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

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Date:   Thu, 6 Dec 2018 20:32:05 -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 8:26 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!

-- 
Jens Axboe