[PATCH v6] block: plug attempts to batch allocate tags multiple times

[PATCH v6] block: plug attempts to batch allocate tags multiple times

[Xue He]

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() 0.78%
------------------------------------------------------------

---
changes since v1:
- Modify multiple batch registrations into a single loop to achieve
  the batch quantity

changes since v2:
- Modify the call location of remainder handling
- Refactoring sbitmap cleanup time

changes since v3:
- Add handle operation in loop
- Add helper sbitmap_find_bits_in_word

changes since v4:
- Split blk-mq.c changes from sbitmap

changes since v5:
- Add workload with perf
- Modify over-counting bug

Signed-off-by: hexue <xue01.he@samsung.com>
---
 block/blk-mq.c | 39 ++++++++++++++++++++++-----------------
 1 file changed, 22 insertions(+), 17 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d626d32f6e57..9e6fca1b5fb7 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -467,26 +467,31 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data)
 	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, &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);
+			data->nr_tags--;
+			nr++;
+		}
+	} while (data->nr_tags);
 
-	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;
+	if (!(data->rq_flags & RQF_SCHED_TAGS))
+		blk_mq_add_active_requests(data->hctx, nr);
 
 	return rq_list_pop(data->cached_rqs);
 }
-- 
2.34.1

Re: [PATCH v6] block: plug attempts to batch allocate tags multiple times

[Ming Lei]

On Mon, Nov 17, 2025 at 08:48:45AM +0000, Xue He wrote:
> 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() 0.78%
> ------------------------------------------------------------
> 
> ---
> changes since v1:
> - Modify multiple batch registrations into a single loop to achieve
>   the batch quantity
> 
> changes since v2:
> - Modify the call location of remainder handling
> - Refactoring sbitmap cleanup time
> 
> changes since v3:
> - Add handle operation in loop
> - Add helper sbitmap_find_bits_in_word
> 
> changes since v4:
> - Split blk-mq.c changes from sbitmap
> 
> changes since v5:
> - Add workload with perf
> - Modify over-counting bug
> 
> Signed-off-by: hexue <xue01.he@samsung.com>
> ---
>  block/blk-mq.c | 39 ++++++++++++++++++++++-----------------
>  1 file changed, 22 insertions(+), 17 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index d626d32f6e57..9e6fca1b5fb7 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -467,26 +467,31 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data)
>  	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, &tag_offset);

You may keep the original batch update on `data->nr_tags` by passing
`data->nr_tags - nr`.

> +		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);
> +			data->nr_tags--;

data->nr_tags-- can be killed.

> +			nr++;
> +		}
> +	} while (data->nr_tags);

Replace the above check with `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 */
>  	percpu_ref_get_many(&data->q->q_usage_counter, nr - 1);
> -	data->nr_tags -= nr;

The above line needs to keep.

> +	if (!(data->rq_flags & RQF_SCHED_TAGS))
> +		blk_mq_add_active_requests(data->hctx, nr);

The above two lines can be kept in original position, then the change
outside loop can be avoided.

With above update, feel free to add:

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming

Re: [PATCH v6] block: plug attempts to batch allocate tags multiple times

[Xue He]

>The above two lines can be kept in original position, then the change
>outside loop can be avoided.
>
>With above update, feel free to add:
>
>Reviewed-by: Ming Lei <ming.lei@redhat.com>
>
>Thanks,
>Ming

Thanks for review, I'll update and add this, then resubmit v7.

Thanks,
Xue He