linux-kernel - Re: [PATCH V7 1/4] blk-mq: refactor the code of issue request directly

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Message-ID: <d7c48fb9-9294-49ff-bb18-67a5db3ea021@oracle.com>
Date:   Thu, 15 Nov 2018 09:35:38 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V7 1/4] blk-mq: refactor the code of issue request
 directly



On 11/14/18 11:22 PM, Jens Axboe wrote:
> On 11/14/18 2:43 AM, Ming Lei wrote:
>> On Wed, Nov 14, 2018 at 05:23:48PM +0800, jianchao.wang wrote:
>>> Hi Ming
>>>
>>> On 11/14/18 5:11 PM, Ming Lei wrote:
>>>>>  
>>>>> -	if (!blk_mq_get_dispatch_budget(hctx))
>>>>> -		goto insert;
>>>>> +	if (unlikely(!blk_mq_get_dispatch_budget(hctx)))
>>>>> +		goto out_unlock;
>>>> The unlikely annotation is a bit misleading, since out-of-budget can
>>>> happen frequently in case of low queue depth, and there are lots of
>>>> such examples.
>>>>
>>>
>>> This could be good for the case for no .get_budget and getting budget success.
>>> In case of out-of-budget, we insert the request which is slow path.
>>
>> In case of low queue depth, it is hard to say that 'insert request' is
>> done in slow path, cause it happens quite frequently.
>>
>> I suggest to remove these two unlikely() since modern CPU's branch prediction
>> should work well enough.
>>
>> Especially the annotation of unlikely() often means that this branch is
>> missed in most of times for all settings, and it is obviously not true
>> in this case.
> 
> Agree, unlikely() should only be used for the error handling case or
> similar that does indeed almost never trigger. It should not be used
> for cases that don't trigger a lot in "most" circumstances.
> 

That's really appreciated for all of your kindly response.
Fair enough with 'unlikely'.
I will remove these two wrong 'unlikely' in next version.

Thanks
Jianchao