blk-mq: Use request queue-wide tags for tagset-wide sbitmap

The tags used for an IO scheduler are currently per hctx.

As such, when q->nr_hw_queues grows, so does the request queue total IO
scheduler tag depth.

This may cause problems for SCSI MQ HBAs whose total driver depth is
fixed.

Ming and Yanhui report higher CPU usage and lower throughput in scenarios
where the fixed total driver tag depth is appreciably lower than the total
scheduler tag depth:
https://lore.kernel.org/linux-block/440dfcfc-1a2c-bd98-1161-cec4d78c6dfc@huawei.com/T/#mc0d6d4f95275a2743d1c8c3e4dc9ff6c9aa3a76b

In that scenario, since the scheduler tag is got first, much contention
is introduced since a driver tag may not be available after we have got
the sched tag.

Improve this scenario by introducing request queue-wide tags for when
a tagset-wide sbitmap is used. The static sched requests are still
allocated per hctx, as requests are initialised per hctx, as in
blk_mq_init_request(..., hctx_idx, ...) ->
set->ops->init_request(.., hctx_idx, ...).

For simplicity of resizing the request queue sbitmap when updating the
request queue depth, just init at the max possible size, so we don't need
to deal with the possibly with swapping out a new sbitmap for old if
we need to grow.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/1620907258-30910-3-git-send-email-john.garry@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 5 insertions, 6 deletions

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index f597d40de10b..86f87346232a 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -13,6 +13,7 @@
 #include <linux/delay.h>
 #include "blk.h"
 #include "blk-mq.h"
+#include "blk-mq-sched.h"
 #include "blk-mq-tag.h"
 
 /*
@@ -590,8 +591,6 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
 	 */
 	if (tdepth > tags->nr_tags) {
 		struct blk_mq_tag_set *set = hctx->queue->tag_set;
-		/* Only sched tags can grow, so clear HCTX_SHARED flag  */
-		unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
 		struct blk_mq_tags *new;
 		bool ret;
 
@@ -602,21 +601,21 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
 		 * We need some sort of upper limit, set it high enough that
 		 * no valid use cases should require more.
 		 */
-		if (tdepth > 16 * BLKDEV_MAX_RQ)
+		if (tdepth > MAX_SCHED_RQ)
 			return -EINVAL;
 
 		new = blk_mq_alloc_rq_map(set, hctx->queue_num, tdepth,
-				tags->nr_reserved_tags, flags);
+				tags->nr_reserved_tags, set->flags);
 		if (!new)
 			return -ENOMEM;
 		ret = blk_mq_alloc_rqs(set, new, hctx->queue_num, tdepth);
 		if (ret) {
-			blk_mq_free_rq_map(new, flags);
+			blk_mq_free_rq_map(new, set->flags);
 			return -ENOMEM;
 		}
 
 		blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
-		blk_mq_free_rq_map(*tagsptr, flags);
+		blk_mq_free_rq_map(*tagsptr, set->flags);
 		*tagsptr = new;
 	} else {
 		/*