Re: [PATCH 1/2] Revert "blk-mq, elevator: Count requests per hctx to improve performance"

From: Ming Lei <ming.lei@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org
Subject: Re: [PATCH 1/2] Revert "blk-mq, elevator: Count requests per hctx to improve performance"
Date: Tue, 12 Jan 2021 10:14:14 +0800	[thread overview]
Message-ID: <20210112021414.GB60605@T590> (raw)
In-Reply-To: <20210111164717.21937-2-jack@suse.cz>

On Mon, Jan 11, 2021 at 05:47:16PM +0100, Jan Kara wrote:
> This reverts commit b445547ec1bbd3e7bf4b1c142550942f70527d95.
> 
> Since both mq-deadline and BFQ completely ignore hctx they are passed to
> their dispatch function and dispatch whatever request they deem fit
> checking whether any request for a particular hctx is queued is just
> pointless since we'll very likely get a request from a different hctx
> anyway. In the following commit we'll deal with lock contention in these
> IO schedulers in presence of multiple HW queues in a different way.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  block/bfq-iosched.c    | 5 -----
>  block/blk-mq.c         | 1 -
>  block/mq-deadline.c    | 6 ------
>  include/linux/blk-mq.h | 4 ----
>  4 files changed, 16 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 9e81d1052091..a99dfaa75a8c 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -4640,9 +4640,6 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
>  {
>  	struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
>  
> -	if (!atomic_read(&hctx->elevator_queued))
> -		return false;
> -
>  	/*
>  	 * Avoiding lock: a race on bfqd->busy_queues should cause at
>  	 * most a call to dispatch for nothing
> @@ -5557,7 +5554,6 @@ static void bfq_insert_requests(struct blk_mq_hw_ctx *hctx,
>  		rq = list_first_entry(list, struct request, queuelist);
>  		list_del_init(&rq->queuelist);
>  		bfq_insert_request(hctx, rq, at_head);
> -		atomic_inc(&hctx->elevator_queued);
>  	}
>  }
>  
> @@ -5925,7 +5921,6 @@ static void bfq_finish_requeue_request(struct request *rq)
>  
>  		bfq_completed_request(bfqq, bfqd);
>  		bfq_finish_requeue_request_body(bfqq);
> -		atomic_dec(&rq->mq_hctx->elevator_queued);
>  
>  		spin_unlock_irqrestore(&bfqd->lock, flags);
>  	} else {
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index f285a9123a8b..57f3482b2c26 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2653,7 +2653,6 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set,
>  		goto free_hctx;
>  
>  	atomic_set(&hctx->nr_active, 0);
> -	atomic_set(&hctx->elevator_queued, 0);
>  	if (node == NUMA_NO_NODE)
>  		node = set->numa_node;
>  	hctx->numa_node = node;
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 800ac902809b..b57470e154c8 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -386,8 +386,6 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
>  	spin_lock(&dd->lock);
>  	rq = __dd_dispatch_request(dd);
>  	spin_unlock(&dd->lock);
> -	if (rq)
> -		atomic_dec(&rq->mq_hctx->elevator_queued);
>  
>  	return rq;
>  }
> @@ -535,7 +533,6 @@ static void dd_insert_requests(struct blk_mq_hw_ctx *hctx,
>  		rq = list_first_entry(list, struct request, queuelist);
>  		list_del_init(&rq->queuelist);
>  		dd_insert_request(hctx, rq, at_head);
> -		atomic_inc(&hctx->elevator_queued);
>  	}
>  	spin_unlock(&dd->lock);
>  }
> @@ -582,9 +579,6 @@ static bool dd_has_work(struct blk_mq_hw_ctx *hctx)
>  {
>  	struct deadline_data *dd = hctx->queue->elevator->elevator_data;
>  
> -	if (!atomic_read(&hctx->elevator_queued))
> -		return false;
> -
>  	return !list_empty_careful(&dd->dispatch) ||
>  		!list_empty_careful(&dd->fifo_list[0]) ||
>  		!list_empty_careful(&dd->fifo_list[1]);
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index d705b174d346..b4b9604bbfd7 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -140,10 +140,6 @@ struct blk_mq_hw_ctx {
>  	 * shared across request queues.
>  	 */
>  	atomic_t		nr_active;
> -	/**
> -	 * @elevator_queued: Number of queued requests on hctx.
> -	 */
> -	atomic_t                elevator_queued;
>  
>  	/** @cpuhp_online: List to store request if CPU is going to die */
>  	struct hlist_node	cpuhp_online;
> -- 
> 2.26.2
> 

Reviewed-by: Ming Lei <ming.lei@redhat.com>

-- 
Ming