[PATCH 1/4] block: add mq_ops->queue_rqs hook

[Hannes Reinecke]

On 12/3/21 10:45 PM, Jens Axboe wrote:
> If we have a list of requests in our plug list, send it to the driver in
> one go, if possible. The driver must set mq_ops->queue_rqs() to support
> this, if not the usual one-by-one path is used.
> 
> Signed-off-by: Jens Axboe <axboe at kernel.dk>
> ---
>   block/blk-mq.c         | 24 +++++++++++++++++++++---
>   include/linux/blk-mq.h |  8 ++++++++
>   2 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 22ec21aa0c22..9ac9174a2ba4 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2513,6 +2513,7 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>   {
>   	struct blk_mq_hw_ctx *this_hctx;
>   	struct blk_mq_ctx *this_ctx;
> +	struct request *rq;
>   	unsigned int depth;
>   	LIST_HEAD(list);
>   
> @@ -2521,7 +2522,26 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>   	plug->rq_count = 0;
>   
>   	if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
> -		blk_mq_run_dispatch_ops(plug->mq_list->q,
> +		struct request_queue *q;
> +
> +		rq = plug->mq_list;
> +		q = rq->q;
> +
> +		/*
> +		 * Peek first request and see if we have a ->queue_rqs() hook.
> +		 * If we do, we can dispatch the whole plug list in one go. We
> +		 * already know at this point that all requests belong to the
> +		 * same queue, caller must ensure that's the case.
> +		 */
> +		if (q->mq_ops->queue_rqs &&
> +		    !(rq->mq_hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {

What is the dependency on shared tags here?
 From what I've seen it's just about submitting requests; the only 
difference to shared tags is the way the tags are allocated.
Care to explain?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
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