Re: blk-mq: takes hours for scsi scanning finish when thousands of LUNs

[Jeff Moyer]

Jens Axboe <axboe@xxxxxxxxx> writes:

> On 10/22/2015 09:53 AM, Jeff Moyer wrote:
>> I think that percolating BLK_MQ_F_TAG_SHARED up to the tag set would
>> allow newly created hctxs to simply inherit the shared state (in
>> blk_mq_init_hctx), and you won't need to freeze every queue in order to
>> guarantee that.
>>
>> I was writing a patch to that effect.  I've now stopped as I want to
>> make sure I'm not off in the weeds.  :)
>
> If that is where the delay is done, then yes, that should fix it and
> be a trivial patch.

It's not quite as trivial as I had hoped.  Jason, can you give the
attached patch a try?  All I've done is boot tested it so far.

Thanks!
Jeff

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7785ae9..8b4c484 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1860,27 +1860,26 @@ static void blk_mq_map_swqueue(struct request_queue *q,
 	}
 }
 
-static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set)
+static void queue_set_hctx_shared(struct request_queue *q, bool shared)
 {
 	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;
+	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;
+	}
+}
+
+static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set, bool shared)
+{
+	struct request_queue *q;
 
 	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;
-		}
+		queue_set_hctx_shared(q, shared);
 		blk_mq_unfreeze_queue(q);
 	}
 }
@@ -1891,7 +1890,13 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
 
 	mutex_lock(&set->tag_list_lock);
 	list_del_init(&q->tag_set_list);
-	blk_mq_update_tag_set_depth(set);
+
+	if (set->tag_list.next == set->tag_list.prev) {
+		/* just transitioned to unshared */
+		set->flags &= ~BLK_MQ_F_TAG_SHARED;
+		/* update existing queue */
+		blk_mq_update_tag_set_depth(set, false);
+	}
 	mutex_unlock(&set->tag_list_lock);
 }
 
@@ -1902,7 +1907,24 @@ static void blk_mq_add_queue_tag_set(struct blk_mq_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);
+
+	if (set->tag_list.next != set->tag_list.prev) {
+		/*
+		 * Only update the tag set state if the state has
+		 * actually changed.
+		 */
+		if (!(set->flags & BLK_MQ_F_TAG_SHARED)) {
+			/* just transitioned to shared tags */
+			set->flags |= BLK_MQ_F_TAG_SHARED;
+			blk_mq_update_tag_set_depth(set, true);
+		} else {
+			/* ensure we didn't race with another addition */
+			struct blk_mq_hw_ctx *hctx = queue_first_hw_ctx(q);
+			if ((hctx->flags & BLK_MQ_F_TAG_SHARED) !=
+							BLK_MQ_F_TAG_SHARED)
+				queue_set_hctx_shared(q, true);
+		}
+	}
 	mutex_unlock(&set->tag_list_lock);
 }
 
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 5e7d43a..12ffc40 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -254,6 +254,9 @@ static inline void *blk_mq_rq_to_pdu(struct request *rq)
 	for ((i) = 0; (i) < (hctx)->nr_ctx &&				\
 	     ({ ctx = (hctx)->ctxs[(i)]; 1; }); (i)++)
 
+#define queue_first_hw_ctx(q)						\
+	(q)->queue_hw_ctx[0]
+
 #define blk_ctx_sum(q, sum)						\
 ({									\
 	struct blk_mq_ctx *__x;						\
--
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