clat (usec): min=0 , max=1932.4K, avg=77824.17, stdev=170339.49
     clat (usec): min=0 , max=2510.2K, avg=78023.96, stdev=146949.15
     clat (usec): min=0 , max=3037.2K, avg=74746.53, stdev=128498.27
     clat (usec): min=0 , max=1363.4K, avg=89830.75, stdev=114279.68
   cpu : usr=53.28%, sys=724.19%, ctx=37988895, majf=17531, minf=23577622
   cpu : usr=49.03%, sys=633.20%, ctx=205935380, majf=18197, minf=27288959
   cpu : usr=55.78%, sys=722.40%, ctx=101525058, majf=19273, minf=28067082
   cpu : usr=56.55%, sys=690.83%, ctx=228205022, majf=18039, minf=26551985

So bio based causes cpu to jump up by some 30%?
What happens if you divide IOPS/CPU?
Any improvement that comes from increasing the cpu share
of the given guest on the host will not scale well on
a typical overcommitted host.

if you add sys and usr time up, the jump is that much.

For ramdisk based device,

bio-based
------------------
 >>> 74.08 + 703.84
777.9200000000001
 >>> 70.92 + 702.81
773.7299999999999
 >>> 72.23 + 695.37
767.6
 >>> 69.69 + 654.13
723.8199999999999
 >>> 53.28 + 724.19
777.47

req-based
------------------
 >>> 53.28 + 724.19
777.47
 >>> 49.03 + 633.20
682.23
 >>> 55.78 + 722.40
778.18
 >>> 56.55 + 690.83
747.38


And for real device(fusion io), the cpu time is smaller with bio path.
bio-based
------------------
 >>> 56.79 + 421.70
478.49
 >>> 61.81 + 455.53
517.3399999999999
 >>> 63.10+455.38
518.48
 >>> 62.04 + 453.58
515.62

req-based
------------------
 >>> 44.08 + 590.71
634.7900000000001
 >>> 48.73 + 610.78
659.51
 >>> 45.58 + 581.16
626.74
 >>> 48.40 + 599.65
648.05

quoted

2) Fio test is performed in a 8 vcpu guest with Fusion-IO based guest using
kvm tool.

Short version:
  With bio-based IO path, sequential read/write, random read/write
  IOPS boost         : 11%, 11%, 13%, 10%
  Latency improvement: 10%, 10%, 12%, 10%
Long Version:
  With bio-based IO path:
   read : io=2048.0MB, bw=58920KB/s, iops=117840 , runt= 35593msec
   write: io=2048.0MB, bw=64308KB/s, iops=128616 , runt= 32611msec
   read : io=3095.7MB, bw=59633KB/s, iops=119266 , runt= 53157msec
   write: io=3095.7MB, bw=62993KB/s, iops=125985 , runt= 50322msec
     clat (usec): min=0 , max=1284.3K, avg=128109.01, stdev=71513.29
     clat (usec): min=94 , max=962339 , avg=116832.95, stdev=65836.80
     clat (usec): min=0 , max=1846.6K, avg=128509.99, stdev=89575.07
     clat (usec): min=0 , max=2256.4K, avg=121361.84, stdev=82747.25
   cpu : usr=56.79%, sys=421.70%, ctx=147335118, majf=21080, minf=19852517
   cpu : usr=61.81%, sys=455.53%, ctx=143269950, majf=16027, minf=24800604
   cpu : usr=63.10%, sys=455.38%, ctx=178373538, majf=16958, minf=24822612
   cpu : usr=62.04%, sys=453.58%, ctx=226902362, majf=16089, minf=23278105
  With request-based IO path:
   read : io=2048.0MB, bw=52896KB/s, iops=105791 , runt= 39647msec
   write: io=2048.0MB, bw=57856KB/s, iops=115711 , runt= 36248msec
   read : io=3095.7MB, bw=52387KB/s, iops=104773 , runt= 60510msec
   write: io=3095.7MB, bw=57310KB/s, iops=114619 , runt= 55312msec
     clat (usec): min=0 , max=1532.6K, avg=142085.62, stdev=109196.84
     clat (usec): min=0 , max=1487.4K, avg=129110.71, stdev=114973.64
     clat (usec): min=0 , max=1388.6K, avg=145049.22, stdev=107232.55
     clat (usec): min=0 , max=1465.9K, avg=133585.67, stdev=110322.95
   cpu : usr=44.08%, sys=590.71%, ctx=451812322, majf=14841, minf=17648641
   cpu : usr=48.73%, sys=610.78%, ctx=418953997, majf=22164, minf=26850689
   cpu : usr=45.58%, sys=581.16%, ctx=714079216, majf=21497, minf=22558223
   cpu : usr=48.40%, sys=599.65%, ctx=656089423, majf=16393, minf=23824409

Is this host or guest cpu? We should probably measure host cpu
as this includes device overhead which could vary by load.

