blk-mq: implement new and more efficient tagging scheme

blk-mq currently uses percpu_ida for tag allocation. But that only
works well if the ratio between tag space and number of CPUs is
sufficiently high. For most devices and systems, that is not the
case. The end result if that we either only utilize the tag space
partially, or we end up attempting to fully exhaust it and run
into lots of lock contention with stealing between CPUs. This is
not optimal.

This new tagging scheme is a hybrid bitmap allocator. It uses
two tricks to both be SMP friendly and allow full exhaustion
of the space:

1) We cache the last allocated (or freed) tag on a per blk-mq
   software context basis. This allows us to limit the space
   we have to search. The key element here is not caching it
   in the shared tag structure, otherwise we end up dirtying
   more shared cache lines on each allocate/free operation.

2) The tag space is split into cache line sized groups, and
   each context will start off randomly in that space. Even up
   to full utilization of the space, this divides the tag users
   efficiently into cache line groups, avoiding dirtying the same
   one both between allocators and between allocator and freeer.

This scheme shows drastically better behaviour, both on small
tag spaces but on large ones as well. It has been tested extensively
to show better performance for all the cases blk-mq cares about.

Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 339 insertions, 76 deletions

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 1f43d6ee956f..467f3a20b355 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -1,64 +1,257 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/random.h>
 
 #include <linux/blk-mq.h>
 #include "blk.h"
 #include "blk-mq.h"
 #include "blk-mq-tag.h"
 
-void blk_mq_wait_for_tags(struct blk_mq_tags *tags, bool reserved)
+void blk_mq_wait_for_tags(struct blk_mq_tags *tags, struct blk_mq_hw_ctx *hctx,
+			  bool reserved)
 {
-	int tag = blk_mq_get_tag(tags, __GFP_WAIT, reserved);
-	blk_mq_put_tag(tags, tag);
+	int tag, zero = 0;
+
+	tag = blk_mq_get_tag(tags, hctx, &zero, __GFP_WAIT, reserved);
+	blk_mq_put_tag(tags, tag, &zero);
+}
+
+static bool bt_has_free_tags(struct blk_mq_bitmap_tags *bt)
+{
+	int i;
+
+	for (i = 0; i < bt->map_nr; i++) {
+		struct blk_mq_bitmap *bm = &bt->map[i];
+		int ret;
+
+		ret = find_first_zero_bit(&bm->word, bm->depth);
+		if (ret < bm->depth)
+			return true;
+	}
+
+	return false;
 }
 
 bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
 {
-	return !tags ||
-		percpu_ida_free_tags(&tags->free_tags, nr_cpu_ids) != 0;
+	if (!tags)
+		return true;
+
+	return bt_has_free_tags(&tags->bitmap_tags);
+}
+
+static int __bt_get_word(struct blk_mq_bitmap *bm, unsigned int last_tag)
+{
+	int tag, org_last_tag, end;
+
+	org_last_tag = last_tag = TAG_TO_BIT(last_tag);
+	end = bm->depth;
+	do {
+restart:
+		tag = find_next_zero_bit(&bm->word, end, last_tag);
+		if (unlikely(tag >= end)) {
+			/*
+			 * We started with an offset, start from 0 to
+			 * exhaust the map.
+			 */
+			if (org_last_tag && last_tag) {
+				end = last_tag;
+				last_tag = 0;
+				goto restart;
+			}
+			return -1;
+		}
+		last_tag = tag + 1;
+	} while (test_and_set_bit_lock(tag, &bm->word));
+
+	return tag;
+}
+
+/*
+ * Straight forward bitmap tag implementation, where each bit is a tag
+ * (cleared == free, and set == busy). The small twist is using per-cpu
+ * last_tag caches, which blk-mq stores in the blk_mq_ctx software queue
+ * contexts. This enables us to drastically limit the space searched,
+ * without dirtying an extra shared cacheline like we would if we stored
+ * the cache value inside the shared blk_mq_bitmap_tags structure. On top
+ * of that, each word of tags is in a separate cacheline. This means that
+ * multiple users will tend to stick to different cachelines, at least
+ * until the map is exhausted.
+ */
+static int __bt_get(struct blk_mq_bitmap_tags *bt, unsigned int *tag_cache)
+{
+	unsigned int last_tag, org_last_tag;
+	int index, i, tag;
+
+	last_tag = org_last_tag = *tag_cache;
+	index = TAG_TO_INDEX(last_tag);
+
+	for (i = 0; i < bt->map_nr; i++) {
+		tag = __bt_get_word(&bt->map[index], last_tag);
+		if (tag != -1) {
+			tag += index * BITS_PER_LONG;
+			goto done;
+		}
+
+		last_tag = 0;
+		if (++index >= bt->map_nr)
+			index = 0;
+	}
+
+	*tag_cache = 0;
+	return -1;
+
+	/*
+	 * Only update the cache from the allocation path, if we ended
+	 * up using the specific cached tag.
+	 */
+done:
+	if (tag == org_last_tag) {
+		last_tag = tag + 1;
+		if (last_tag >= bt->depth - 1)
+			last_tag = 0;
+
+		*tag_cache = last_tag;
+	}
+
+	return tag;
+}
+
+static inline void bt_index_inc(unsigned int *index)
+{
+	*index = (*index + 1) & (BT_WAIT_QUEUES - 1);
+}
+
+static struct bt_wait_state *bt_wait_ptr(struct blk_mq_bitmap_tags *bt,
+					 struct blk_mq_hw_ctx *hctx)
+{
+	struct bt_wait_state *bs;
+
+	if (!hctx)
+		return &bt->bs[0];
+
+	bs = &bt->bs[hctx->wait_index];
+	bt_index_inc(&hctx->wait_index);
+	return bs;
 }
 
