blk-mq: improve support for shared tags maps

This adds support for active queue tracking, meaning that the
blk-mq tagging maintains a count of active users of a tag set.
This allows us to maintain a notion of fairness between users,
so that we can distribute the tag depth evenly without starving
some users while allowing others to try unfair deep queues.

If sharing of a tag set is detected, each hardware queue will
track the depth of its own queue. And if this exceeds the total
depth divided by the number of active queues, the user is actively
throttled down.

The active queue count is done lazily to avoid bouncing that data
between submitter and completer. Each hardware queue gets marked
active when it allocates its first tag, and gets marked inactive
when 1) the last tag is cleared, and 2) the queue timeout grace
period has passed.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq-sysfs.c

@@ -208,6 +208,11 @@ static ssize_t blk_mq_hw_sysfs_tags_show(struct blk_mq_hw_ctx *hctx, char *page)
 	return blk_mq_tag_sysfs_show(hctx->tags, page);
 }
 
+static ssize_t blk_mq_hw_sysfs_active_show(struct blk_mq_hw_ctx *hctx, char *page)
+{
+	return sprintf(page, "%u\n", atomic_read(&hctx->nr_active));
+}
+
 static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
 {
 	unsigned int i, first = 1;
@@ -267,6 +272,10 @@ static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_dispatched = {
 	.attr = {.name = "dispatched", .mode = S_IRUGO },
 	.show = blk_mq_hw_sysfs_dispatched_show,
 };
+static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_active = {
+	.attr = {.name = "active", .mode = S_IRUGO },
+	.show = blk_mq_hw_sysfs_active_show,
+};
 static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_pending = {
 	.attr = {.name = "pending", .mode = S_IRUGO },
 	.show = blk_mq_hw_sysfs_rq_list_show,
@@ -287,6 +296,7 @@ static struct attribute *default_hw_ctx_attrs[] = {
 	&blk_mq_hw_sysfs_pending.attr,
 	&blk_mq_hw_sysfs_tags.attr,
 	&blk_mq_hw_sysfs_cpus.attr,
+	&blk_mq_hw_sysfs_active.attr,
 	NULL,
 };
 

block/blk-mq-tag.c

@@ -7,13 +7,12 @@
 #include "blk-mq.h"
 #include "blk-mq-tag.h"
 
-void blk_mq_wait_for_tags(struct blk_mq_tags *tags, struct blk_mq_hw_ctx *hctx,
-			  bool reserved)
+void blk_mq_wait_for_tags(struct blk_mq_hw_ctx *hctx, bool reserved)
 {
 	int tag, zero = 0;
 
-	tag = blk_mq_get_tag(tags, hctx, &zero, __GFP_WAIT, reserved);
-	blk_mq_put_tag(tags, tag, &zero);
+	tag = blk_mq_get_tag(hctx, &zero, __GFP_WAIT, reserved);
+	blk_mq_put_tag(hctx, tag, &zero);
 }
 
 static bool bt_has_free_tags(struct blk_mq_bitmap_tags *bt)
@@ -40,6 +39,84 @@ bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
 	return bt_has_free_tags(&tags->bitmap_tags);
 }
 
