[PATCH -next RFC v3 4/8] blk-mq: don't preempt tag under heavy load

[Yu Kuai]

Tag preemption is the default behaviour, specifically blk_mq_get_tag()
will try to get tag unconditionally, which means a new io can preempt
tag even if there are lots of ios that are waiting for tags.

Such behaviour doesn't make sense when the disk is under heavy load,
because it will intensify competition without improving performance,
especially for huge io as split ios are unlikely to be issued
continuously.

The ideal way to disable tag preemption is to track how many tags are
available, and wait directly in blk_mq_get_tag() if free tags are
very little. However, this is out of reality because fast path is
affected.

As 'ws_active' is only updated in slow path, this patch disable tag
preemption if 'ws_active' is greater than 8, which means there are many
threads waiting for tags already.

Once tag preemption is disabled, there is a situation that can cause
performance degradation(or io hung in extreme scenarios): the waitqueue
doesn't have 'wake_batch' threads, thus wake up on this waitqueue might
cause the concurrency of ios to be decreased. The next patch will fix this
problem.

This patch also add a detection in blk_mq_timeout_work(), just in case
io hung is triggered due to waiters can't awakened in some corner cases.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 block/blk-mq-tag.c | 36 +++++++++++++++++++++++++-----------
 block/blk-mq.c     | 29 +++++++++++++++++++++++++++++
 block/blk-mq.h     |  2 ++
 3 files changed, 56 insertions(+), 11 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 5ad85063e91e..a6c5ec846a5e 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -127,6 +127,13 @@ unsigned long blk_mq_get_tags(struct blk_mq_alloc_data *data, int nr_tags,
 	return ret;
 }
 
+static inline bool preempt_tag(struct blk_mq_alloc_data *data,
+			       struct sbitmap_queue *bt)
+{
+	return data->preempt ||
+	       atomic_read(&bt->ws_active) <= SBQ_WAIT_QUEUES;
+}
+
 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 {
 	struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
@@ -148,12 +155,14 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		tag_offset = tags->nr_reserved_tags;
 	}
 
-	tag = __blk_mq_get_tag(data, bt);
-	if (tag != BLK_MQ_NO_TAG)
-		goto found_tag;
+	if (data->flags & BLK_MQ_REQ_NOWAIT || preempt_tag(data, bt)) {
+		tag = __blk_mq_get_tag(data, bt);
+		if (tag != BLK_MQ_NO_TAG)
+			goto found_tag;
 
-	if (data->flags & BLK_MQ_REQ_NOWAIT)
-		return BLK_MQ_NO_TAG;
+		if (data->flags & BLK_MQ_REQ_NOWAIT)
+			return BLK_MQ_NO_TAG;
+	}
 
 	do {
 		struct sbitmap_queue *bt_prev;
@@ -169,21 +178,26 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		 * Retry tag allocation after running the hardware queue,
 		 * as running the queue may also have found completions.
 		 */
-		tag = __blk_mq_get_tag(data, bt);
-		if (tag != BLK_MQ_NO_TAG)
-			break;
+		if (preempt_tag(data, bt)) {
+			tag = __blk_mq_get_tag(data, bt);
+			if (tag != BLK_MQ_NO_TAG)
+				break;
+		}
 
 		if (!ws)
 			ws = bt_wait_ptr(bt, data->hctx);
 		sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
 
-		tag = __blk_mq_get_tag(data, bt);
-		if (tag != BLK_MQ_NO_TAG)
-			break;
+		if (preempt_tag(data, bt)) {
+			tag = __blk_mq_get_tag(data, bt);
+			if (tag != BLK_MQ_NO_TAG)
+				break;
+		}
 
 		bt_prev = bt;
 		io_schedule();
 
+		data->preempt = true;
 		sbitmap_finish_wait(bt, ws, &wait);
 
 		data->ctx = blk_mq_get_ctx(data->q);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ed3ed86f7dd2..32beacbad5e2 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1446,6 +1446,34 @@ static bool blk_mq_check_expired(struct request *rq, void *priv, bool reserved)
 	return true;
 }
 
+static void blk_mq_check_tag_waiters(struct blk_mq_hw_ctx *hctx)
+{
+	bool warn = false;
+	struct blk_mq_tags *tags = hctx->tags;
+
+again:
+	if (atomic_read(&tags->bitmap_tags.ws_active)) {
+		warn = true;
+		sbitmap_queue_wake_all(&tags->bitmap_tags);
+	}
+
+	if (atomic_read(&tags->breserved_tags.ws_active)) {
+		warn = true;
+		sbitmap_queue_wake_all(&tags->breserved_tags);
+	}
+
+	if (hctx->sched_tags && tags != hctx->sched_tags) {
+		tags = hctx->sched_tags;
+		goto again;
+	}
+
+	/*
+	 * This is problematic because someone is still waiting for tag while
+	 * no tag is used.
+	 */
+	WARN_ON_ONCE(warn);
+}
+
 static void blk_mq_timeout_work(struct work_struct *work)
 {
 	struct request_queue *q =
@@ -1482,6 +1510,7 @@ static void blk_mq_timeout_work(struct work_struct *work)
 		 * each hctx as idle.
 		 */
 		queue_for_each_hw_ctx(q, hctx, i) {
+			blk_mq_check_tag_waiters(hctx);
 			/* the hctx may be unmapped, so check it here */
 			if (blk_mq_hw_queue_mapped(hctx))
 				blk_mq_tag_idle(hctx);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 2615bd58bad3..1a85bd1045d8 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -156,6 +156,8 @@ struct blk_mq_alloc_data {
 
 	/* allocate multiple requests/tags in one go */
 	unsigned int nr_tags;
+	/* true if blk_mq_get_tag() will try to preempt tag */
+	bool preempt;
 	struct request **cached_rq;
 
 	/* input & output parameter */
-- 
2.31.1