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

[Omar Sandoval <osandov@osandov.com>]

On Tue, Nov 27, 2018 at 04:59:14PM -0700, Jens Axboe wrote:
> On 11/27/18 4:49 PM, Jens Axboe wrote:
> > On 11/27/18 4:31 PM, Omar Sandoval wrote:
> >> On Mon, Nov 26, 2018 at 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@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:
> > 
> > Heh yes, good point, it should be 3 at least. But if you look at the
> > later mq plug patch, we only sort for that one if we have multiple
> > queues. So the logic should be something ala:
> > 
> > if (plug->rq_count > 2 && plug->has_multiple_queues)
> > 
> > since that's the only case we want to sort for.
> 
> How about something like this?
> 
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index be9233400314..d107d016b92b 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->multiple_queues = false;
>  
>  	/*
>  	 * Store ordering should not be needed here, since a potential
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 7b7dff85cf6c..02daa32c5d77 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1677,7 +1677,8 @@ 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->rq_count > 2 && plug->multiple_queues)
> +		list_sort(NULL, &list, plug_rq_cmp);
>  
>  	this_q = NULL;
>  	this_hctx = NULL;
> @@ -1866,6 +1867,20 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
>  	}
>  }
>  
> +static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
> +{
> +	list_add_tail(&rq->queuelist, &plug->mq_list);
> +	plug->rq_count++;
> +	if (!plug->multiple_queues && !list_is_singular(&plug->mq_list)) {
> +		struct request *tmp;
> +
> +		tmp = list_first_entry(&plug->mq_list, struct request,
> +						queuelist);
> +		if (tmp->q != rq->q)
> +			plug->multiple_queues = true;

Actually, I thought we want to sort whenever there are different
software/hardware queues on the plug list, even if they're on the same
queue?

> +	}
> +}
> +
>  static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>  {
>  	const int is_sync = op_is_sync(bio->bi_opf);
> @@ -1932,8 +1947,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>  			trace_block_plug(q);
>  		}
>  
> -		list_add_tail(&rq->queuelist, &plug->mq_list);
> -		plug->rq_count++;
> +		blk_add_rq_to_plug(plug, rq);
>  	} else if (plug && !blk_queue_nomerges(q)) {
>  		blk_mq_bio_to_request(rq, bio);
>  
> @@ -1948,8 +1962,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>  			same_queue_rq = NULL;
>  		if (same_queue_rq)
>  			list_del_init(&same_queue_rq->queuelist);
> -		list_add_tail(&rq->queuelist, &plug->mq_list);
> -		plug->rq_count++;
> +		blk_add_rq_to_plug(plug, rq);
>  
>  		blk_mq_put_ctx(data.ctx);
>  
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 02732cae6080..08d940f85fa0 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1131,6 +1131,7 @@ struct blk_plug {
>  	struct list_head mq_list; /* blk-mq requests */
>  	struct list_head cb_list; /* md requires an unplug callback */
>  	unsigned short rq_count;
> +	bool multiple_queues;
>  };
>  #define BLK_MAX_REQUEST_COUNT 16
>  #define BLK_PLUG_FLUSH_SIZE (128 * 1024)