[PATCH] blk-mq: determine in advance whether batch alloc can be performed

[PATCH] blk-mq: determine in advance whether batch alloc can be performed

[Liu Song]

From: Liu Song <liusong@linux.alibaba.com>

Some conditions for judging whether batch alloc can be performed are
included in "blk_mq_get_tags", and this function is only used by
"__blk_mq_alloc_requests_batch".

This patch introduced a helper "can_do_batch_alloc" to prepend the
judgment condition and avoid unnecessary function calls.

Signed-off-by: Liu Song <liusong@linux.alibaba.com>
---
 block/blk-mq-tag.c |  3 ---
 block/blk-mq.c     | 11 ++++++++++-
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 8e3b36d..cda3987 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -115,9 +115,6 @@ unsigned long blk_mq_get_tags(struct blk_mq_alloc_data *data, int nr_tags,
 	struct sbitmap_queue *bt = &tags->bitmap_tags;
 	unsigned long ret;
 
-	if (data->shallow_depth ||data->flags & BLK_MQ_REQ_RESERVED ||
-	    data->hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)
-		return 0;
 	ret = __sbitmap_queue_get_batch(bt, nr_tags, offset);
 	*offset += tags->nr_reserved_tags;
 	return ret;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3c1e6b6..330c618 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -437,6 +437,15 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 	return rq_list_pop(data->cached_rq);
 }
 
+static inline bool can_do_batch_alloc(struct blk_mq_alloc_data *data)
+{
+	if (data->nr_tags > 1 && !(data->shallow_depth ||
+		data->flags & BLK_MQ_REQ_RESERVED ||
+		data->hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED))
+		return true;
+	return false;
+}
+
 static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 {
 	struct request_queue *q = data->q;
@@ -480,7 +489,7 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 	/*
 	 * Try batched alloc if we want more than 1 tag.
 	 */
-	if (data->nr_tags > 1) {
+	if (can_do_batch_alloc(data)) {
 		rq = __blk_mq_alloc_requests_batch(data, alloc_time_ns);
 		if (rq)
 			return rq;
-- 
1.8.3.1

Re: [PATCH] blk-mq: determine in advance whether batch alloc can be performed

[Jens Axboe]

On 8/25/22 7:26 PM, Liu Song wrote:
> From: Liu Song <liusong@linux.alibaba.com>
> 
> Some conditions for judging whether batch alloc can be performed are
> included in "blk_mq_get_tags", and this function is only used by
> "__blk_mq_alloc_requests_batch".
> 
> This patch introduced a helper "can_do_batch_alloc" to prepend the
> judgment condition and avoid unnecessary function calls.

Curious if you saw any differences from this? Or do you just consider
it a cleanup?

-- 
Jens Axboe

Re: [PATCH] blk-mq: determine in advance whether batch alloc can be performed

[Liu Song]

On 2022/8/26 21:33, Jens Axboe wrote:
> On 8/25/22 7:26 PM, Liu Song wrote:
>> From: Liu Song <liusong@linux.alibaba.com>
>>
>> Some conditions for judging whether batch alloc can be performed are
>> included in "blk_mq_get_tags", and this function is only used by
>> "__blk_mq_alloc_requests_batch".
>>
>> This patch introduced a helper "can_do_batch_alloc" to prepend the
>> judgment condition and avoid unnecessary function calls.
> Curious if you saw any differences from this? Or do you just consider
> it a cleanup?
>
Hi

This patch aggregates the judgment conditions for whether batch alloc 
can be used,
which can be considered as clean up.

If it can be determined at the first judgment that batch alloc cannot be 
used, the
overhead of calling "blk_mq_get_tags" once will be reduced, are some 
differences.


Thanks