linux-kernel - Re: [PATCH 2/6] change alloc function in pcpu_alloc

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Message-ID: <20100413154820.GC25756@csn.ul.ie>
Date:	Tue, 13 Apr 2010 16:48:20 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Bob Liu <lliubbo@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH 2/6] change alloc function in pcpu_alloc_pages

On Wed, Apr 14, 2010 at 12:24:59AM +0900, Minchan Kim wrote:
> alloc_pages_node is called with cpu_to_node(cpu).
> I think cpu_to_node(cpu) never returns -1.
> (But I am not sure we need double check.)
> 
> So we can use alloc_pages_exact_node instead of alloc_pages_node.
> It could avoid comparison and branch as 6484eb3e2a81807722 tried.
> 

Well, numa_node_id() is implemented as

#ifndef numa_node_id
#define numa_node_id()          (cpu_to_node(raw_smp_processor_id()))
#endif

and the mapping table on x86 at least is based on possible CPUs in
init_cpu_to_node() leaves the mapping as 0 if the APIC is bad or the numa
node is reported in apicid_to_node as -1. It would appear on power that
the node will be 0 for possible CPUs as well.

Hence, I believe this to be safe but a confirmation from Tejun would be
nice. I would continue digging but this looks like an initialisation path
so I'll move on to the next patch rather than spending more time.

> Cc: Tejun Heo <tj@...nel.org>
> Cc: Christoph Lameter <cl@...ux-foundation.org>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> ---
>  mm/percpu.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 768419d..ec3e671 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -720,7 +720,7 @@ static int pcpu_alloc_pages(struct pcpu_chunk *chunk,
>  		for (i = page_start; i < page_end; i++) {
>  			struct page **pagep = &pages[pcpu_page_idx(cpu, i)];
>  
> -			*pagep = alloc_pages_node(cpu_to_node(cpu), gfp, 0);
> +			*pagep = alloc_pages_exact_node(cpu_to_node(cpu), gfp, 0);
>  			if (!*pagep) {
>  				pcpu_free_pages(chunk, pages, populated,
>  						page_start, page_end);
> -- 
> 1.7.0.5
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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