linux-kernel - Re: [PATCH V7 1/4] blk-mq: refactor the code of issue request directly

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Date:   Wed, 14 Nov 2018 17:11:55 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V7 1/4] blk-mq: refactor the code of issue request
 directly

On Wed, Nov 14, 2018 at 04:45:28PM +0800, Jianchao Wang wrote:
> Merge blk_mq_try_issue_directly and __blk_mq_try_issue_directly
> into one interface to unify the interfaces to issue requests
> directly. The merged interface takes over the requests totally,
> it could insert, end or do nothing based on the return value of
> .queue_rq and 'bypass' parameter. Then caller needn't any other
> handling any more.
> 
> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
> ---
>  block/blk-mq.c | 93 ++++++++++++++++++++++++++++------------------------------
>  1 file changed, 45 insertions(+), 48 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 411be60..14b4d06 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1766,78 +1766,75 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	return ret;
>  }
>  
> -static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> +static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  						struct request *rq,
>  						blk_qc_t *cookie,
> -						bool bypass_insert)
> +						bool bypass)
>  {
>  	struct request_queue *q = rq->q;
>  	bool run_queue = true;
> +	blk_status_t ret = BLK_STS_RESOURCE;
> +	int srcu_idx;
>  
> +	hctx_lock(hctx, &srcu_idx);
>  	/*
> -	 * RCU or SRCU read lock is needed before checking quiesced flag.
> +	 * hctx_lock is needed before checking quiesced flag.
>  	 *
> -	 * When queue is stopped or quiesced, ignore 'bypass_insert' from
> -	 * blk_mq_request_issue_directly(), and return BLK_STS_OK to caller,
> -	 * and avoid driver to try to dispatch again.
> +	 * When queue is stopped or quiesced, ignore 'bypass', insert
> +	 * 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)) {
> +	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))) {
>  		run_queue = false;
> -		bypass_insert = false;
> -		goto insert;
> +		bypass = false;
> +		goto out_unlock;
>  	}
>  
> -	if (q->elevator && !bypass_insert)
> -		goto insert;
> +	/*
> +	 * Bypass the potential scheduler on the bottom device.
> +	 */
> +	if (unlikely(q->elevator && !bypass))
> +		goto out_unlock;
>  
> -	if (!blk_mq_get_dispatch_budget(hctx))
> -		goto insert;
> +	if (unlikely(!blk_mq_get_dispatch_budget(hctx)))
> +		goto out_unlock;

The unlikely annotation is a bit misleading, since out-of-budget can
happen frequently in case of low queue depth, and there are lots of
such examples.

>  
> -	if (!blk_mq_get_driver_tag(rq)) {
> +	if (unlikely(!blk_mq_get_driver_tag(rq))) {

Same with above.

Thanks,
Ming