[PATCH V3 0/3] Improve virtio-blk performance

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

Split the mapping code in blk_rq_map_sg() to a helper
__blk_segment_map_sg(), so that other mapping function, e.g.
blk_bio_map_sg(), can share the code.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Tejun Heo <tj@kernel.org>
Cc: Shaohua Li <shli@kernel.org>
Cc: linux-kernel@vger.kernel.org
Suggested-by: Tejun Heo <tj@kernel.org>
Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Asias He <redacted>
---
 block/blk-merge.c |   80 ++++++++++++++++++++++++++++++-----------------------
 1 file changed, 45 insertions(+), 35 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 160035f..576b68e 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c

@@ -110,6 +110,49 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
 	return 0;
 }
 
+static void
+__blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
+		     struct scatterlist *sglist, struct bio_vec **bvprv,
+		     struct scatterlist **sg, int *nsegs, int *cluster)
+{
+
+	int nbytes = bvec->bv_len;
+
+	if (*bvprv && *cluster) {
+		if ((*sg)->length + nbytes > queue_max_segment_size(q))
+			goto new_segment;
+
+		if (!BIOVEC_PHYS_MERGEABLE(*bvprv, bvec))
+			goto new_segment;
+		if (!BIOVEC_SEG_BOUNDARY(q, *bvprv, bvec))
+			goto new_segment;
+
+		(*sg)->length += nbytes;
+	} else {
+new_segment:
+		if (!*sg)
+			*sg = sglist;
+		else {
+			/*
+			 * If the driver previously mapped a shorter
+			 * list, we could see a termination bit
+			 * prematurely unless it fully inits the sg
+			 * table on each mapping. We KNOW that there
+			 * must be more entries here or the driver
+			 * would be buggy, so force clear the
+			 * termination bit to avoid doing a full
+			 * sg_init_table() in drivers for each command.
+			 */
+			(*sg)->page_link &= ~0x02;
+			*sg = sg_next(*sg);
+		}
+
+		sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
+		(*nsegs)++;
+	}
+	*bvprv = bvec;
+}
+
 /*
  * map a request to scatterlist, return number of sg entries setup. Caller
  * must make sure sg can hold rq->nr_phys_segments entries

@@ -131,41 +174,8 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 	bvprv = NULL;
 	sg = NULL;
 	rq_for_each_segment(bvec, rq, iter) {
-		int nbytes = bvec->bv_len;
-
-		if (bvprv && cluster) {
-			if (sg->length + nbytes > queue_max_segment_size(q))
-				goto new_segment;
-
-			if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
-				goto new_segment;
-			if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
-				goto new_segment;
-
-			sg->length += nbytes;
-		} else {
-new_segment:
-			if (!sg)
-				sg = sglist;
-			else {
-				/*
-				 * If the driver previously mapped a shorter
-				 * list, we could see a termination bit
-				 * prematurely unless it fully inits the sg
-				 * table on each mapping. We KNOW that there
-				 * must be more entries here or the driver
-				 * would be buggy, so force clear the
-				 * termination bit to avoid doing a full
-				 * sg_init_table() in drivers for each command.
-				 */
-				sg->page_link &= ~0x02;
-				sg = sg_next(sg);
-			}
-
-			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
-			nsegs++;
-		}
-		bvprv = bvec;
+		__blk_segment_map_sg(q, bvec, sglist, &bvprv, &sg,
+				     &nsegs, &cluster);
 	} /* segments in rq */
 
 

-- 
1.7.10.4

Add a helper to map a bio to a scatterlist, modelled after
blk_rq_map_sg.

This helper is useful for any driver that wants to create
a scatterlist from its ->make_request_fn method.

Changes in v2:
 - Use __blk_segment_map_sg to avoid duplicated code
 - Add cocbook style function comment

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Tejun Heo <tj@kernel.org>
Cc: Shaohua Li <shli@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Minchan Kim <redacted>
Signed-off-by: Asias He <redacted>
---
 block/blk-merge.c      |   37 +++++++++++++++++++++++++++++++++++++
 include/linux/blkdev.h |    2 ++
 2 files changed, 39 insertions(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 576b68e..e76279e 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c

@@ -209,6 +209,43 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 }
 EXPORT_SYMBOL(blk_rq_map_sg);
 
+/**
+ * blk_bio_map_sg - map a bio to a scatterlist
+ * @q: request_queue in question
+ * @bio: bio being mapped
+ * @sglist: scatterlist being mapped
+ *
+ * Note:
+ *    Caller must make sure sg can hold bio->bi_phys_segments entries
+ *
+ * Will return the number of sg entries setup
+ */
+int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
+		   struct scatterlist *sglist)
+{
+	struct bio_vec *bvec, *bvprv;
+	struct scatterlist *sg;
+	int nsegs, cluster;
+	unsigned long i;
+
+	nsegs = 0;
+	cluster = blk_queue_cluster(q);
+
+	bvprv = NULL;
+	sg = NULL;
+	bio_for_each_segment(bvec, bio, i) {
+		__blk_segment_map_sg(q, bvec, sglist, &bvprv, &sg,
+				     &nsegs, &cluster);
+	} /* segments in bio */
+
+	if (sg)
+		sg_mark_end(sg);
+
+	BUG_ON(bio->bi_phys_segments && nsegs > bio->bi_phys_segments);
+	return nsegs;
+}
+EXPORT_SYMBOL(blk_bio_map_sg);
+
 static inline int ll_new_hw_segment(struct request_queue *q,
 				    struct request *req,
 				    struct bio *bio)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 07954b0..87fb56c 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h