-static unsigned int __blk_mq_get_tag(struct blk_mq_tags *tags, gfp_t gfp)
+static int bt_get(struct blk_mq_bitmap_tags *bt, struct blk_mq_hw_ctx *hctx,
+		  unsigned int *last_tag, gfp_t gfp)
 {
+	struct bt_wait_state *bs;
+	DEFINE_WAIT(wait);
 	int tag;
 
-	tag = percpu_ida_alloc(&tags->free_tags, (gfp & __GFP_WAIT) ?
-			       TASK_UNINTERRUPTIBLE : TASK_RUNNING);
-	if (tag < 0)
-		return BLK_MQ_TAG_FAIL;
-	return tag + tags->nr_reserved_tags;
+	tag = __bt_get(bt, last_tag);
+	if (tag != -1)
+		return tag;
+
+	if (!(gfp & __GFP_WAIT))
+		return -1;
+
+	bs = bt_wait_ptr(bt, hctx);
+	do {
+		bool was_empty;
+
+		was_empty = list_empty(&wait.task_list);
+		prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
+
+		tag = __bt_get(bt, last_tag);
+		if (tag != -1)
+			break;
+
+		if (was_empty)
+			atomic_set(&bs->wait_cnt, bt->wake_cnt);
+
+		io_schedule();
+	} while (1);
+
+	finish_wait(&bs->wait, &wait);
+	return tag;
+}
+
+static unsigned int __blk_mq_get_tag(struct blk_mq_tags *tags,
+				     struct blk_mq_hw_ctx *hctx,
+				     unsigned int *last_tag, gfp_t gfp)
+{
+	int tag;
+
+	tag = bt_get(&tags->bitmap_tags, hctx, last_tag, gfp);
+	if (tag >= 0)
+		return tag + tags->nr_reserved_tags;
+
+	return BLK_MQ_TAG_FAIL;
 }
 
 static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_tags *tags,
 					      gfp_t gfp)
 {
-	int tag;
+	int tag, zero = 0;
 
 	if (unlikely(!tags->nr_reserved_tags)) {
 		WARN_ON_ONCE(1);
 		return BLK_MQ_TAG_FAIL;
 	}
 
-	tag = percpu_ida_alloc(&tags->reserved_tags, (gfp & __GFP_WAIT) ?
-			       TASK_UNINTERRUPTIBLE : TASK_RUNNING);
+	tag = bt_get(&tags->breserved_tags, NULL, &zero, gfp);
 	if (tag < 0)
 		return BLK_MQ_TAG_FAIL;
+
 	return tag;
 }
 
-unsigned int blk_mq_get_tag(struct blk_mq_tags *tags, gfp_t gfp, bool reserved)
+unsigned int blk_mq_get_tag(struct blk_mq_tags *tags,
+			    struct blk_mq_hw_ctx *hctx, unsigned int *last_tag,
+			    gfp_t gfp, bool reserved)
 {
 	if (!reserved)
-		return __blk_mq_get_tag(tags, gfp);
+		return __blk_mq_get_tag(tags, hctx, last_tag, gfp);
 
 	return __blk_mq_get_reserved_tag(tags, gfp);
 }
 
+static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt)
+{
+	int i, wake_index;
+
+	wake_index = bt->wake_index;
+	for (i = 0; i < BT_WAIT_QUEUES; i++) {
+		struct bt_wait_state *bs = &bt->bs[wake_index];
+
+		if (waitqueue_active(&bs->wait)) {
+			if (wake_index != bt->wake_index)
+				bt->wake_index = wake_index;
+
+			return bs;
+		}
+
+		bt_index_inc(&wake_index);
+	}
+
+	return NULL;
+}
+
+static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
+{
+	const int index = TAG_TO_INDEX(tag);
+	struct bt_wait_state *bs;
+
+	clear_bit(TAG_TO_BIT(tag), &bt->map[index].word);
+
+	bs = bt_wake_ptr(bt);
+	if (bs && atomic_dec_and_test(&bs->wait_cnt)) {
+		smp_mb__after_clear_bit();
+		atomic_set(&bs->wait_cnt, bt->wake_cnt);
+		bt_index_inc(&bt->wake_index);
+		wake_up(&bs->wait);
+	}
+}
+
 static void __blk_mq_put_tag(struct blk_mq_tags *tags, unsigned int tag)
 {
 	BUG_ON(tag >= tags->nr_tags);
 
