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

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Date:	Mon, 17 Jun 2013 14:37:02 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
cc:	Zhang Yanfei <zhangyanfei.yes@...il.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 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.
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