linux-kernel - Re: [Patch V3 2/9] kernel/profile.c: Replace cpu_to_mem() with cpu_to

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Date:	Fri, 9 Oct 2015 10:35:51 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Tang Chen <tangchen@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-hotplug@...r.kernel.org, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [Patch V3 2/9] kernel/profile.c: Replace cpu_to_mem() with
 cpu_to_node()

On 2015/8/20 8:00, David Rientjes wrote:
> On Wed, 19 Aug 2015, Jiang Liu wrote:
> 
>> On 2015/8/18 8:31, David Rientjes wrote:
>>> On Mon, 17 Aug 2015, Jiang Liu wrote:
>>>
>>>> Function profile_cpu_callback() allocates memory without specifying
>>>> __GFP_THISNODE flag, so replace cpu_to_mem() with cpu_to_node()
>>>> because cpu_to_mem() may cause suboptimal memory allocation if
>>>> there's no free memory on the node returned by cpu_to_mem().
>>>>
>>>
>>> Why is cpu_to_node() better with regard to free memory and NUMA locality?
>> Hi David,
>> 	Thanks for review. This is a special case pointed out by Tejun.
>> For the imagined topology, A<->B<->X<->C<->D, where A, B, C, D has
>> memory and X is memoryless.
>> Possible fallback lists are:
>> B: [ B, A, C, D]
>> X: [ B, C, A, D]
>> C: [ C, D, B, A]
>>
>> cpu_to_mem(X) will either return B or C. Let's assume it returns B.
>> Then we will use "B: [ B, A, C, D]" to allocate memory for X, which
>> is not the optimal fallback list for X. And cpu_to_node(X) returns
>> X, and "X: [ B, C, A, D]" is the optimal fallback list for X.
> 
> Ok, that makes sense, but I would prefer that this 
> alloc_pages_exact_node() change to alloc_pages_node() since, as you 
> mention in your commit message, __GFP_THISNODE is not set.
Hi David,
	Sorry for slow response due to personal reasons!
	Function alloc_pages_exact_node() has been renamed as
__alloc_pages_node() by commit 96db800f5d73, and __alloc_pages_node()
is a slightly optimized version of alloc_pages_node() which doesn't
fallback to current node for nid == NUMA_NO_NODE case. So it would
be better to keep using __alloc_pages_node() because cpu_to_node()
always returns valid node id.
Thanks!
Gerry

> 
> In the longterm, if we setup both zonelists correctly (no __GFP_THISNODE 
> and with __GFP_THISNODE), then I'm not sure there's any reason to ever use 
> cpu_to_mem() for alloc_pages().
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