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

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Date:   Thu, 6 Dec 2018 20:47:12 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
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 12/6/18 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.

-- 
Jens Axboe