-	percpu_ida_free(&tags->free_tags, tag - tags->nr_reserved_tags);
+	bt_clear_tag(&tags->bitmap_tags, tag);
 }
 
 static void __blk_mq_put_reserved_tag(struct blk_mq_tags *tags,
@@ -66,22 +259,41 @@ static void __blk_mq_put_reserved_tag(struct blk_mq_tags *tags,
 {
 	BUG_ON(tag >= tags->nr_reserved_tags);
 
-	percpu_ida_free(&tags->reserved_tags, tag);
+	bt_clear_tag(&tags->breserved_tags, tag);
 }
 
-void blk_mq_put_tag(struct blk_mq_tags *tags, unsigned int tag)
+void blk_mq_put_tag(struct blk_mq_tags *tags, unsigned int tag,
+		    unsigned int *last_tag)
 {
-	if (tag >= tags->nr_reserved_tags)
-		__blk_mq_put_tag(tags, tag);
-	else
+	if (tag >= tags->nr_reserved_tags) {
+		const int real_tag = tag - tags->nr_reserved_tags;
+
+		__blk_mq_put_tag(tags, real_tag);
+		*last_tag = real_tag;
+	} else
 		__blk_mq_put_reserved_tag(tags, tag);
 }
 
-static int __blk_mq_tag_iter(unsigned id, void *data)
+static void bt_for_each_free(struct blk_mq_bitmap_tags *bt,
+			     unsigned long *free_map, unsigned int off)
 {
-	unsigned long *tag_map = data;
-	__set_bit(id, tag_map);
-	return 0;
+	int i;
+
+	for (i = 0; i < bt->map_nr; i++) {
+		struct blk_mq_bitmap *bm = &bt->map[i];
+		int bit = 0;
+
+		do {
+			bit = find_next_zero_bit(&bm->word, bm->depth, bit);
+			if (bit >= bm->depth)
+				break;
+
+			__set_bit(bit + off, free_map);
+			bit++;
+		} while (1);
+
+		off += BITS_PER_LONG;
+	}
 }
 
 void blk_mq_tag_busy_iter(struct blk_mq_tags *tags,
@@ -95,21 +307,98 @@ void blk_mq_tag_busy_iter(struct blk_mq_tags *tags,
 	if (!tag_map)
 		return;
 
-	percpu_ida_for_each_free(&tags->free_tags, __blk_mq_tag_iter, tag_map);
+	bt_for_each_free(&tags->bitmap_tags, tag_map, tags->nr_reserved_tags);
 	if (tags->nr_reserved_tags)
-		percpu_ida_for_each_free(&tags->reserved_tags, __blk_mq_tag_iter,
-			tag_map);
+		bt_for_each_free(&tags->breserved_tags, tag_map, 0);
 
 	fn(data, tag_map);
 	kfree(tag_map);
 }
 
+static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
+{
+	unsigned int i, used;
+
+	for (i = 0, used = 0; i < bt->map_nr; i++) {
+		struct blk_mq_bitmap *bm = &bt->map[i];
+
+		used += bitmap_weight(&bm->word, bm->depth);
+	}
+
+	return bt->depth - used;
+}
+
+static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
+			int node, bool reserved)
+{
+	int i;
+
+	/*
+	 * Depth can be zero for reserved tags, that's not a failure
+	 * condition.
+	 */
+	if (depth) {
+		int nr, i, map_depth;
+
+		nr = ALIGN(depth, BITS_PER_LONG) / BITS_PER_LONG;
+		bt->map = kzalloc_node(nr * sizeof(struct blk_mq_bitmap),
+						GFP_KERNEL, node);
+		if (!bt->map)
+			return -ENOMEM;
+
+		bt->map_nr = nr;
+		map_depth = depth;
+		for (i = 0; i < nr; i++) {
+			bt->map[i].depth = min(map_depth, BITS_PER_LONG);
+			map_depth -= BITS_PER_LONG;
+		}
+	}
+
+	bt->bs = kzalloc(BT_WAIT_QUEUES * sizeof(*bt->bs), GFP_KERNEL);
+	if (!bt->bs) {
+		kfree(bt->map);
+		return -ENOMEM;
+	}
+
+	for (i = 0; i < BT_WAIT_QUEUES; i++)
+		init_waitqueue_head(&bt->bs[i].wait);
+
+	bt->wake_cnt = BT_WAIT_BATCH;
+	if (bt->wake_cnt > depth / 4)
+		bt->wake_cnt = max(1U, depth / 4);
+
+	bt->depth = depth;
+	return 0;
+}
+
+static void bt_free(struct blk_mq_bitmap_tags *bt)
+{
+	kfree(bt->map);
+	kfree(bt->bs);
+}
+
+static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
+						   int node)
+{
+	unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
+
+	if (bt_alloc(&tags->bitmap_tags, depth, node, false))
+		goto enomem;
+	if (bt_alloc(&tags->breserved_tags, tags->nr_reserved_tags, node, true))
+		goto enomem;
+
+	return tags;
+enomem:
+	bt_free(&tags->bitmap_tags);
+	kfree(tags);
+	return NULL;
+}
+
 struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
 				     unsigned int reserved_tags, int node)
 {
 	unsigned int nr_tags, nr_cache;
 	struct blk_mq_tags *tags;
-	int ret;
 
