[PATCH 0/2] virtio-scsi queue for 3.17

[PATCH 0/2] virtio-scsi queue for 3.17

[Paolo Bonzini]

Christoph asked me to rebase the two virtio-scsi patches from
http://thread.gmane.org/gmane.linux.kernel/1717796 that do not apply
anymore, patch 2 and 6.

These are on top of his drivers-for-3.16 branch, more precisely commit
8faeb529b2da (virtio-scsi: fix various bad behavior on aborted requests).

Paolo

Ming Lei (1):
  virtio-scsi: replace target spinlock with seqcount

Venkatesh Srinivas (1):
  virtio-scsi: Implement change_queue_depth for virtscsi targets

 drivers/scsi/virtio_scsi.c | 75 ++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 62 insertions(+), 13 deletions(-)

-- 
1.8.3.1

[PATCH 1/2] virtio-scsi: replace target spinlock with seqcount

[Paolo Bonzini]

From: Ming Lei <ming.lei@canonical.com>

The spinlock of tgt_lock is only for serializing read and write
req_vq, one lockless seqcount is enough for the purpose.

On one 16core VM with vhost-scsi backend, the patch can improve
IOPS with 3% on random read test.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
[Add initialization in virtscsi_target_alloc. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 drivers/scsi/virtio_scsi.c | 42 +++++++++++++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 13 deletions(-)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 308256b5e4cb..f67a872de14b 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -27,6 +27,7 @@
 #include <scsi/scsi_host.h>
 #include <scsi/scsi_device.h>
 #include <scsi/scsi_cmnd.h>
+#include <linux/seqlock.h>
 
 #define VIRTIO_SCSI_MEMPOOL_SZ 64
 #define VIRTIO_SCSI_EVENT_LEN 8
@@ -75,18 +76,16 @@ struct virtio_scsi_vq {
  * queue, and also lets the driver optimize the IRQ affinity for the virtqueues
  * (each virtqueue's affinity is set to the CPU that "owns" the queue).
  *
- * tgt_lock is held to serialize reading and writing req_vq. Reading req_vq
- * could be done locklessly, but we do not do it yet.
+ * tgt_seq is held to serialize reading and writing req_vq.
  *
  * Decrements of reqs are never concurrent with writes of req_vq: before the
  * decrement reqs will be != 0; after the decrement the virtqueue completion
  * routine will not use the req_vq so it can be changed by a new request.
- * Thus they can happen outside the tgt_lock, provided of course we make reqs
+ * Thus they can happen outside the tgt_seq, provided of course we make reqs
  * an atomic_t.
  */
 struct virtio_scsi_target_state {
-	/* This spinlock never held at the same time as vq_lock. */
-	spinlock_t tgt_lock;
+	seqcount_t tgt_seq;
 
 	/* Count of outstanding requests. */
 	atomic_t reqs;
@@ -559,19 +558,33 @@ static struct virtio_scsi_vq *virtscsi_pick_vq(struct virtio_scsi *vscsi,
 	unsigned long flags;
 	u32 queue_num;
 
-	spin_lock_irqsave(&tgt->tgt_lock, flags);
+	local_irq_save(flags);
+	if (atomic_inc_return(&tgt->reqs) > 1) {
+		unsigned long seq;
+
+		do {
+			seq = read_seqcount_begin(&tgt->tgt_seq);
+			vq = tgt->req_vq;
+		} while (read_seqcount_retry(&tgt->tgt_seq, seq));
+	} else {
+		/* no writes can be concurrent because of atomic_t */
+		write_seqcount_begin(&tgt->tgt_seq);
+
+		/* keep previous req_vq if a reader just arrived */
+		if (unlikely(atomic_read(&tgt->reqs) > 1)) {
+			vq = tgt->req_vq;
+			goto unlock;
+		}
 
-	if (atomic_inc_return(&tgt->reqs) > 1)
-		vq = tgt->req_vq;
-	else {
 		queue_num = smp_processor_id();
 		while (unlikely(queue_num >= vscsi->num_queues))
 			queue_num -= vscsi->num_queues;
-
 		tgt->req_vq = vq = &vscsi->req_vqs[queue_num];
+ unlock:
+		write_seqcount_end(&tgt->tgt_seq);
 	}
+	local_irq_restore(flags);
 
