linux-kernel - Re: [PATCH V5 2/6] blk-mq: refactor the code of issue request directly

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Date:   Tue, 13 Nov 2018 12:12:42 +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 V5 2/6] blk-mq: refactor the code of issue request
 directly

Hi Jens

On 11/13/18 11:22 AM, Jens Axboe wrote:
> On 11/12/18 2:23 AM, Jianchao Wang wrote:
>> Merge blk_mq_try_issue_directly and __blk_mq_try_issue_directly
>> into one interface to unify the interfaces to issue requests
>> directly. The merged interface takes over the requests totally,
>> it could insert, end or do nothing based on the return value of
>> .queue_rq and 'bypass' parameter. Then caller needn't any other
>> handling any more.
>>
>> To make code clearer, introduce new helpers enum mq_issue_decision
>> and blk_mq_make_decision to decide how to handle the non-issued
>> requests.
>>
>> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
>> ---
>>  block/blk-mq.c | 108 +++++++++++++++++++++++++++++++++------------------------
>>  1 file changed, 63 insertions(+), 45 deletions(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 364a53f..48b7a7c 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -1766,77 +1766,95 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
>>  	return ret;
>>  }
>>  
>> -static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>> +enum mq_issue_decision {
>> +	MQ_ISSUE_INSERT_QUEUE,
>> +	MQ_ISSUE_END_REQUEST,
>> +	MQ_ISSUE_DO_NOTHING,
>> +};
>> +
>> +static inline enum mq_issue_decision
>> +	blk_mq_make_dicision(blk_status_t ret, bool bypass)
>> +{
>> +	enum mq_issue_decision dec;
>> +
>> +	switch(ret) {
>> +	case BLK_STS_OK:
>> +		dec = MQ_ISSUE_DO_NOTHING;
>> +		break;
>> +	case BLK_STS_DEV_RESOURCE:
>> +	case BLK_STS_RESOURCE:
>> +		dec = bypass ? MQ_ISSUE_DO_NOTHING : MQ_ISSUE_INSERT_QUEUE;
>> +		break;
>> +	default:
>> +		dec = bypass ? MQ_ISSUE_DO_NOTHING : MQ_ISSUE_END_REQUEST;
>> +		break;
>> +	}
>> +
>> +	return dec;
>> +}
> 
> You seem to mix and match decision and dicision, the former is the
> right spelling.
> 

Oops, it's my bad that not find this even in V5 respin.

> But more importantly, not sure I like where this is going, wrapping
> the return value in some other status code. That also makes it a bit
> fragile in terms of adding other status codes, another spot to update.
> Like the decent distinction between RESOURCE and DEV_RESOURCE.
> 
> Maybe it is cleaner to just handle this in the caller still?
> 
Actually enum mq_issue_decision is introduced to reduce the depth of
'if' branch. And it is indeed not good to introduce another status value.

I will discard enum mq_issue_decision and blk_mq_make_decision in next version.

Thanks
Jianchao