 	if (total_tags > BLK_MQ_TAG_MAX) {
 		pr_err("blk-mq: tag depth too large\n");
@@ -121,72 +410,46 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
 		return NULL;
 
 	nr_tags = total_tags - reserved_tags;
-	nr_cache = nr_tags / num_possible_cpus();
-
-	if (nr_cache < BLK_MQ_TAG_CACHE_MIN)
-		nr_cache = BLK_MQ_TAG_CACHE_MIN;
-	else if (nr_cache > BLK_MQ_TAG_CACHE_MAX)
-		nr_cache = BLK_MQ_TAG_CACHE_MAX;
+	nr_cache = nr_tags / num_online_cpus();
 
 	tags->nr_tags = total_tags;
 	tags->nr_reserved_tags = reserved_tags;
-	tags->nr_max_cache = nr_cache;
-	tags->nr_batch_move = max(1u, nr_cache / 2);
-
-	ret = __percpu_ida_init(&tags->free_tags, tags->nr_tags -
-				tags->nr_reserved_tags,
-				tags->nr_max_cache,
-				tags->nr_batch_move);
-	if (ret)
-		goto err_free_tags;
-
-	if (reserved_tags) {
-		/*
-		 * With max_cahe and batch set to 1, the allocator fallbacks to
-		 * no cached. It's fine reserved tags allocation is slow.
-		 */
-		ret = __percpu_ida_init(&tags->reserved_tags, reserved_tags,
-				1, 1);
-		if (ret)
-			goto err_reserved_tags;
-	}
 
-	return tags;
-
-err_reserved_tags:
-	percpu_ida_destroy(&tags->free_tags);
-err_free_tags:
-	kfree(tags);
-	return NULL;
+	return blk_mq_init_bitmap_tags(tags, node);
 }
 
 void blk_mq_free_tags(struct blk_mq_tags *tags)
 {
-	percpu_ida_destroy(&tags->free_tags);
-	percpu_ida_destroy(&tags->reserved_tags);
+	bt_free(&tags->bitmap_tags);
+	bt_free(&tags->breserved_tags);
 	kfree(tags);
 }
 
+void blk_mq_tag_init_last_tag(struct blk_mq_tags *tags, unsigned int *tag)
+{
+	unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
+
+	if (depth > 1)
+		*tag = prandom_u32() % (depth - 1);
+	else
+		*tag = 0;
+}
+
 ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)
 {
 	char *orig_page = page;
-	unsigned int cpu;
+	unsigned int free, res;
 
 	if (!tags)
 		return 0;
 
-	page += sprintf(page, "nr_tags=%u, reserved_tags=%u, batch_move=%u,"
-			" max_cache=%u\n", tags->nr_tags, tags->nr_reserved_tags,
-			tags->nr_batch_move, tags->nr_max_cache);
+	page += sprintf(page, "nr_tags=%u, reserved_tags=%u\n",
+			tags->nr_tags, tags->nr_reserved_tags);
 
-	page += sprintf(page, "nr_free=%u, nr_reserved=%u\n",
-			percpu_ida_free_tags(&tags->free_tags, nr_cpu_ids),
-			percpu_ida_free_tags(&tags->reserved_tags, nr_cpu_ids));
+	free = bt_unused_tags(&tags->bitmap_tags);
+	res = bt_unused_tags(&tags->breserved_tags);
 
-	for_each_possible_cpu(cpu) {
-		page += sprintf(page, "  cpu%02u: nr_free=%u\n", cpu,
-				percpu_ida_free_tags(&tags->free_tags, cpu));
-	}
+	page += sprintf(page, "nr_free=%u, nr_reserved=%u\n", free, res);
 
 	return page - orig_page;
 }