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[dm-devel] Questions regarding request based dm-multipath and blk-layer/elevator



Hi

I have couple of questions regd request based dm.

1. With request based multipath, we have 2 elevators in the path to the
device. Doesn't 2 idling I/O schedulers affect performance? Is it better to
use the noop elevator for the dm device? What is suggested?
I can send a patch to set noop as default for rq based dm, if it would be
better.

2. The blk-layer limit nr_requests is the maximum nr of requests per-queue.
Currently we set it to the default maximum(128) and leave it.

Instead would it be better to set it to a higher number based on the number of
paths(underlying devices) and their nr_requests? In bio-based dm-multipath, we
were queueing upto the sum of all the underlying queue's nr_requests.

For example the attached patch would set it to the sum of nr_requests of all
the targets. May be it is better to do it from the user-space daemon,
multipathd? Or just 128 requests is enough as in the end, it is going to be a
single LUN? Or should we simply use the nr_request from one of the underlying
device? Or the maximum nr_request amoung the underlying devices?

Thanks
Nikanth

diff --git a/block/blk-core.c b/block/blk-core.c
index 9fe174d..fc33c2d 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -91,6 +91,44 @@ void blk_queue_congestion_threshold(struct request_queue *q)
 	q->nr_congestion_off = nr;
 }
 
+unsigned long
+__blk_queue_set_nr_requests(struct request_queue *q, unsigned long nr)
+{
+	struct request_list *rl = &q->rq;
+
+	if (nr < BLKDEV_MIN_RQ)
+		nr = BLKDEV_MIN_RQ;
+
+	q->nr_requests = nr;
+	blk_queue_congestion_threshold(q);
+
+	if (rl->count[BLK_RW_SYNC] >= queue_congestion_on_threshold(q))
+		blk_set_queue_congested(q, BLK_RW_SYNC);
+	else if (rl->count[BLK_RW_SYNC] < queue_congestion_off_threshold(q))
+		blk_clear_queue_congested(q, BLK_RW_SYNC);
+
+	if (rl->count[BLK_RW_ASYNC] >= queue_congestion_on_threshold(q))
+		blk_set_queue_congested(q, BLK_RW_ASYNC);
+	else if (rl->count[BLK_RW_ASYNC] < queue_congestion_off_threshold(q))
+		blk_clear_queue_congested(q, BLK_RW_ASYNC);
+
+	if (rl->count[BLK_RW_SYNC] >= q->nr_requests) {
+		blk_set_queue_full(q, BLK_RW_SYNC);
+	} else if (rl->count[BLK_RW_SYNC]+1 <= q->nr_requests) {
+		blk_clear_queue_full(q, BLK_RW_SYNC);
+		wake_up(&rl->wait[BLK_RW_SYNC]);
+	}
+
+	if (rl->count[BLK_RW_ASYNC] >= q->nr_requests) {
+		blk_set_queue_full(q, BLK_RW_ASYNC);
+	} else if (rl->count[BLK_RW_ASYNC]+1 <= q->nr_requests) {
+		blk_clear_queue_full(q, BLK_RW_ASYNC);
+		wake_up(&rl->wait[BLK_RW_ASYNC]);
+	}
+	return nr;
+}
+EXPORT_SYMBOL(__blk_queue_set_nr_requests);
+
 /**
  * blk_get_backing_dev_info - get the address of a queue's backing_dev_info
  * @bdev:	device
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 306759b..615198c 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -39,7 +39,6 @@ static ssize_t queue_requests_show(struct request_queue *q, char *page)
 static ssize_t
 queue_requests_store(struct request_queue *q, const char *page, size_t count)
 {
-	struct request_list *rl = &q->rq;
 	unsigned long nr;
 	int ret;
 
