linux-kernel - Re: [PATCH] blk-mq: remove needless goto from blk_mq_get_driver

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Date:   Fri, 25 Oct 2019 14:15:40 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     André Almeida <andrealmeid@...labora.com>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     kernel@...labora.com, krisman@...labora.com
Subject: Re: [PATCH] blk-mq: remove needless goto from blk_mq_get_driver_tag

On 10/24/19 12:10 PM, André Almeida wrote:
> On 10/24/19 12:32 AM, Jens Axboe wrote:
>> On 10/23/19 7:34 PM, Bart Van Assche wrote:
>>> On 2019-10-22 10:41, André Almeida wrote:
>>>> The only usage of the label "done" is when (rq->tag != -1) at the
>>>> begging of the function. Rather than jumping to label, we can just
>>>> remove this label and execute the code at the "if". Besides that,
>>>> the code that would be executed after the label "done" is the return of
>>>> the logical expression (rq->tag != -1) but since we are already inside
>>>> the if, we now that this is true. Remove the label and replace the goto
>>>> with the proper result of the label.
>>>>
>>>> Signed-off-by: André Almeida <andrealmeid@...labora.com>
>>>> ---
>>>> Hello,
>>>>
>>>> I've used `blktest` to check if this change add any regression. I have
>>>> used `./check block` and I got the same results with and without this
>>>> patch (a bunch of "passed" and three "not run" because of the virtual
>>>> scsi capabilities). Please let me know if there would be a better way to
>>>> test changes at block stack.
>>>>
>>>> This commit was rebase at linux-block/for-5.5/block.
>>>>
>>>> Thanks,
>>>> 	André
>>>> ---
>>>>    block/blk-mq.c | 3 +--
>>>>    1 file changed, 1 insertion(+), 2 deletions(-)
>>>>
>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>> index 8538dc415499..1e067b78ab97 100644
>>>> --- a/block/blk-mq.c
>>>> +++ b/block/blk-mq.c
>>>> @@ -1036,7 +1036,7 @@ bool blk_mq_get_driver_tag(struct request *rq)
>>>>    	bool shared;
>>>>    
>>>>    	if (rq->tag != -1)
>>>> -		goto done;
>>>> +		return true;
>>>>    
>>>>    	if (blk_mq_tag_is_reserved(data.hctx->sched_tags, rq->internal_tag))
>>>>    		data.flags |= BLK_MQ_REQ_RESERVED;
>>>> @@ -1051,7 +1051,6 @@ bool blk_mq_get_driver_tag(struct request *rq)
>>>>    		data.hctx->tags->rqs[rq->tag] = rq;
>>>>    	}
>>>>    
>>>> -done:
>>>>    	return rq->tag != -1;
>>>>    }
>>>
>>> Do we really need code changes like the above? I'm not aware of any text
>>> in the Documentation/ directory that forbids the use of goto statements.
> 
> goto are allowed, but the coding style document[1] provides some
> rationale for using goto, including that "If there is no cleanup needed
> then just return directly". Seems like this code used to do some stuff
> in the the past, but since 8ab6bb9ee8d0 "blk-mq: cleanup
> blk_mq_get_driver_tag()" it is just a return.
> 
>>
>> Agree, it looks fine as-is. It's also a fast path, so I'd never get rid
>> of that without looking at the generated code.
>>
> 
> You can have a look at the generated code for x86, here's the
> original[2] and here is the modified[3]. The only improvement at the
> assembly is that we get rid of this duplicated cmp instruction:
> 
>      2736:   83 f8 ff                cmp    eax,0xffffffff
>      2739:   75 4b                   jne    2786
>      ...
>      2786:   83 f8 ff                cmp    eax,0xffffffff
>      2789:   0f 95 c0                setne  al

Well, that's a win. I'll apply it.

-- 
Jens Axboe