It's guest cpu. Yes, host cpu is also interesting.

quoted

How to use:
-----------------------------
Add 'virtio_blk.use_bio=1' to kernel cmdline or 'modprobe virtio_blk
use_bio=1' to enable ->make_request_fn() based I/O path.

Cc: Rusty Russell <redacted>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Rusty Russell <redacted>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Minchan Kim <redacted>
Signed-off-by: Asias He <redacted>
---
  drivers/block/virtio_blk.c |  203 +++++++++++++++++++++++++++++++++++---------
  1 file changed, 163 insertions(+), 40 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c0bbeb4..95cfeed 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c

@@ -14,6 +14,9 @@

  #define PART_BITS 4

+static bool use_bio;
+module_param(use_bio, bool, S_IRUGO);
+
  static int major;
  static DEFINE_IDA(vd_index_ida);

@@ -23,6 +26,7 @@ struct virtio_blk
  {
  	struct virtio_device *vdev;
  	struct virtqueue *vq;
+	wait_queue_head_t queue_wait;

  	/* The disk structure for the kernel. */
  	struct gendisk *disk;

@@ -51,53 +55,87 @@ struct virtio_blk
  struct virtblk_req
  {
  	struct request *req;
+	struct bio *bio;
  	struct virtio_blk_outhdr out_hdr;
  	struct virtio_scsi_inhdr in_hdr;
  	u8 status;
+	struct scatterlist sg[];
  };

-static void blk_done(struct virtqueue *vq)
+static inline int virtblk_result(struct virtblk_req *vbr)
+{
+	switch (vbr->status) {
+	case VIRTIO_BLK_S_OK:
+		return 0;
+	case VIRTIO_BLK_S_UNSUPP:
+		return -ENOTTY;
+	default:
+		return -EIO;
+	}
+}
+
+static inline void virtblk_request_done(struct virtio_blk *vblk,
+					struct virtblk_req *vbr)
+{
+	struct request *req = vbr->req;
+	int error = virtblk_result(vbr);
+
+	if (req->cmd_type == REQ_TYPE_BLOCK_PC) {
+		req->resid_len = vbr->in_hdr.residual;
+		req->sense_len = vbr->in_hdr.sense_len;
+		req->errors = vbr->in_hdr.errors;
+	} else if (req->cmd_type == REQ_TYPE_SPECIAL) {
+		req->errors = (error != 0);
+	}
+
+	__blk_end_request_all(req, error);
+	mempool_free(vbr, vblk->pool);
+}
+
+static inline void virtblk_bio_done(struct virtio_blk *vblk,
+				    struct virtblk_req *vbr)
+{
+	bio_endio(vbr->bio, virtblk_result(vbr));
+	mempool_free(vbr, vblk->pool);
+}
+
+static void virtblk_done(struct virtqueue *vq)
  {
  	struct virtio_blk *vblk = vq->vdev->priv;
+	unsigned long bio_done = 0, req_done = 0;
  	struct virtblk_req *vbr;
-	unsigned int len;
  	unsigned long flags;
+	unsigned int len;

