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

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Message-ID: <42861ece-5519-bb59-e3a7-8fea1c731f19@kernel.dk>
Date:   Wed, 14 Nov 2018 08:22:44 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Ming Lei <ming.lei@...hat.com>,
        "jianchao.wang" <jianchao.w.wang@...cle.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 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.


-- 
Jens Axboe