@@ -47,37 +46,11 @@ queue_requests_store(struct request_queue *q, const char *page, size_t count)
 		return -EINVAL;
 
 	ret = queue_var_store(&nr, page, count);
-	if (nr < BLKDEV_MIN_RQ)
-		nr = BLKDEV_MIN_RQ;
 
 	spin_lock_irq(q->queue_lock);
-	q->nr_requests = nr;
-	blk_queue_congestion_threshold(q);
-
-	if (rl->count[BLK_RW_SYNC] >= queue_congestion_on_threshold(q))
-		blk_set_queue_congested(q, BLK_RW_SYNC);
-	else if (rl->count[BLK_RW_SYNC] < queue_congestion_off_threshold(q))
-		blk_clear_queue_congested(q, BLK_RW_SYNC);
-
-	if (rl->count[BLK_RW_ASYNC] >= queue_congestion_on_threshold(q))
-		blk_set_queue_congested(q, BLK_RW_ASYNC);
-	else if (rl->count[BLK_RW_ASYNC] < queue_congestion_off_threshold(q))
-		blk_clear_queue_congested(q, BLK_RW_ASYNC);
-
-	if (rl->count[BLK_RW_SYNC] >= q->nr_requests) {
-		blk_set_queue_full(q, BLK_RW_SYNC);
-	} else if (rl->count[BLK_RW_SYNC]+1 <= q->nr_requests) {
-		blk_clear_queue_full(q, BLK_RW_SYNC);
-		wake_up(&rl->wait[BLK_RW_SYNC]);
-	}
-
-	if (rl->count[BLK_RW_ASYNC] >= q->nr_requests) {
-		blk_set_queue_full(q, BLK_RW_ASYNC);
-	} else if (rl->count[BLK_RW_ASYNC]+1 <= q->nr_requests) {
-		blk_clear_queue_full(q, BLK_RW_ASYNC);
-		wake_up(&rl->wait[BLK_RW_ASYNC]);
-	}
+	__blk_queue_set_nr_requests(q, nr);
 	spin_unlock_irq(q->queue_lock);
+
 	return ret;
 }
 
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 9924ea2..c6deff9 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -526,6 +526,18 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
 }
 EXPORT_SYMBOL_GPL(dm_set_device_limits);
 
+int dm_set_device_nr_requests(struct dm_target *ti, struct dm_dev *dev,
+			 sector_t start, sector_t len, void *data)
+{
+	unsigned long *nr_requests = data;
+	struct block_device *bdev = dev->bdev;
+	struct request_queue *q = bdev_get_queue(bdev);
+
+	*nr_requests += q->nr_requests;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(dm_set_device_nr_requests);
+
 int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
 		  struct dm_dev **result)
 {
@@ -983,12 +995,13 @@ struct dm_target *dm_table_find_target(struct dm_table *t, sector_t sector)
  * Establish the new table's queue_limits and validate them.
  */
 int dm_calculate_queue_limits(struct dm_table *table,
-			      struct queue_limits *limits)
+	      struct queue_limits *limits, unsigned long *nr_requests)
 {
 	struct dm_target *uninitialized_var(ti);
 	struct queue_limits ti_limits;
 	unsigned i = 0;
 
+	*nr_requests = 0;
 	blk_set_default_limits(limits);
 
 	while (i < dm_table_get_num_targets(table)) {
@@ -1005,6 +1018,13 @@ int dm_calculate_queue_limits(struct dm_table *table,
 		ti->type->iterate_devices(ti, dm_set_device_limits,
 					  &ti_limits);
 
+		/*
+		 * Combine queue nr_requests limit of all the devices this
+		 * target uses.
+		 */
+		ti->type->iterate_devices(ti, dm_set_device_nr_requests,
+					  nr_requests);
+
 		/* Set I/O hints portion of queue limits */
 		if (ti->type->io_hints)
 			ti->type->io_hints(ti, &ti_limits);
@@ -1074,7 +1094,7 @@ no_integrity:
 }
 
 void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
-			       struct queue_limits *limits)
+	       struct queue_limits *limits, unsigned long nr_requests)
 {
 	/*
 	 * Copy table's limits to the DM device's request_queue
@@ -1098,8 +1118,10 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 	 * Those bios are passed to request-based dm at the resume time.
 	 */
 	smp_mb();
-	if (dm_table_request_based(t))
+	if (dm_table_request_based(t)) {
 		queue_flag_set_unlocked(QUEUE_FLAG_STACKABLE, q);
+		__blk_queue_set_nr_requests(q, nr_requests);
+	}
 }
 
