linux-kernel - Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued

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Message-ID: <4b7dcb10-9ec5-9a00-8d16-009e83ef7b5b@oracle.com>
Date:   Wed, 13 Sep 2017 09:39:44 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is
 requeued



On 09/13/2017 09:24 AM, Ming Lei wrote:
> On Wed, Sep 13, 2017 at 09:01:25AM +0800, jianchao.wang wrote:
>> Hi ming
>>
>> On 09/12/2017 06:23 PM, Ming Lei wrote:
>>>> @@ -1029,14 +1029,20 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
>>>>  		if (list_empty(list))
>>>>  			bd.last = true;
>>>>  		else {
>>>> -			struct request *nxt;
>>>> -
>>>>  			nxt = list_first_entry(list, struct request, queuelist);
>>>>  			bd.last = !blk_mq_get_driver_tag(nxt, NULL, false);
>>>>  		}
>>>>  
>>>>  		ret = q->mq_ops->queue_rq(hctx, &bd);
>>>>  		if (ret == BLK_STS_RESOURCE) {
>>>> +			/*
>>>> +			 * If an I/O scheduler has been configured and we got a
>>>> +			 * driver tag for the next request already, free it again.
>>>> +			 */
>>>> +			if (!list_empty(list)) {
>>>> +				nxt = list_first_entry(list, struct request, queuelist);
>>>> +				blk_mq_put_driver_tag(nxt);
>>>> +			}
>>> The following way might be more simple and clean:
>>>
>>> 			if (nxt)
>>> 				blk_mq_put_driver_tag(nxt);
>>>
>>> meantime 'nxt' need to be cleared inside the 'if (list_empty(list))'
>>> before .queue_rq().
>>
>> I had ever thought about that, but to avoid add extra command in the 
>> fast path, I made the patch above.
> 
> Got it, so how about changing to the following way simply:
> 
>  			if (nxt && !list_empty(list))
>  				blk_mq_put_driver_tag(nxt);
> 
It seems that we even could change it as following:
                        if (!list_empty(list))
  				blk_mq_put_driver_tag(nxt);


thanks
jianchao