+static inline void bt_index_inc(unsigned int *index)
+{
+	*index = (*index + 1) & (BT_WAIT_QUEUES - 1);
+}
+
+/*
+ * If a previously inactive queue goes active, bump the active user count.
+ */
+bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
+{
+	if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
+	    !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
+		atomic_inc(&hctx->tags->active_queues);
+
+	return true;
+}
+
+/*
+ * If a previously busy queue goes inactive, potential waiters could now
+ * be allowed to queue. Wake them up and check.
+ */
+void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
+{
+	struct blk_mq_tags *tags = hctx->tags;
+	struct blk_mq_bitmap_tags *bt;
+	int i, wake_index;
+
+	if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
+		return;
+
+	atomic_dec(&tags->active_queues);
+
+	/*
+	 * Will only throttle depth on non-reserved tags
+	 */
+	bt = &tags->bitmap_tags;
+	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))
+			wake_up(&bs->wait);
+
+		bt_index_inc(&wake_index);
+	}
+}
+
+/*
+ * For shared tag users, we track the number of currently active users
+ * and attempt to provide a fair share of the tag depth for each of them.
+ */
+static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
+				  struct blk_mq_bitmap_tags *bt)
+{
+	unsigned int depth, users;
+
+	if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_SHARED))
+		return true;
+	if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
+		return true;
+
+	/*
+	 * Don't try dividing an ant
+	 */
+	if (bt->depth == 1)
+		return true;
+
+	users = atomic_read(&hctx->tags->active_queues);
+	if (!users)
+		return true;
+
+	/*
+	 * Allow at least some tags
+	 */
+	depth = max((bt->depth + users - 1) / users, 4U);
+	return atomic_read(&hctx->nr_active) < depth;
+}
+
 static int __bt_get_word(struct blk_mq_bitmap *bm, unsigned int last_tag)
 {
 	int tag, org_last_tag, end;
@@ -78,11 +155,15 @@ static int __bt_get_word(struct blk_mq_bitmap *bm, unsigned int last_tag)
  * 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)
+static int __bt_get(struct blk_mq_hw_ctx *hctx, struct blk_mq_bitmap_tags *bt,
+		    unsigned int *tag_cache)
 {
 	unsigned int last_tag, org_last_tag;
 	int index, i, tag;
 
+	if (!hctx_may_queue(hctx, bt))
+		return -1;
+
 	last_tag = org_last_tag = *tag_cache;
 	index = TAG_TO_INDEX(bt, last_tag);
 
@@ -117,11 +198,6 @@ static int __bt_get(struct blk_mq_bitmap_tags *bt, unsigned int *tag_cache)
 	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)
 {
@@ -142,7 +218,7 @@ static int bt_get(struct blk_mq_bitmap_tags *bt, struct blk_mq_hw_ctx *hctx,
 	DEFINE_WAIT(wait);
 	int tag;
 
-	tag = __bt_get(bt, last_tag);
+	tag = __bt_get(hctx, bt, last_tag);
 	if (tag != -1)
 		return tag;
 
@@ -156,7 +232,7 @@ static int bt_get(struct blk_mq_bitmap_tags *bt, struct blk_mq_hw_ctx *hctx,
 		was_empty = list_empty(&wait.task_list);
 		prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
 
-		tag = __bt_get(bt, last_tag);
+		tag = __bt_get(hctx, bt, last_tag);
 		if (tag != -1)
 			break;
 
@@ -200,14 +276,13 @@ static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_tags *tags,
 	return tag;
 }
 
-unsigned int blk_mq_get_tag(struct blk_mq_tags *tags,
-			    struct blk_mq_hw_ctx *hctx, unsigned int *last_tag,
+unsigned int blk_mq_get_tag(struct blk_mq_hw_ctx *hctx, unsigned int *last_tag,
 			    gfp_t gfp, bool reserved)
 {
 	if (!reserved)
-		return __blk_mq_get_tag(tags, hctx, last_tag, gfp);
+		return __blk_mq_get_tag(hctx->tags, hctx, last_tag, gfp);
 
-	return __blk_mq_get_reserved_tag(tags, gfp);
+	return __blk_mq_get_reserved_tag(hctx->tags, gfp);
 }
 
 static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt)
@@ -265,9 +340,11 @@ static void __blk_mq_put_reserved_tag(struct blk_mq_tags *tags,
 	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_hw_ctx *hctx, unsigned int tag,
 		    unsigned int *last_tag)
 {
+	struct blk_mq_tags *tags = hctx->tags;
+
 	if (tag >= tags->nr_reserved_tags) {
 		const int real_tag = tag - tags->nr_reserved_tags;
 
@@ -465,6 +542,7 @@ ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)
 	res = bt_unused_tags(&tags->breserved_tags);
 
 	page += sprintf(page, "nr_free=%u, nr_reserved=%u\n", free, res);
+	page += sprintf(page, "active_queues=%u\n", atomic_read(&tags->active_queues));
 
 	return page - orig_page;
 }

block/blk-mq-tag.h

@@ -38,6 +38,8 @@ struct blk_mq_tags {
 	unsigned int nr_tags;
 	unsigned int nr_reserved_tags;
 
