[PATCH 3/3] xen-blkfront: dynamic configuration of per-vbd resources

[Bob Liu]

The current VBD layer reserves buffer space for each attached device based on
three statically configured settings which are read at boot time.
 * max_indirect_segs: Maximum amount of segments.
 * max_ring_page_order: Maximum order of pages to be used for the shared ring.
 * max_queues: Maximum of queues(rings) to be used.

But the storage backend, workload, and guest memory result in very different
tuning requirements. It's impossible to centrally predict application
characteristics so it's best to leave allow the settings can be dynamiclly
adjusted based on workload inside the Guest.

Usage:
Show current values:
cat /sys/devices/vbd-xxx/max_indirect_segs
cat /sys/devices/vbd-xxx/max_ring_page_order
cat /sys/devices/vbd-xxx/max_queues

Write new values:
echo <new value> > /sys/devices/vbd-xxx/max_indirect_segs
echo <new value> > /sys/devices/vbd-xxx/max_ring_page_order
echo <new value> > /sys/devices/vbd-xxx/max_queues

Signed-off-by: Bob Liu <bob.liu@xxxxxxxxxx>
--
v2: Add device lock and other comments from Konrad.
---
 drivers/block/xen-blkfront.c | 285 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 283 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 10f46a8..9a5ed22 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -46,6 +46,7 @@
 #include <linux/scatterlist.h>
 #include <linux/bitmap.h>
 #include <linux/list.h>
+#include <linux/delay.h>
 
 #include <xen/xen.h>
 #include <xen/xenbus.h>
@@ -212,6 +213,11 @@ struct blkfront_info
 	/* Save uncomplete reqs and bios for migration. */
 	struct list_head requests;
 	struct bio_list bio_list;
+	/* For dynamic configuration. */
+	unsigned int reconfiguring:1;
+	int new_max_indirect_segments;
+	int new_max_ring_page_order;
+	int new_max_queues;
 };
 
 static unsigned int nr_minors;
@@ -1350,6 +1356,31 @@ static void blkif_free(struct blkfront_info *info, int suspend)
 	for (i = 0; i < info->nr_rings; i++)
 		blkif_free_ring(&info->rinfo[i]);
 
+	/* Remove old xenstore nodes. */
+	if (info->nr_ring_pages > 1)
+		xenbus_rm(XBT_NIL, info->xbdev->nodename, "ring-page-order");
+
+	if (info->nr_rings == 1) {
+		if (info->nr_ring_pages == 1) {
+			xenbus_rm(XBT_NIL, info->xbdev->nodename, "ring-ref");
+		} else {
+			for (i = 0; i < info->nr_ring_pages; i++) {
+				char ring_ref_name[RINGREF_NAME_LEN];
+
+				snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
+				xenbus_rm(XBT_NIL, info->xbdev->nodename, ring_ref_name);
+			}
+		}
+	} else {
+		xenbus_rm(XBT_NIL, info->xbdev->nodename, "multi-queue-num-queues");
+
+		for (i = 0; i < info->nr_rings; i++) {
+			char queuename[QUEUE_NAME_LEN];
+
+			snprintf(queuename, QUEUE_NAME_LEN, "queue-%u", i);
+			xenbus_rm(XBT_NIL, info->xbdev->nodename, queuename);
+		}
+	}
 	kfree(info->rinfo);
 	info->rinfo = NULL;
 	info->nr_rings = 0;
@@ -1772,6 +1803,10 @@ static int talk_to_blkback(struct xenbus_device *dev,
 		info->nr_ring_pages = 1;
 	else {
 		ring_page_order = min(xen_blkif_max_ring_order, max_page_order);
+		if (info->new_max_ring_page_order) {
+			BUG_ON(info->new_max_ring_page_order > max_page_order);
+			ring_page_order = info->new_max_ring_page_order;
+		}
 		info->nr_ring_pages = 1 << ring_page_order;
 	}
 
@@ -1895,6 +1930,10 @@ static int negotiate_mq(struct blkfront_info *info)
 		backend_max_queues = 1;
 
