Re: [PATCH v5 RESEND] block: plug attempts to batch allocate tags multiple times

[Ming Lei]

On Thu, Nov 13, 2025 at 08:02:02AM +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.
> 
> ------------------------------------------------------------
> Perf:
> base code: __blk_mq_alloc_requests() 1.31%
> patch: __blk_mq_alloc_requests() 0.7%
> ------------------------------------------------------------

Can you include the workload with perf together?

> 
> ---
> 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
> 
> Signed-off-by: hexue <xue01.he@xxxxxxxxxxx>
> ---
>  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 09f579414161..64cd0a3c7cbf 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++;
> +		}
> +		if (!(data->rq_flags & RQF_SCHED_TAGS))
> +			blk_mq_add_active_requests(data->hctx, nr);

Here not only less-efficient, but also a over-counting bug, please
move the above two lines after `percpu_ref_get_many`.


Thanks, 
Ming