+	atomic_t active_queues;
+
 	struct blk_mq_bitmap_tags bitmap_tags;
 	struct blk_mq_bitmap_tags breserved_tags;
 
@@ -49,9 +51,9 @@ struct blk_mq_tags {
 extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags, unsigned int reserved_tags, int node);
 extern void blk_mq_free_tags(struct blk_mq_tags *tags);
 
-extern 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);
-extern void blk_mq_wait_for_tags(struct blk_mq_tags *tags, struct blk_mq_hw_ctx *hctx, bool reserved);
-extern void blk_mq_put_tag(struct blk_mq_tags *tags, unsigned int tag, unsigned int *last_tag);
+extern unsigned int blk_mq_get_tag(struct blk_mq_hw_ctx *hctx, unsigned int *last_tag, gfp_t gfp, bool reserved);
+extern void blk_mq_wait_for_tags(struct blk_mq_hw_ctx *hctx, bool reserved);
+extern void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, unsigned int tag, unsigned int *last_tag);
 extern void blk_mq_tag_busy_iter(struct blk_mq_tags *tags, void (*fn)(void *data, unsigned long *), void *data);
 extern bool blk_mq_has_free_tags(struct blk_mq_tags *tags);
 extern ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page);
@@ -68,4 +70,23 @@ enum {
 	BLK_MQ_TAG_MAX		= BLK_MQ_TAG_FAIL - 1,
 };
 
+extern bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *);
+extern void __blk_mq_tag_idle(struct blk_mq_hw_ctx *);
+
+static inline bool blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
+{
+	if (!(hctx->flags & BLK_MQ_F_TAG_SHARED))
+		return false;
+
+	return __blk_mq_tag_busy(hctx);
+}
+
+static inline void blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
+{
+	if (!(hctx->flags & BLK_MQ_F_TAG_SHARED))
+		return;
+
+	__blk_mq_tag_idle(hctx);
+}
+
 #endif

block/blk-mq.c

@@ -80,9 +80,16 @@ static struct request *__blk_mq_alloc_request(struct blk_mq_hw_ctx *hctx,
 	struct request *rq;
 	unsigned int tag;
 
-	tag = blk_mq_get_tag(hctx->tags, hctx, &ctx->last_tag, gfp, reserved);
+	tag = blk_mq_get_tag(hctx, &ctx->last_tag, gfp, reserved);
 	if (tag != BLK_MQ_TAG_FAIL) {
 		rq = hctx->tags->rqs[tag];
+
+		rq->cmd_flags = 0;
+		if (blk_mq_tag_busy(hctx)) {
+			rq->cmd_flags = REQ_MQ_INFLIGHT;
+			atomic_inc(&hctx->nr_active);
+		}
+
 		rq->tag = tag;
 		return rq;
 	}
@@ -190,7 +197,7 @@ static void blk_mq_rq_ctx_init(struct request_queue *q, struct blk_mq_ctx *ctx,
 	/* csd/requeue_work/fifo_time is initialized before use */
 	rq->q = q;
 	rq->mq_ctx = ctx;
-	rq->cmd_flags = rw_flags;
+	rq->cmd_flags |= rw_flags;
 	rq->cmd_type = 0;
 	/* do not touch atomic flags, it needs atomic ops against the timer */
 	rq->cpu = -1;
@@ -262,7 +269,7 @@ static struct request *blk_mq_alloc_request_pinned(struct request_queue *q,
 			break;
 		}
 
-		blk_mq_wait_for_tags(hctx->tags, hctx, reserved);
+		blk_mq_wait_for_tags(hctx, reserved);
 	} while (1);
 
 	return rq;
@@ -303,8 +310,11 @@ static void __blk_mq_free_request(struct blk_mq_hw_ctx *hctx,
 	const int tag = rq->tag;
 	struct request_queue *q = rq->q;
 
