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Date:   Fri, 7 Dec 2018 11:41:56 +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 11:34 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.

Thanks
Jianchao

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