 	info->nr_rings = min(backend_max_queues, xen_blkif_max_queues);
+	if (info->new_max_queues) {
+		BUG_ON(info->new_max_queues > backend_max_queues);
+		info->nr_rings = info->new_max_queues;
+	}
 	/* We need at least one ring. */
 	if (!info->nr_rings)
 		info->nr_rings = 1;
@@ -2352,11 +2391,227 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
 			    NULL);
 	if (err)
 		info->max_indirect_segments = 0;
-	else
+	else {
 		info->max_indirect_segments = min(indirect_segments,
 						  xen_blkif_max_segments);
+		if (info->new_max_indirect_segments) {
+			BUG_ON(info->new_max_indirect_segments > indirect_segments);
+			info->max_indirect_segments = info->new_max_indirect_segments;
+		}
+	}
+}
+
+static ssize_t max_ring_page_order_show(struct device *dev,
+					struct device_attribute *attr, char *page)
+{
+	struct blkfront_info *info = dev_get_drvdata(dev);
+
+	return sprintf(page, "%u\n", get_order(info->nr_ring_pages * XEN_PAGE_SIZE));
+}
+
+static ssize_t max_indirect_segs_show(struct device *dev,
+				      struct device_attribute *attr, char *page)
+{
+	struct blkfront_info *info = dev_get_drvdata(dev);
+
+	return sprintf(page, "%u\n", info->max_indirect_segments);
+}
+
+static ssize_t max_queues_show(struct device *dev,
+			       struct device_attribute *attr, char *page)
+{
+	struct blkfront_info *info = dev_get_drvdata(dev);
+
+	return sprintf(page, "%u\n", info->nr_rings);
+}
+
+static ssize_t dynamic_reconfig_device(struct blkfront_info *info, ssize_t count)
+{
+	unsigned int i;
+	int err = -EBUSY;
+
+	/*
+	 * Make sure no migration in parallel, device lock is actually a
+	 * mutex.
+	 */
+	if (!device_trylock(&info->xbdev->dev)) {
+		pr_err("Fail to acquire dev:%s lock, may be in migration.\n",
+			dev_name(&info->xbdev->dev));
+		return err;
+	}
+
+	/*
+	 * Prevent new requests and guarantee no uncompleted reqs.
+	 */
+	blk_mq_freeze_queue(info->rq);
+	if (part_in_flight(&info->gd->part0))
+		goto out;
+
+	/*
+	 * Front 				Backend
+	 * Switch to XenbusStateClosed
+	 *					frontend_changed():
+	 *					 case XenbusStateClosed:
+	 *						xen_blkif_disconnect()
+	 *						Switch to XenbusStateClosed
+	 * blkfront_resume():
+	 *					frontend_changed():
+	 *						reconnect
+	 * Wait until XenbusStateConnected
+	 */
+	info->reconfiguring = true;
+	xenbus_switch_state(info->xbdev, XenbusStateClosed);
+
+	/* Poll every 100ms, 1 minute timeout. */
+	for (i = 0; i < 600; i++) {
+		/*
+		 * Wait backend enter XenbusStateClosed, blkback_changed()
+		 * will clear reconfiguring.
+		 */
+		if (!info->reconfiguring)
+			goto resume;
+		schedule_timeout_interruptible(msecs_to_jiffies(100));
+	}
+	goto out;
+
+resume:
+	if (blkfront_resume(info->xbdev))
+		goto out;
+
+	/* Poll every 100ms, 1 minute timeout. */
+	for (i = 0; i < 600; i++) {
+		/* Wait blkfront enter StateConnected which is done by blkif_recover(). */
+		if (info->xbdev->state == XenbusStateConnected) {
+			err = count;
+			goto out;
+		}
+		schedule_timeout_interruptible(msecs_to_jiffies(100));
+	}
+
+out:
+	blk_mq_unfreeze_queue(info->rq);
+	device_unlock(&info->xbdev->dev);
+
+	return err;
+}
+
+static ssize_t max_indirect_segs_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t count)
+{
+	ssize_t ret;
+	unsigned int max_segs = 0, backend_max_segs = 0;
+	struct blkfront_info *info = dev_get_drvdata(dev);
+	int err;
+
+	ret = kstrtouint(buf, 10, &max_segs);
+	if (ret < 0)
+		return ret;
+
+	if (max_segs == info->max_indirect_segments)
+		return count;
+
+	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+			    "feature-max-indirect-segments", "%u", &backend_max_segs,
+			    NULL);
+	if (err) {
+		pr_err("Backend %s doesn't support feature-indirect-segments.\n",
+			info->xbdev->otherend);
+		return -EOPNOTSUPP;
+	}
+
+	if (max_segs > backend_max_segs) {
+		pr_err("Invalid max indirect segment (%u), backend-max: %u.\n",
+			max_segs, backend_max_segs);
+		return -EINVAL;
+	}
+
+	info->new_max_indirect_segments = max_segs;
+
+	return dynamic_reconfig_device(info, count);
+}
+
+static ssize_t max_ring_page_order_store(struct device *dev,
+				 struct device_attribute *attr,
+				 const char *buf, size_t count)
+{
+	ssize_t ret;
+	unsigned int max_order = 0, backend_max_order = 0;
+	struct blkfront_info *info = dev_get_drvdata(dev);
+	int err;
+
+	ret = kstrtouint(buf, 10, &max_order);
+	if (ret < 0)
+		return ret;
+
+	if ((1 << max_order) == info->nr_ring_pages)
+		return count;
+
+	if (max_order > XENBUS_MAX_RING_GRANT_ORDER) {
+		pr_err("Invalid max_ring_page_order (%u), max: %u.\n",
+				max_order, XENBUS_MAX_RING_GRANT_ORDER);
+		return -EINVAL;
+	}
+
+	err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
+			   "max-ring-page-order", "%u", &backend_max_order);
+	if (err != 1) {
+		pr_err("Backend %s doesn't support feature multi-page-ring.\n",
+			info->xbdev->otherend);
+		return -EOPNOTSUPP;
+	}
+	if (max_order > backend_max_order) {
+		pr_err("Invalid max_ring_page_order (%u), backend supports max: %u.\n",
+			max_order, backend_max_order);
+		return -EINVAL;
+	}
+	info->new_max_ring_page_order = max_order;
+
+	return dynamic_reconfig_device(info, count);
 }
 
