[PATCH 1/2] blk-mq-sched: Allocate sched reserved tags as specified in the original queue tagset

[Sagi Grimberg]

>> Can't we just not go through the scheduler for reserved tags? Obviously
>> there is no point in scheduling them...
>
> Right, that would be possible. But I'd rather not treat any requests
> differently, it's a huge pain in the ass that flush request currently
> insert with a driver tag already allocated. So it's not because
> scheduling will add anything at all, it's more that I'd like to move
> flush requests to use regular inserts as well and not deal with some
> request being "special" in any way.
>
> The below should hopefully work. Totally untested...
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 54c84363c1b2..e48bc2c72615 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -181,7 +181,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
>  void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, struct blk_mq_tags *tags,
>  		    struct blk_mq_ctx *ctx, unsigned int tag)
>  {
> -	if (tag >= tags->nr_reserved_tags) {
> +	if (!blk_mq_tag_is_reserved(tags, tag)) {
>  		const int real_tag = tag - tags->nr_reserved_tags;
>
>  		BUG_ON(real_tag >= tags->nr_tags);
> diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
> index 63497423c5cd..5cb51e53cc03 100644
> --- a/block/blk-mq-tag.h
> +++ b/block/blk-mq-tag.h
> @@ -85,4 +85,10 @@ static inline void blk_mq_tag_set_rq(struct blk_mq_hw_ctx *hctx,
>  	hctx->tags->rqs[tag] = rq;
>  }
>
> +static inline bool blk_mq_tag_is_reserved(struct blk_mq_tags *tags,
> +					  unsigned int tag)
> +{
> +	return tag < tags->nr_reserved_tags;
> +}
> +
>  #endif
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 9611cd9920e9..293e79c1ee95 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -853,6 +853,9 @@ bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
>  		return true;
>  	}
>
> +	if (blk_mq_tag_is_reserved(data.hctx->sched_tags, rq->internal_tag))
> +		data.flags |= BLK_MQ_REQ_RESERVED;
> +
>  	rq->tag = blk_mq_get_tag(&data);
>  	if (rq->tag >= 0) {
>  		if (blk_mq_tag_busy(data.hctx)) {
>
>

Both patches look they'd work, I'll test. Thanks.