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Date:   Wed, 17 Jan 2018 23:36:09 -0500
From:   Mike Snitzer <snitzer@redhat.com>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        dm-devel@redhat.com, Christoph Hellwig <hch@infradead.org>,
        Bart Van Assche <bart.vanassche@sandisk.com>,
        linux-kernel@vger.kernel.org
Subject: Re: blk-mq: don't dispatch request in blk_mq_request_direct_issue if
 queue is busy
Message-ID: <20180118043608.GA8809@redhat.com>
References: <20180118040659.20202-1-ming.lei@redhat.com>
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On Wed, Jan 17 2018 at 11:06pm -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> If we run into blk_mq_request_direct_issue(), when queue is busy, we
> don't want to dispatch this request into hctx->dispatch_list, and
> what we need to do is to return the queue busy info to caller, so
> that caller can deal with it well.
> 
> Fixes: 396eaf21ee ("blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback")
> Reported-by: Laurence Oberman <loberman@redhat.com>
> Reviewed-by: Mike Snitzer <snitzer@redhat.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4d4af8d712da..1af7fa70993b 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1856,15 +1856,6 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	return ret;
>  }
>  
> -static void __blk_mq_fallback_to_insert(struct request *rq,
> -					bool run_queue, bool bypass_insert)
> -{
> -	if (!bypass_insert)
> -		blk_mq_sched_insert_request(rq, false, run_queue, false);
> -	else
> -		blk_mq_request_bypass_insert(rq, run_queue);
> -}
> -
>  static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  						struct request *rq,
>  						blk_qc_t *cookie,
> @@ -1873,9 +1864,16 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	struct request_queue *q = rq->q;
>  	bool run_queue = true;
>  
> -	/* RCU or SRCU read lock is needed before checking quiesced flag */
> +	/*
> +	 * RCU or SRCU read lock is needed before checking quiesced flag.
> +	 *
> +	 * When queue is stopped or quiesced, ignore 'bypass_insert' from
> +	 * blk_mq_request_direct_issue(), and return BLK_STS_OK to caller,
> +	 * and avoid driver to try to dispatch again.
> +	 */
>  	if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)) {
>  		run_queue = false;
> +		bypass_insert = false;
>  		goto insert;
>  	}
>  
> @@ -1892,10 +1890,10 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  
>  	return __blk_mq_issue_directly(hctx, rq, cookie);
>  insert:
> -	__blk_mq_fallback_to_insert(rq, run_queue, bypass_insert);
>  	if (bypass_insert)
>  		return BLK_STS_RESOURCE;
>  
> +	blk_mq_sched_insert_request(rq, false, run_queue, false);
>  	return BLK_STS_OK;
>  }

OK so you're just leveraging blk_mq_sched_insert_request()'s
ability to resort to__blk_mq_insert_request() if !q->elevator.