linux-kernel - [PATCH v2] block: plug attempts to batch allocate tags multiple times

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Message-Id: <20250918075511.8197-1-xue01.he@samsung.com>
Date: Thu, 18 Sep 2025 07:55:11 +0000
From: Xue He <xue01.he@...sung.com>
To: axboe@...nel.dk, akpm@...ux-foundation.org
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, hexue
	<xue01.he@...sung.com>
Subject: [PATCH v2] block: plug attempts to batch allocate tags multiple
 times

In the existing plug mechanism, tags are allocated in batches based on
the number of requests. However, testing has shown that the plug only
attempts batch allocation of tags once at the beginning of a batch of
I/O operations. Since the tag_mask does not always have enough available
tags to satisfy the requested number, a full batch allocation is not
guaranteed to succeed each time. The remaining tags are then allocated
individually (occurs frequently), leading to multiple single-tag
allocation overheads.

This patch aims to retry batch allocation of tags when the initial batch
allocation fails to reach the requested number, thereby reducing the
overhead of individual allocation attempts.

--------------------------------------------------------------------
perf:
base code: __blk_mq_alloc_requests() 1.33%
patch:__blk_mq_alloc_requests() 0.72%
-------------------------------------------------------------------

Signed-off-by: hexue <xue01.he@...sung.com>
---
 block/blk-mq.c | 43 +++++++++++++++++++++++--------------------
 lib/sbitmap.c  |  7 ++++---
 2 files changed, 27 insertions(+), 23 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ba3a4b77f578..3ed8da176831 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -456,28 +456,31 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data)
 	struct blk_mq_tags *tags;
 	struct request *rq;
 	unsigned long tag_mask;
-	int i, nr = 0;
 
-	tag_mask = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
-	if (unlikely(!tag_mask))
-		return NULL;
+	do {
+		int i, nr = 0;
 
-	tags = blk_mq_tags_from_data(data);
-	for (i = 0; tag_mask; i++) {
-		if (!(tag_mask & (1UL << i)))
-			continue;
-		tag = tag_offset + i;
-		prefetch(tags->static_rqs[tag]);
-		tag_mask &= ~(1UL << i);
-		rq = blk_mq_rq_ctx_init(data, tags, tag);
-		rq_list_add_head(data->cached_rqs, rq);
-		nr++;
-	}
-	if (!(data->rq_flags & RQF_SCHED_TAGS))
-		blk_mq_add_active_requests(data->hctx, nr);
-	/* caller already holds a reference, add for remainder */
-	percpu_ref_get_many(&data->q->q_usage_counter, nr - 1);
-	data->nr_tags -= nr;
+		tag_mask = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
+		if (unlikely(!tag_mask))
+			return NULL;
+
+		tags = blk_mq_tags_from_data(data);
+		for (i = 0; tag_mask; i++) {
+			if (!(tag_mask & (1UL << i)))
+				continue;
+			tag = tag_offset + i;
+			prefetch(tags->static_rqs[tag]);
+			tag_mask &= ~(1UL << i);
+			rq = blk_mq_rq_ctx_init(data, tags, tag);
+			rq_list_add_head(data->cached_rqs, rq);
+			nr++;
+		}
+		if (!(data->rq_flags & RQF_SCHED_TAGS))
+			blk_mq_add_active_requests(data->hctx, nr);
+		/* caller already holds a reference, add for remainder */
+		percpu_ref_get_many(&data->q->q_usage_counter, nr - 1);
+		data->nr_tags -= nr;
+	} while (data->nr_tags);
 
 	return rq_list_pop(data->cached_rqs);
 }
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 4d188d05db15..4ac303842aec 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -534,10 +534,11 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
 		unsigned int map_depth = __map_depth(sb, index);
 		unsigned long val;
 
-		sbitmap_deferred_clear(map, 0, 0, 0);
 		val = READ_ONCE(map->word);
-		if (val == (1UL << (map_depth - 1)) - 1)
-			goto next;
+		if (val == (1UL << (map_depth - 1)) - 1) {
+			if (!sbitmap_deferred_clear(map, map_depth, 0, 0))
+				goto next;
+		}
 
 		nr = find_first_zero_bit(&val, map_depth);
 		if (nr + nr_tags <= map_depth) {
-- 
2.34.1