+	if (rq->cmd_flags & REQ_MQ_INFLIGHT)
+		atomic_dec(&hctx->nr_active);
+
 	clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
-	blk_mq_put_tag(hctx->tags, tag, &ctx->last_tag);
+	blk_mq_put_tag(hctx, tag, &ctx->last_tag);
 	blk_mq_queue_exit(q);
 }
 
@@ -571,8 +581,13 @@ static void blk_mq_rq_timer(unsigned long data)
 	queue_for_each_hw_ctx(q, hctx, i)
 		blk_mq_hw_ctx_check_timeout(hctx, &next, &next_set);
 
-	if (next_set)
-		mod_timer(&q->timeout, round_jiffies_up(next));
+	if (next_set) {
+		next = blk_rq_timeout(round_jiffies_up(next));
+		mod_timer(&q->timeout, next);
+	} else {
+		queue_for_each_hw_ctx(q, hctx, i)
+			blk_mq_tag_idle(hctx);
+	}
 }
 
 /*
@@ -1439,6 +1454,56 @@ static void blk_mq_map_swqueue(struct request_queue *q)
 	}
 }
 
+static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set)
+{
+	struct blk_mq_hw_ctx *hctx;
+	struct request_queue *q;
+	bool shared;
+	int i;
+
+	if (set->tag_list.next == set->tag_list.prev)
+		shared = false;
+	else
+		shared = true;
+
+	list_for_each_entry(q, &set->tag_list, tag_set_list) {
+		blk_mq_freeze_queue(q);
+
+		queue_for_each_hw_ctx(q, hctx, i) {
+			if (shared)
+				hctx->flags |= BLK_MQ_F_TAG_SHARED;
+			else
+				hctx->flags &= ~BLK_MQ_F_TAG_SHARED;
+		}
+		blk_mq_unfreeze_queue(q);
+	}
+}
+
+static void blk_mq_del_queue_tag_set(struct request_queue *q)
+{
+	struct blk_mq_tag_set *set = q->tag_set;
+
+	blk_mq_freeze_queue(q);
+
+	mutex_lock(&set->tag_list_lock);
+	list_del_init(&q->tag_set_list);
+	blk_mq_update_tag_set_depth(set);
+	mutex_unlock(&set->tag_list_lock);
+
+	blk_mq_unfreeze_queue(q);
+}
+
+static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
+				     struct request_queue *q)
+{
+	q->tag_set = set;
+
+	mutex_lock(&set->tag_list_lock);
+	list_add_tail(&q->tag_set_list, &set->tag_list);
+	blk_mq_update_tag_set_depth(set);
+	mutex_unlock(&set->tag_list_lock);
+}
+
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 {
 	struct blk_mq_hw_ctx **hctxs;
@@ -1464,6 +1529,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 		if (!zalloc_cpumask_var(&hctxs[i]->cpumask, GFP_KERNEL))
 			goto err_hctxs;
 
+		atomic_set(&hctxs[i]->nr_active, 0);
 		hctxs[i]->numa_node = NUMA_NO_NODE;
 		hctxs[i]->queue_num = i;
 	}
@@ -1516,6 +1582,8 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	list_add_tail(&q->all_q_node, &all_q_list);
 	mutex_unlock(&all_q_mutex);
 
+	blk_mq_add_queue_tag_set(set, q);
+
 	return q;
 
 err_flush_rq:
@@ -1543,6 +1611,8 @@ void blk_mq_free_queue(struct request_queue *q)
 	struct blk_mq_hw_ctx *hctx;
 	int i;
 
+	blk_mq_del_queue_tag_set(q);
+
 	queue_for_each_hw_ctx(q, hctx, i) {
 		kfree(hctx->ctx_map);
 		kfree(hctx->ctxs);
@@ -1635,6 +1705,9 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
 			goto out_unwind;
 	}
 
+	mutex_init(&set->tag_list_lock);
+	INIT_LIST_HEAD(&set->tag_list);
+
 	return 0;
 
 out_unwind:

block/blk-timeout.c

@@ -166,6 +166,17 @@ void blk_abort_request(struct request *req)
 }
 EXPORT_SYMBOL_GPL(blk_abort_request);
 
