linux-kernel - Re: [PATCH] mm: Add unlikely for current

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Date:	Tue, 18 Jun 2013 05:56:47 +0800
From:	Zhang Yanfei <zhangyanfei.yes@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux MM <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Add unlikely for current_order test

On 06/18/2013 05:37 AM, David Rientjes wrote:
> On Mon, 17 Jun 2013, Zhang Yanfei wrote:
> 
>>> I don't understand the justification at all, current_order being unlikely 
>>> greater than or equal to pageblock_order / 2 doesn't imply at all that 
>>> it's unlikely that current_order is greater than or equal to 
>>> pageblock_order.
>>>
>>
>> hmmm... I am confused. Since current_order is >= pageblock_order / 2 is unlikely,
>> why current_order is >= pageblock_order isn't unlikely. Or there are other
>> tips?
>>
>> Actually, I am also a little confused about why current_order should be
>> unlikely greater than or equal to pageblock_order / 2. When borrowing pages
>> with other migrate_type, we always search from MAX_ORDER-1, which is greater
>> or equal to pageblock_order.
>>
> 
> Look at what is being done in the function: current_order loops down from 
> MAX_ORDER-1 to the order passed.  It is not at all "unlikely" that 
> current_order is greater than pageblock_order, or pageblock_order / 2.
> 
> MAX_ORDER is typically 11 and pageblock_order is typically 9 on x86.  
> Integer division truncates, so pageblock_order / 2 is 4.  For the first 
> eight iterations, it's guaranteed that current_order >= pageblock_order / 
> 2 if it even gets that far!
> 
> So just remove the unlikely() entirely, it's completely bogus.

I see. Thanks!

I will send another patch.

-- 
Thanks.
Zhang Yanfei
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