@@ -883,6 +883,8 @@ extern void blk_queue_flush_queueable(struct request_queue *q, bool queueable);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
 
 extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
+extern int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
+			  struct scatterlist *sglist);
 extern void blk_dump_rq_flags(struct request *, char *);
 extern long nr_blockdev_pages(void);
 

-- 
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
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 774c31d..e137190 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);
 

@@ -400,6 +515,8 @@ 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;

@@ -429,10 +546,12 @@ 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;
 

@@ -440,7 +559,10 @@ 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;

@@ -453,12 +575,14 @@ 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);

@@ -471,7 +595,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 	vblk->index = index;
 
 	/* configure queue flush support */
-	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
+	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH) && !use_bio)
 		blk_queue_flush(q, REQ_FLUSH);
 
 	/* If disk is read-only in the host, the guest should obey */

@@ -544,7 +668,6 @@ 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 Fri, 13 Jul 2012 16:38:51 +0800, Asias He [off-list ref] wrote:

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.

This patch conflicts with Paolo's Bonzini's 'virtio-blk: allow toggling
host cache between writeback and writethrough' which is also queued (see
linux-next).

I'm not sure what the correct behavior for bio & cacheflush is, if any.

But as to the patch itself: it's a hack.

1) Leaving the guest's admin to turn on the switch is a terrible choice.
2) The block layer should stop merging and sorting when a device is
   fast, not the driver.
3) I pointed out that slow disks have low IOPS, so why is this
   conditional?  Sure, more guest exits, but it's still a small number
   for a slow device.
4) The only case where we want merging is on a slow device when the host
   isn't doing it.

Now, despite this, I'm prepared to commit it.  But in my mind it's a
hack: we should aim for use_bio to be based on a feature bit fed from
the host, and use the module parameter only if we want to override it.

Cheers,
Rusty.

On Fri, 13 Jul 2012 16:38:51 +0800, Asias He [off-list ref] wrote:

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

Acked-by: Rusty Russell <redacted>

I just hope we can do better than a module option in future.

Thanks,
Rusty.

On 07/27/2012 08:33 AM, Rusty Russell wrote:

On Fri, 13 Jul 2012 16:38:51 +0800, Asias He [off-list ref] wrote:

quoted

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.

This patch conflicts with Paolo's Bonzini's 'virtio-blk: allow toggling
host cache between writeback and writethrough' which is also queued (see
linux-next).

Rebased against Paolo's patch in V4.

I'm not sure what the correct behavior for bio & cacheflush is, if any.

REQ_FLUSH is not supported in the bio path.

But as to the patch itself: it's a hack.

1) Leaving the guest's admin to turn on the switch is a terrible choice.
2) The block layer should stop merging and sorting when a device is
    fast, not the driver.
3) I pointed out that slow disks have low IOPS, so why is this
    conditional?  Sure, more guest exits, but it's still a small number
    for a slow device.
4) The only case where we want merging is on a slow device when the host
    isn't doing it.

Now, despite this, I'm prepared to commit it.  But in my mind it's a
hack: we should aim for use_bio to be based on a feature bit fed from
the host, and use the module parameter only if we want to override it.

OK. A feature bit from host sound like a choice but a switch is also 
needed on host side. And for other OS, e.g. Windows, the bio thing does 
not apply at all.

Anyway, I have to admit that adding a module parameter here is not the 
best choice. Let's think more.

-- 
Asias

quoted

I'm not sure what the correct behavior for bio & cacheflush is, if
any.

REQ_FLUSH is not supported in the bio path.

Ouch, that's correct:

@@ -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);

then it is not safe against power losses.

Paolo

On Sat, Jul 28, 2012 at 10:38:41AM +0800, Asias He wrote:

On 07/27/2012 08:33 AM, Rusty Russell wrote:

quoted

On Fri, 13 Jul 2012 16:38:51 +0800, Asias He [off-list ref] wrote:

quoted

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.

This patch conflicts with Paolo's Bonzini's 'virtio-blk: allow toggling
host cache between writeback and writethrough' which is also queued (see
linux-next).

Rebased against Paolo's patch in V4.

quoted

I'm not sure what the correct behavior for bio & cacheflush is, if any.

REQ_FLUSH is not supported in the bio path.

quoted

But as to the patch itself: it's a hack.

1) Leaving the guest's admin to turn on the switch is a terrible choice.
2) The block layer should stop merging and sorting when a device is
   fast, not the driver.
3) I pointed out that slow disks have low IOPS, so why is this
   conditional?  Sure, more guest exits, but it's still a small number
   for a slow device.
4) The only case where we want merging is on a slow device when the host
   isn't doing it.