-	spin_unlock_irqrestore(&tgt->tgt_lock, flags);
 	return vq;
 }
 
@@ -667,14 +680,17 @@ static int virtscsi_abort(struct scsi_cmnd *sc)
 
 static int virtscsi_target_alloc(struct scsi_target *starget)
 {
+	struct Scsi_Host *sh = dev_to_shost(starget->dev.parent);
+	struct virtio_scsi *vscsi = shost_priv(sh);
+
 	struct virtio_scsi_target_state *tgt =
 				kmalloc(sizeof(*tgt), GFP_KERNEL);
 	if (!tgt)
 		return -ENOMEM;
 
-	spin_lock_init(&tgt->tgt_lock);
+	seqcount_init(&tgt->tgt_seq);
 	atomic_set(&tgt->reqs, 0);
-	tgt->req_vq = NULL;
+	tgt->req_vq = &vscsi->req_vqs[0];
 
 	starget->hostdata = tgt;
 	return 0;
-- 
1.8.3.1

[PATCH 2/2] virtio-scsi: Implement change_queue_depth for virtscsi targets

[Paolo Bonzini]

From: Venkatesh Srinivas <venkateshs@google.com>

change_queue_depth allows changing per-target queue depth via sysfs.

It also allows the SCSI midlayer to ramp down the number of concurrent
inflight requests in response to a SCSI BUSY status response and allows
the midlayer to ramp the count back up to the device maximum when the
BUSY condition has resolved.

Signed-off-by: Venkatesh Srinivas <venkateshs@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 drivers/scsi/virtio_scsi.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index f67a872de14b..8fd0b21ce13a 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -27,6 +27,7 @@
 #include <scsi/scsi_host.h>
 #include <scsi/scsi_device.h>
 #include <scsi/scsi_cmnd.h>
+#include <scsi/scsi_tcq.h>
 #include <linux/seqlock.h>
 
 #define VIRTIO_SCSI_MEMPOOL_SZ 64
@@ -654,6 +655,36 @@ static int virtscsi_device_reset(struct scsi_cmnd *sc)
 	return virtscsi_tmf(vscsi, cmd);
 }
 
+/**
+ * virtscsi_change_queue_depth() - Change a virtscsi target's queue depth
+ * @sdev:	Virtscsi target whose queue depth to change
+ * @qdepth:	New queue depth
+ * @reason:	Reason for the queue depth change.
+ */
+static int virtscsi_change_queue_depth(struct scsi_device *sdev,
+				       int qdepth,
+				       int reason)
+{
+	struct Scsi_Host *shost = sdev->host;
+	int max_depth = shost->cmd_per_lun;
+
+	switch (reason) {
+	case SCSI_QDEPTH_QFULL: /* Drop qdepth in response to BUSY state */
+		scsi_track_queue_full(sdev, qdepth);
+		break;
+	case SCSI_QDEPTH_RAMP_UP: /* Raise qdepth after BUSY state resolved */
+	case SCSI_QDEPTH_DEFAULT: /* Manual change via sysfs */
+		scsi_adjust_queue_depth(sdev,
+					scsi_get_tag_type(sdev),
+					min(max_depth, qdepth));
+		break;
+	default:
+		return -EOPNOTSUPP;
+	}
+
+	return sdev->queue_depth;
+}
+
 static int virtscsi_abort(struct scsi_cmnd *sc)
 {
 	struct virtio_scsi *vscsi = shost_priv(sc->device->host);
@@ -709,6 +740,7 @@ static struct scsi_host_template virtscsi_host_template_single = {
 	.this_id = -1,
 	.cmd_size = sizeof(struct virtio_scsi_cmd),
 	.queuecommand = virtscsi_queuecommand_single,
+	.change_queue_depth = virtscsi_change_queue_depth,
 	.eh_abort_handler = virtscsi_abort,
 	.eh_device_reset_handler = virtscsi_device_reset,
 
@@ -726,6 +758,7 @@ static struct scsi_host_template virtscsi_host_template_multi = {
 	.this_id = -1,
 	.cmd_size = sizeof(struct virtio_scsi_cmd),
 	.queuecommand = virtscsi_queuecommand_multi,
+	.change_queue_depth = virtscsi_change_queue_depth,
 	.eh_abort_handler = virtscsi_abort,
 	.eh_device_reset_handler = virtscsi_device_reset,
 
-- 
1.8.3.1

Re: [PATCH 0/2] virtio-scsi queue for 3.17

[Christoph Hellwig]

Thanks Paolo,

I've applied the patches.