[PATCH RFC 1/3] block/mq-deadline: Revert "block/mq-deadline: Fix the tag reservation code"

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

This reverts commit 39823b47bbd40502632ffba90ebb34fff7c8b5e8.

Because tag reservation is not fixed and will introduce performance
problem.

1) Set min_shallow_depth to 1 will end up setting wake_batch to 1,
   deadline has no reason to do this. And this will cause performance
   degradation in some high concurrency test, for both IO bandwidth
   and cpu usage.
2) async_depth is nr_requests, hence shallow_depth will always set to
   1 << bt->sb.shift. For consequence, no tag can be reserved.

The next patch will fix tag reservation again.

Fixes: 39823b47bbd4 ("block/mq-deadline: Fix the tag reservation code")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 block/mq-deadline.c | 20 +++-----------------
 1 file changed, 3 insertions(+), 17 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 91b3789f710e..1f0d175a941e 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -487,20 +487,6 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
 	return rq;
 }
 
-/*
- * 'depth' is a number in the range 1..INT_MAX representing a number of
- * requests. Scale it with a factor (1 << bt->sb.shift) / q->nr_requests since
- * 1..(1 << bt->sb.shift) is the range expected by sbitmap_get_shallow().
- * Values larger than q->nr_requests have the same effect as q->nr_requests.
- */
-static int dd_to_word_depth(struct blk_mq_hw_ctx *hctx, unsigned int qdepth)
-{
-	struct sbitmap_queue *bt = &hctx->sched_tags->bitmap_tags;
-	const unsigned int nrr = hctx->queue->nr_requests;
-
-	return ((qdepth << bt->sb.shift) + nrr - 1) / nrr;
-}
-
 /*
  * Called by __blk_mq_alloc_request(). The shallow_depth value set by this
  * function is used by __blk_mq_get_tag().
@@ -517,7 +503,7 @@ static void dd_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
 	 * Throttle asynchronous requests and writes such that these requests
 	 * do not block the allocation of synchronous requests.
 	 */
-	data->shallow_depth = dd_to_word_depth(data->hctx, dd->async_depth);
+	data->shallow_depth = dd->async_depth;
 }
 
 /* Called by blk_mq_update_nr_requests(). */
@@ -527,9 +513,9 @@ static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
 	struct deadline_data *dd = q->elevator->elevator_data;
 	struct blk_mq_tags *tags = hctx->sched_tags;
 
-	dd->async_depth = q->nr_requests;
+	dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
 
-	sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, 1);
+	sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, dd->async_depth);
 }
 
 /* Called by blk_mq_init_hctx() and blk_mq_init_sched(). */
-- 
2.39.2