  	spin_lock_irqsave(vblk->disk->queue->queue_lock, flags);
  	while ((vbr = virtqueue_get_buf(vblk->vq, &len)) != NULL) {
-		int error;
-
-		switch (vbr->status) {
-		case VIRTIO_BLK_S_OK:
-			error = 0;
-			break;
-		case VIRTIO_BLK_S_UNSUPP:
-			error = -ENOTTY;
-			break;
-		default:
-			error = -EIO;
-			break;
-		}
-
-		switch (vbr->req->cmd_type) {
-		case REQ_TYPE_BLOCK_PC:
-			vbr->req->resid_len = vbr->in_hdr.residual;
-			vbr->req->sense_len = vbr->in_hdr.sense_len;
-			vbr->req->errors = vbr->in_hdr.errors;
-			break;
-		case REQ_TYPE_SPECIAL:
-			vbr->req->errors = (error != 0);
-			break;
-		default:
-			break;
+		if (vbr->bio) {
+			virtblk_bio_done(vblk, vbr);
+			bio_done++;
+		} else {
+			virtblk_request_done(vblk, vbr);
+			req_done++;
  		}
-
-		__blk_end_request_all(vbr->req, error);
-		mempool_free(vbr, vblk->pool);
  	}
  	/* In case queue is stopped waiting for more buffers. */
-	blk_start_queue(vblk->disk->queue);
+	if (req_done)
+		blk_start_queue(vblk->disk->queue);
  	spin_unlock_irqrestore(vblk->disk->queue->queue_lock, flags);
+
+	if (bio_done)
+		wake_up(&vblk->queue_wait);
+}
+
+static inline struct virtblk_req *virtblk_alloc_req(struct virtio_blk *vblk,
+						    gfp_t gfp_mask)
+{
+	struct virtblk_req *vbr;
+
+	vbr = mempool_alloc(vblk->pool, gfp_mask);
+	if (vbr && use_bio)
+		sg_init_table(vbr->sg, vblk->sg_elems);
+
+	return vbr;
  }

  static bool do_req(struct request_queue *q, struct virtio_blk *vblk,

@@ -106,13 +144,13 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
  	unsigned long num, out = 0, in = 0;
  	struct virtblk_req *vbr;

-	vbr = mempool_alloc(vblk->pool, GFP_ATOMIC);
+	vbr = virtblk_alloc_req(vblk, GFP_ATOMIC);
  	if (!vbr)
  		/* When another request finishes we'll try again. */
  		return false;

  	vbr->req = req;
-
+	vbr->bio = NULL;
  	if (req->cmd_flags & REQ_FLUSH) {
  		vbr->out_hdr.type = VIRTIO_BLK_T_FLUSH;
  		vbr->out_hdr.sector = 0;

@@ -172,7 +210,8 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
  		}
  	}

-	if (virtqueue_add_buf(vblk->vq, vblk->sg, out, in, vbr, GFP_ATOMIC)<0) {
+	if (virtqueue_add_buf(vblk->vq, vblk->sg, out, in, vbr,
+			      GFP_ATOMIC) < 0) {
  		mempool_free(vbr, vblk->pool);
  		return false;
  	}

@@ -180,7 +219,7 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
  	return true;
  }

-static void do_virtblk_request(struct request_queue *q)
+static void virtblk_request(struct request_queue *q)
  {
  	struct virtio_blk *vblk = q->queuedata;
  	struct request *req;

@@ -203,6 +242,82 @@ static void do_virtblk_request(struct request_queue *q)
  		virtqueue_kick(vblk->vq);
  }

+static void virtblk_add_buf_wait(struct virtio_blk *vblk,
+				 struct virtblk_req *vbr,
+				 unsigned long out,
+				 unsigned long in)
+{
+	DEFINE_WAIT(wait);
+
+	for (;;) {
+		prepare_to_wait_exclusive(&vblk->queue_wait, &wait,
+					  TASK_UNINTERRUPTIBLE);
+
+		spin_lock_irq(vblk->disk->queue->queue_lock);
+		if (virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
+				      GFP_ATOMIC) < 0) {
+			spin_unlock_irq(vblk->disk->queue->queue_lock);
+			io_schedule();
+		} else {
+			virtqueue_kick(vblk->vq);
+			spin_unlock_irq(vblk->disk->queue->queue_lock);
+			break;
+		}
+
+	}
+
+	finish_wait(&vblk->queue_wait, &wait);
+}
+
+static void virtblk_make_request(struct request_queue *q, struct bio *bio)
+{
+	struct virtio_blk *vblk = q->queuedata;
+	unsigned int num, out = 0, in = 0;
+	struct virtblk_req *vbr;
+
+	BUG_ON(bio->bi_phys_segments + 2 > vblk->sg_elems);
+	BUG_ON(bio->bi_rw & (REQ_FLUSH | REQ_FUA));
+
+	vbr = virtblk_alloc_req(vblk, GFP_NOIO);
+	if (!vbr) {
+		bio_endio(bio, -ENOMEM);
+		return;
+	}
+
+	vbr->bio = bio;
+	vbr->req = NULL;
+	vbr->out_hdr.type = 0;
+	vbr->out_hdr.sector = bio->bi_sector;
+	vbr->out_hdr.ioprio = bio_prio(bio);
+
+	sg_set_buf(&vbr->sg[out++], &vbr->out_hdr, sizeof(vbr->out_hdr));
+
+	num = blk_bio_map_sg(q, bio, vbr->sg + out);
+
+	sg_set_buf(&vbr->sg[num + out + in++], &vbr->status,
+		   sizeof(vbr->status));
+
+	if (num) {
+		if (bio->bi_rw & REQ_WRITE) {
+			vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
+			out += num;
+		} else {
+			vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
+			in += num;
+		}
+	}
+
+	spin_lock_irq(vblk->disk->queue->queue_lock);
+	if (unlikely(virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
+				       GFP_ATOMIC) < 0)) {
+		spin_unlock_irq(vblk->disk->queue->queue_lock);
+		virtblk_add_buf_wait(vblk, vbr, out, in);
+		return;
+	}
+	virtqueue_kick(vblk->vq);
+	spin_unlock_irq(vblk->disk->queue->queue_lock);
+}
+
  /* return id (s/n) string for *disk to *id_str
   */
  static int virtblk_get_id(struct gendisk *disk, char *id_str)