+static ssize_t max_queues_store(struct device *dev,
+				struct device_attribute *attr,
+				const char *buf, size_t count)
+{
+	ssize_t ret;
+	unsigned int max_queues = 0, backend_max_queues = 0;
+	struct blkfront_info *info = dev_get_drvdata(dev);
+	int err;
+
+	ret = kstrtouint(buf, 10, &max_queues);
+	if (ret < 0)
+		return ret;
+
+	if (max_queues == info->nr_rings)
+		return count;
+
+	if (max_queues > num_online_cpus()) {
+		pr_err("Invalid max_queues (%u), can't bigger than online cpus: %u.\n",
+			max_queues, num_online_cpus());
+		return -EINVAL;
+	}
+
+	err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
+			   "multi-queue-max-queues", "%u", &backend_max_queues);
+	if (err != 1) {
+		pr_err("Backend %s doesn't support block multi queue.\n",
+			info->xbdev->otherend);
+		return -EOPNOTSUPP;
+	}
+	if (max_queues > backend_max_queues) {
+		pr_err("Invalid max_queues (%u), backend supports max: %u.\n",
+			max_queues, backend_max_queues);
+		return -EINVAL;
+	}
+	info->new_max_queues = max_queues;
+
+	return dynamic_reconfig_device(info, count);
+}
+
+static DEVICE_ATTR_RW(max_queues);
+static DEVICE_ATTR_RW(max_ring_page_order);
+static DEVICE_ATTR_RW(max_indirect_segs);
+
 /*
  * Invoked when the backend is finally 'ready' (and has told produced
  * the details about the physical device - #sectors, size, etc).
@@ -2443,6 +2698,22 @@ static void blkfront_connect(struct blkfront_info *info)
 		return;
 	}
 
+	err = device_create_file(&info->xbdev->dev, &dev_attr_max_ring_page_order);
+	if (err)
+		goto fail;
+
+	err = device_create_file(&info->xbdev->dev, &dev_attr_max_indirect_segs);
+	if (err) {
+		device_remove_file(&info->xbdev->dev, &dev_attr_max_ring_page_order);
+		goto fail;
+	}
+
+	err = device_create_file(&info->xbdev->dev, &dev_attr_max_queues);
+	if (err) {
+		device_remove_file(&info->xbdev->dev, &dev_attr_max_ring_page_order);
+		device_remove_file(&info->xbdev->dev, &dev_attr_max_indirect_segs);
+		goto fail;
+	}
 	xenbus_switch_state(info->xbdev, XenbusStateConnected);
 
 	/* Kick pending requests. */
@@ -2453,6 +2724,12 @@ static void blkfront_connect(struct blkfront_info *info)
 	add_disk(info->gd);
 
 	info->is_ready = 1;
+	return;
+
+fail:
+	blkif_free(info, 0);
+	xlvbd_release_gendisk(info);
+	return;
 }
 
 /**
@@ -2500,8 +2777,12 @@ static void blkback_changed(struct xenbus_device *dev,
 		break;
 
 	case XenbusStateClosed:
-		if (dev->state == XenbusStateClosed)
+		if (dev->state == XenbusStateClosed) {
+			/* Clear reconfiguring. */
+			if (info->reconfiguring)
+				info->reconfiguring = false;
 			break;
+		}
 		/* Missed the backend's Closing state -- fallthrough */
 	case XenbusStateClosing:
 		if (info)
-- 
2.7.4