[PATCH 2/8] block: improve logic around when to sort a plug list

[osandov@osandov.com (Omar Sandoval)]

On Mon, Nov 26, 2018@09:35:50AM -0700, Jens Axboe wrote:
> Do it for the nr_hw_queues == 1 case, but only do it for the multi queue
> case if we have requests for multiple devices in the plug.
> 
> Signed-off-by: Jens Axboe <axboe at kernel.dk>
> ---
>  block/blk-core.c       | 1 +
>  block/blk-mq.c         | 7 +++++--
>  include/linux/blkdev.h | 1 +
>  3 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index be9233400314..c9758d185357 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1780,6 +1780,7 @@ void blk_start_plug(struct blk_plug *plug)
>  	INIT_LIST_HEAD(&plug->mq_list);
>  	INIT_LIST_HEAD(&plug->cb_list);
>  	plug->rq_count = 0;
> +	plug->do_sort = false;
>  
>  	/*
>  	 * Store ordering should not be needed here, since a potential
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 99c66823d52f..6a249bf6ed00 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1678,7 +1678,8 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>  	list_splice_init(&plug->mq_list, &list);
>  	plug->rq_count = 0;
>  
> -	list_sort(NULL, &list, plug_rq_cmp);
> +	if (plug->do_sort)
> +		list_sort(NULL, &list, plug_rq_cmp);
>  
>  	this_q = NULL;
>  	this_hctx = NULL;
> @@ -1935,6 +1936,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>  
>  		list_add_tail(&rq->queuelist, &plug->mq_list);
>  		plug->rq_count++;
> +		plug->do_sort = true;
>  	} else if (plug && !blk_queue_nomerges(q)) {
>  		blk_mq_bio_to_request(rq, bio);
>  
> @@ -1958,7 +1960,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>  			data.hctx = same_queue_rq->mq_hctx;
>  			blk_mq_try_issue_directly(data.hctx, same_queue_rq,
>  					&cookie);
> -		}
> +		} else if (plug->rq_count > 1)
> +			plug->do_sort = true;

If plug->rq_count == 2, there's no benefit to sorting, either. The
nr_hw_queues == 1 case could also avoid sorting in that case. So maybe
this whole patch could just be replaced with:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7b7dff85cf6c..de93c14e4700 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1675,9 +1675,10 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	unsigned int depth;
 
 	list_splice_init(&plug->mq_list, &list);
-	plug->rq_count = 0;
 
-	list_sort(NULL, &list, plug_rq_cmp);
+	if (plug->rq_count > 2)
+		list_sort(NULL, &list, plug_rq_cmp);
+	plug->rq_count = 0;
 
 	this_q = NULL;
 	this_hctx = NULL;

That will also handle the multi-queue case since we only plug one
request per multi-queue device. Thoughts?