Now, despite this, I'm prepared to commit it.  But in my mind it's a
hack: we should aim for use_bio to be based on a feature bit fed from
the host, and use the module parameter only if we want to override it.

OK. A feature bit from host sound like a choice but a switch is also
needed on host side.

qemu automatically gives you the ability to control
any feature bit.

And for other OS, e.g. Windows, the bio thing
does not apply at all.

Let's try to define when it's a good idea. Is it a hint to guest that
backend handles small accesses efficiently so ok to disable batching?

Anyway, I have to admit that adding a module parameter here is not
the best choice. Let's think more.

-- 
Asias

On 07/29/2012 08:59 PM, Michael S. Tsirkin wrote:

On Sat, Jul 28, 2012 at 10:38:41AM +0800, Asias He wrote:

quoted

On 07/27/2012 08:33 AM, Rusty Russell wrote:

quoted

On Fri, 13 Jul 2012 16:38:51 +0800, Asias He [off-list ref] wrote:

quoted

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.

This patch conflicts with Paolo's Bonzini's 'virtio-blk: allow toggling
host cache between writeback and writethrough' which is also queued (see
linux-next).

Rebased against Paolo's patch in V4.

quoted

I'm not sure what the correct behavior for bio & cacheflush is, if any.

REQ_FLUSH is not supported in the bio path.

quoted

But as to the patch itself: it's a hack.

1) Leaving the guest's admin to turn on the switch is a terrible choice.
2) The block layer should stop merging and sorting when a device is
    fast, not the driver.
3) I pointed out that slow disks have low IOPS, so why is this
    conditional?  Sure, more guest exits, but it's still a small number
    for a slow device.
4) The only case where we want merging is on a slow device when the host
    isn't doing it.

Now, despite this, I'm prepared to commit it.  But in my mind it's a
hack: we should aim for use_bio to be based on a feature bit fed from
the host, and use the module parameter only if we want to override it.

OK. A feature bit from host sound like a choice but a switch is also
needed on host side.

qemu automatically gives you the ability to control
any feature bit.

Automatically?

quoted

And for other OS, e.g. Windows, the bio thing
does not apply at all.

Let's try to define when it's a good idea. Is it a hint to guest that
backend handles small accesses efficiently so ok to disable batching?

Yes. It's also a hint for latency reduction.

quoted

Anyway, I have to admit that adding a module parameter here is not
the best choice. Let's think more.

--
Asias

-- 
Asias

On 07/28/2012 02:42 PM, Paolo Bonzini wrote:

Yes. Something like this:

   qemu -drive file=foo.img,cache=writeback/unsafe

is not safe against power losses also?

I think we can add REQ_FLUSH & REQ_FUA support to bio path and that 
deserves another patch.

-- 
Asias

Il 30/07/2012 06:43, Asias He ha scritto:

quoted

Yes. Something like this:

  qemu -drive file=foo.img,cache=writeback/unsafe

is not safe against power losses also?

cache=writeback and cache=none are safe, cache=unsafe isn't.

I think we can add REQ_FLUSH & REQ_FUA support to bio path and that
deserves another patch.

You only need to add REQ_FLUSH support.  The virtio-blk protocol does
not support REQ_FUA, because there's no easy way to do it in userspace.

Paolo

On Mon, Jul 30, 2012 at 09:31:06AM +0200, Paolo Bonzini wrote:

You only need to add REQ_FLUSH support.  The virtio-blk protocol does
not support REQ_FUA, because there's no easy way to do it in userspace.

A bio-based driver needs to handle both REQ_FLUSH and REQ_FUA as it does
not get the sequencing of REQ_FUA into REQ_FLUSH that request based drivers
can request.  To what the REQ_FUA request gets translated is a different story.

On Mon, Jul 30, 2012 at 12:43:12PM +0800, Asias He wrote:

I think we can add REQ_FLUSH & REQ_FUA support to bio path and that 
deserves another patch.

Adding it is a requirement for merging the code.

On 07/30/2012 09:43 PM, Christoph Hellwig wrote:

On Mon, Jul 30, 2012 at 12:43:12PM +0800, Asias He wrote:

quoted

I think we can add REQ_FLUSH & REQ_FUA support to bio path and that
deserves another patch.

Adding it is a requirement for merging the code.

OK. Will add that.

-- 
Asias

On 07/30/2012 09:44 PM, Christoph Hellwig wrote:

On Mon, Jul 30, 2012 at 09:31:06AM +0200, Paolo Bonzini wrote:

quoted

You only need to add REQ_FLUSH support.  The virtio-blk protocol does
not support REQ_FUA, because there's no easy way to do it in userspace.

A bio-based driver needs to handle both REQ_FLUSH and REQ_FUA as it does
not get the sequencing of REQ_FUA into REQ_FLUSH that request based drivers
can request.  To what the REQ_FUA request gets translated is a different story.

I just sent out V5 to support both REQ_FLUSH AND REQ_FUA.
Thanks, Christoph!

-- 
Asias