@@ -360,7 +475,7 @@ static int init_vq(struct virtio_blk *vblk)
  	int err = 0;

  	/* We expect one virtqueue, for output. */
-	vblk->vq = virtio_find_single_vq(vblk->vdev, blk_done, "requests");
+	vblk->vq = virtio_find_single_vq(vblk->vdev, virtblk_done, "requests");
  	if (IS_ERR(vblk->vq))
  		err = PTR_ERR(vblk->vq);

@@ -414,7 +529,7 @@ static void virtblk_update_cache_mode(struct virtio_device *vdev)
  	u8 writeback = virtblk_get_cache_mode(vdev);
  	struct virtio_blk *vblk = vdev->priv;

-	if (writeback)
+	if (writeback && !use_bio)
  		blk_queue_flush(vblk->disk->queue, REQ_FLUSH);
  	else
  		blk_queue_flush(vblk->disk->queue, 0);

@@ -477,6 +592,8 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
  	struct virtio_blk *vblk;
  	struct request_queue *q;
  	int err, index;
+	int pool_size;
+
  	u64 cap;
  	u32 v, blk_size, sg_elems, opt_io_size;
  	u16 min_io_size;

@@ -506,10 +623,12 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
  		goto out_free_index;
  	}

+	init_waitqueue_head(&vblk->queue_wait);
  	vblk->vdev = vdev;
  	vblk->sg_elems = sg_elems;
  	sg_init_table(vblk->sg, vblk->sg_elems);
  	mutex_init(&vblk->config_lock);
+
  	INIT_WORK(&vblk->config_work, virtblk_config_changed_work);
  	vblk->config_enable = true;

@@ -517,7 +636,10 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
  	if (err)
  		goto out_free_vblk;

-	vblk->pool = mempool_create_kmalloc_pool(1,sizeof(struct virtblk_req));
+	pool_size = sizeof(struct virtblk_req);
+	if (use_bio)
+		pool_size += sizeof(struct scatterlist) * sg_elems;
+	vblk->pool = mempool_create_kmalloc_pool(1, pool_size);
  	if (!vblk->pool) {
  		err = -ENOMEM;
  		goto out_free_vq;

@@ -530,12 +652,14 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
  		goto out_mempool;
  	}

-	q = vblk->disk->queue = blk_init_queue(do_virtblk_request, NULL);
+	q = vblk->disk->queue = blk_init_queue(virtblk_request, NULL);
  	if (!q) {
  		err = -ENOMEM;
  		goto out_put_disk;
  	}

+	if (use_bio)
+		blk_queue_make_request(q, virtblk_make_request);
  	q->queuedata = vblk;

  	virtblk_name_format("vd", index, vblk->disk->disk_name, DISK_NAME_LEN);

@@ -620,7 +744,6 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
  	if (!err && opt_io_size)
  		blk_queue_io_opt(q, blk_size * opt_io_size);

-
  	add_disk(vblk->disk);
  	err = device_create_file(disk_to_dev(vblk->disk), &dev_attr_serial);
  	if (err)
--

1.7.10.4