 unsigned int dm_table_get_num_targets(struct dm_table *t)
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index d21e128..2d45689 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2052,7 +2052,7 @@ static void __set_size(struct mapped_device *md, sector_t size)
  * Returns old map, which caller must destroy.
  */
 static struct dm_table *__bind(struct mapped_device *md, struct dm_table *t,
-			       struct queue_limits *limits)
+		       struct queue_limits *limits, unsigned long nr_requests)
 {
 	struct dm_table *old_map;
 	struct request_queue *q = md->queue;
@@ -2083,11 +2083,13 @@ static struct dm_table *__bind(struct mapped_device *md, struct dm_table *t,
 
 	__bind_mempools(md, t);
 
-	write_lock_irqsave(&md->map_lock, flags);
+	spin_lock_irqsave(q->queue_lock, flags);
+	write_lock(&md->map_lock);
 	old_map = md->map;
 	md->map = t;
-	dm_table_set_restrictions(t, q, limits);
-	write_unlock_irqrestore(&md->map_lock, flags);
+	dm_table_set_restrictions(t, q, limits, nr_requests);
+	write_unlock(&md->map_lock);
+	spin_unlock_irqrestore(q->queue_lock, flags);
 
 	return old_map;
 }
@@ -2390,6 +2392,7 @@ struct dm_table *dm_swap_table(struct mapped_device *md, struct dm_table *table)
 	struct dm_table *map = ERR_PTR(-EINVAL);
 	struct queue_limits limits;
 	int r;
+	unsigned long nr_requests;
 
 	mutex_lock(&md->suspend_lock);
 
@@ -2397,7 +2400,7 @@ struct dm_table *dm_swap_table(struct mapped_device *md, struct dm_table *table)
 	if (!dm_suspended_md(md))
 		goto out;
 
-	r = dm_calculate_queue_limits(table, &limits);
+	r = dm_calculate_queue_limits(table, &limits, &nr_requests);
 	if (r) {
 		map = ERR_PTR(r);
 		goto out;
@@ -2410,7 +2413,7 @@ struct dm_table *dm_swap_table(struct mapped_device *md, struct dm_table *table)
 		goto out;
 	}
 
-	map = __bind(md, table, &limits);
+	map = __bind(md, table, &limits, nr_requests);
 
 out:
 	mutex_unlock(&md->suspend_lock);
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index bad1724..9e25c48 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -50,9 +50,9 @@ void dm_table_event_callback(struct dm_table *t,
 struct dm_target *dm_table_get_target(struct dm_table *t, unsigned int index);
 struct dm_target *dm_table_find_target(struct dm_table *t, sector_t sector);
 int dm_calculate_queue_limits(struct dm_table *table,
-			      struct queue_limits *limits);
+		      struct queue_limits *limits, unsigned long *nr_requests);
 void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
-			       struct queue_limits *limits);
+		       struct queue_limits *limits, unsigned long nr_requests);
 struct list_head *dm_table_get_devices(struct dm_table *t);
 void dm_table_presuspend_targets(struct dm_table *t);
 void dm_table_postsuspend_targets(struct dm_table *t);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 6690e8b..366bba5 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -934,6 +934,9 @@ extern void blk_limits_io_min(struct queue_limits *limits, unsigned int min);
 extern void blk_queue_io_min(struct request_queue *q, unsigned int min);
 extern void blk_limits_io_opt(struct queue_limits *limits, unsigned int opt);
 extern void blk_queue_io_opt(struct request_queue *q, unsigned int opt);
+extern unsigned long __blk_queue_set_nr_requests(struct request_queue *q,
+							unsigned long nr);
+
 extern void blk_set_default_limits(struct queue_limits *lim);
 extern int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 			    sector_t offset);
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 1381cd9..b0f9443 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -108,6 +108,11 @@ void dm_error(const char *message);
  */
 int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
 			 sector_t start, sector_t len, void *data);
+/*
+ * Combine device nr_requests.
+ */
+int dm_set_device_nr_requests(struct dm_target *ti, struct dm_dev *dev,
+			 sector_t start, sector_t len, void *data);
 
 struct dm_dev {
 	struct block_device *bdev;


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