linux-kernel - Re: [PATCH V11 0/4] blk-mq: refactor code of issue directly

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Date:   Mon, 10 Dec 2018 09:18:54 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     ming.lei@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V11 0/4] blk-mq: refactor code of issue directly



On 12/7/18 11:47 AM, Jens Axboe wrote:
> On 12/6/18 8:46 PM, jianchao.wang wrote:
>>
>>
>> On 12/7/18 11:42 AM, Jens Axboe wrote:
>>> On 12/6/18 8:41 PM, jianchao.wang wrote:
>>>>
>>>>
>>>> On 12/7/18 11:34 AM, Jens Axboe wrote:
>>>>> On 12/6/18 8:32 PM, Jens Axboe wrote:
>>>>>> On 12/6/18 8:26 PM, jianchao.wang wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 12/7/18 11:16 AM, Jens Axboe wrote:
>>>>>>>> On 12/6/18 8:09 PM, Jianchao Wang wrote:
>>>>>>>>> Hi Jens
>>>>>>>>>
>>>>>>>>> Please consider this patchset for 4.21.
>>>>>>>>>
>>>>>>>>> It refactors the code of issue request directly to unify the interface
>>>>>>>>> and make the code clearer and more readable.
>>>>>>>>>
>>>>>>>>> This patch set is rebased on the recent for-4.21/block and add the 1st
>>>>>>>>> patch which inserts the non-read-write request to hctx dispatch
>>>>>>>>> list to avoid to involve merge and io scheduler when bypass_insert
>>>>>>>>> is true, otherwise, inserting is ignored, BLK_STS_RESOURCE is returned
>>>>>>>>> and the caller will fail forever.
>>>>>>>>>
>>>>>>>>> The 2nd patch refactors the code of issue request directly to unify the
>>>>>>>>> helper interface which could handle all the cases.
>>>>>>>>>
>>>>>>>>> The 3rd patch make blk_mq_sched_insert_requests issue requests directly
>>>>>>>>> with 'bypass' false, then it needn't to handle the non-issued requests
>>>>>>>>> any more.
>>>>>>>>>
>>>>>>>>> The 4th patch replace and kill the blk_mq_request_issue_directly.
>>>>>>>>
>>>>>>>> Sorry to keep iterating on this, but let's default to inserting to
>>>>>>>> the dispatch list if we ever see busy from a direct dispatch. I'm fine
>>>>>>>> with doing that for 4.21, as suggested by Ming, I just didn't want to
>>>>>>>> fiddle with it for 4.20. This will prevent any merging on the request
>>>>>>>> going forward, which I think is a much safer default.
>>>>>>>>
>>>>>>>> You do this already for some cases. Let's do it unconditionally for
>>>>>>>> a request that was ever subjected to ->queue_rq() and we didn't either
>>>>>>>> error or finish after the fact.
>>>>>>>>
>>>>>>> I have done it in this version if I get your point correctly.
>>>>>>> Please refer to the following fragment in the 2nd patch.
>>>>>>>
>>>>>>> +	/*
>>>>>>> +	 * If the request is issued unsuccessfully with
>>>>>>> +	 * BLK_STS_DEV_RESOURCE or BLK_STS_RESOURCE, insert
>>>>>>> +	 * the request to hctx dispatch list due to attached
>>>>>>> +	 * lldd resource.
>>>>>>> +	 */
>>>>>>> +	force = true;
>>>>>>> +	ret = __blk_mq_issue_directly(hctx, rq, cookie, last);
>>>>>>> +out_unlock:
>>>>>>> +	hctx_unlock(hctx, srcu_idx);
>>>>>>> +out:
>>>>>>> +	switch (ret) {
>>>>>>> +	case BLK_STS_OK:
>>>>>>> +		break;
>>>>>>> +	case BLK_STS_DEV_RESOURCE:
>>>>>>> +	case BLK_STS_RESOURCE:
>>>>>>> +		if (force) {
>>>>>>> +			blk_mq_request_bypass_insert(rq, run_queue);
>>>>>>> +			ret = bypass ? BLK_STS_OK : ret;
>>>>>>> +		} else if (!bypass) {
>>>>>>> +			blk_mq_sched_insert_request(rq, false,
>>>>>>> +						    run_queue, false);
>>>>>>> +		}
>>>>>>> +		break;
>>>>>>> +	default:
>>>>>>
>>>>>> You are right, I missed that you set force = true before doing the
>>>>>> issue. So this looks good to me!
>>>>>
>>>>> I applied your series. With this, we should be good to remove the
>>>>> REQ_NOMERGE logic that was added for the corruption case, and the
>>>>> blk_rq_can_direct_dispatch() as well?
>>>>>
>>>>
>>>> Yes, it should be that.
>>>> Every thing rejected by .queue_rq is ended or inserted into hctx dispatch
>>>> list. And also direct-issue path is unified with normal path.
>>>
>>> Why are we doing that return value dance, depending on whether this
>>> is a bypass insert or not? That seems confusing.
>>>
>>
>> For the 'bypass == false' case, it need to know whether the request is issued
>> successfully. This is for the 3rd patch.
>> I used to use the returned cookie to identify the result, but you don't like it.
>> So I have to use this return value.
> 
> Makes sense, but could probably do with a comment. I'm going to let the
> series float for a day or two to ensure others get a chance to review it,
> then we can move forward.
> 

Do I need a respin about the comment ?

Thanks
Jianchao