linux-kernel - Re: [PATCH] mm: don't invoke __alloc_pages_direct

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Date:	Sun, 8 Jul 2012 15:53:35 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, JoonSoo Kim <js1304@...il.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: don't invoke __alloc_pages_direct_compact when order
 0

On Sun, 8 Jul 2012, JoonSoo Kim wrote:

> >> __alloc_pages_direct_compact has many arguments so invoking it is very costly.
> >> And in almost invoking case, order is 0, so return immediately.
> >>
> >
> > If "zero cost" is "very costly", then this might make sense.
> >
> > __alloc_pages_direct_compact() is inlined by gcc.
> 
> In my kernel image, __alloc_pages_direct_compact() is not inlined by gcc.

Adding Andrew and Mel to the thread since this would require that we 
revert 11e33f6a55ed ("page allocator: break up the allocator entry point 
into fast and slow paths") which would obviously not be a clean revert 
since there have been several changes to these functions over the past 
three years.

I'm stunned (and skeptical) that __alloc_pages_direct_compact() is not 
inlined by your gcc, especially since the kernel must be compiled with 
optimization (either -O1 or -O2 which causes these functions to be 
inlined).  What version of gcc are you using and on what architecture?  
Please do "make mm/page_alloc.s" and send it to me privately, I'll file 
this and fix it up on gcc-bugs.

I'll definitely be following up on this.
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