Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH V7 1/4] blk-mq: refactor the code of issue request
 directly
To:     Jens Axboe <axboe@kernel.dk>, Ming Lei <ming.lei@redhat.com>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1542185131-15029-1-git-send-email-jianchao.w.wang@oracle.com>
 <1542185131-15029-2-git-send-email-jianchao.w.wang@oracle.com>
 <20181114091153.GB20550@ming.t460p>
 <17b51550-d45b-9861-b172-74ee0e256d98@oracle.com>
 <20181114094343.GA13193@ming.t460p>
 <42861ece-5519-bb59-e3a7-8fea1c731f19@kernel.dk>
From:   "jianchao.wang" <jianchao.w.wang@oracle.com>
Message-ID: <d7c48fb9-9294-49ff-bb18-67a5db3ea021@oracle.com>
Date:   Thu, 15 Nov 2018 09:35:38 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.2.1
MIME-Version: 1.0
In-Reply-To: <42861ece-5519-bb59-e3a7-8fea1c731f19@kernel.dk>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


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