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

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Date:   Thu, 6 Dec 2018 20:42:28 -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 8:41 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.

-- 
Jens Axboe