block: plug attempts to batch allocate tags multiple times

This patch aims to enable batch allocation of sufficient tags after
batch IO submission with plug mechanism, thereby avoiding the need for
frequent individual requests when the initial allocation is
insufficient.
-----------------------------------------------------------
HW:
16 CPUs/16 poll queues
Disk: Samsung PM9A3 Gen4 3.84T

CMD:
[global]
ioengine=io_uring
group_reporting=1
time_based=1
runtime=1m
refill_buffers=1
norandommap=1
randrepeat=0
fixedbufs=1
registerfiles=1
rw=randread
iodepth=128
iodepth_batch_submit=32
iodepth_batch_complete_min=32
iodepth_batch_complete_max=128
iodepth_low=32
bs=4k
numjobs=1
direct=1
hipri=1

[job1]
filename=/dev/nvme0n1
name=batch_test
------------------------------------------------------------
Perf:
base code: __blk_mq_alloc_requests() 1.47%
patch: __blk_mq_alloc_requests() 0.75%
------------------------------------------------------------

Signed-off-by: hexue <[e-mail]>
Reviewed-by: Ming Lei <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-mq.c

···
 	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 {
+		tag_mask = blk_mq_get_tags(data, data->nr_tags - nr, &tag_offset);
+		if (unlikely(!tag_mask)) {
+			if (nr == 0)
+				return NULL;
+			break;
+		}
+		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++;
+		}
+	} while (data->nr_tags > nr);
 
-	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 */