+unsigned long blk_rq_timeout(unsigned long timeout)
+{
+	unsigned long maxt;
+
+	maxt = round_jiffies_up(jiffies + BLK_MAX_TIMEOUT);
+	if (time_after(timeout, maxt))
+		timeout = maxt;
+
+	return timeout;
+}
+
 /**
  * blk_add_timer - Start timeout timer for a single request
  * @req:	request that is about to start running.
@@ -200,7 +211,7 @@ void blk_add_timer(struct request *req)
 	 * than an existing one, modify the timer. Round up to next nearest
 	 * second.
 	 */
-	expiry = round_jiffies_up(req->deadline);
+	expiry = blk_rq_timeout(round_jiffies_up(req->deadline));
 
 	if (!timer_pending(&q->timeout) ||
 	    time_before(expiry, q->timeout.expires)) {

block/blk.h

@@ -9,6 +9,9 @@
 /* Number of requests a "batching" process may submit */
 #define BLK_BATCH_REQ	32
 
+/* Max future timer expiry for timeouts */
+#define BLK_MAX_TIMEOUT		(5 * HZ)
+
 extern struct kmem_cache *blk_requestq_cachep;
 extern struct kmem_cache *request_cachep;
 extern struct kobj_type blk_queue_ktype;
@@ -37,6 +40,7 @@ bool __blk_end_bidi_request(struct request *rq, int error,
 void blk_rq_timed_out_timer(unsigned long data);
 void blk_rq_check_expired(struct request *rq, unsigned long *next_timeout,
 			  unsigned int *next_set);
+unsigned long blk_rq_timeout(unsigned long timeout);
 void blk_add_timer(struct request *req);
 void blk_delete_timer(struct request *);
 

include/linux/blk-mq.h

@@ -48,6 +48,8 @@ struct blk_mq_hw_ctx {
 	unsigned int		numa_node;
 	unsigned int		cmd_size;	/* per-request extra data */
 
+	atomic_t		nr_active;
+
 	struct blk_mq_cpu_notifier	cpu_notifier;
 	struct kobject		kobj;
 };
@@ -64,6 +66,9 @@ struct blk_mq_tag_set {
 	void			*driver_data;
 
 	struct blk_mq_tags	**tags;
+
+	struct mutex		tag_list_lock;
+	struct list_head	tag_list;
 };
 
 typedef int (queue_rq_fn)(struct blk_mq_hw_ctx *, struct request *);
@@ -126,8 +131,10 @@ enum {
 
 	BLK_MQ_F_SHOULD_MERGE	= 1 << 0,
 	BLK_MQ_F_SHOULD_SORT	= 1 << 1,
+	BLK_MQ_F_TAG_SHARED	= 1 << 2,
 
 	BLK_MQ_S_STOPPED	= 0,
+	BLK_MQ_S_TAG_ACTIVE	= 1,
 
 	BLK_MQ_MAX_DEPTH	= 2048,
 

include/linux/blk_types.h

@@ -190,6 +190,7 @@ enum rq_flag_bits {
 	__REQ_PM,		/* runtime pm request */
 	__REQ_END,		/* last of chain of requests */
 	__REQ_HASHED,		/* on IO scheduler merge hash */
+	__REQ_MQ_INFLIGHT,	/* track inflight for MQ */
 	__REQ_NR_BITS,		/* stops here */
 };
 
@@ -243,5 +244,6 @@ enum rq_flag_bits {
 #define REQ_PM			(1ULL << __REQ_PM)
 #define REQ_END			(1ULL << __REQ_END)
 #define REQ_HASHED		(1ULL << __REQ_HASHED)
+#define REQ_MQ_INFLIGHT		(1ULL << __REQ_MQ_INFLIGHT)
 
 #endif /* __LINUX_BLK_TYPES_H */

include/linux/blkdev.h

@@ -481,6 +481,9 @@ struct request_queue {
 	wait_queue_head_t	mq_freeze_wq;
 	struct percpu_counter	mq_usage_counter;
 	struct list_head	all_q_node;
+
+	struct blk_mq_tag_set	*tag_set;
+	struct list_head	tag_set_list;
 };
 
 #define QUEUE_FLAG_QUEUED	1	/* uses generic tag queueing */