linux-kernel - Re: [Patch V3 3/9] sgi-xp: Replace cpu_to_node() with cpu_to

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Message-ID: <56174AC9.4090104@linux.intel.com>
Date:	Fri, 9 Oct 2015 13:04:09 +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>,
	Cliff Whickman <cpw@....com>,
	Robin Holt <robinmholt@...il.com>,
	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 3/9] sgi-xp: Replace cpu_to_node() with cpu_to_mem() to
 support memoryless node

On 2015/8/20 14:36, Jiang Liu wrote:
> On 2015/8/20 8:02, David Rientjes wrote:
>> On Wed, 19 Aug 2015, Jiang Liu wrote:
>>
>>>> Why not simply fix build_zonelists_node() so that the __GFP_THISNODE 
>>>> zonelists are set up to reference the zones of cpu_to_mem() for memoryless 
>>>> nodes?
>>>>
>>>> It seems much better than checking and maintaining every __GFP_THISNODE 
>>>> user to determine if they are using a memoryless node or not.  I don't 
>>>> feel that this solution is maintainable in the longterm.
>>> Hi David,
>>> 	There are some usage cases, such as memory migration,
>>> expect the page allocator rejecting memory allocation requests
>>> if there is no memory on local node. So we have:
>>> 1) alloc_pages_node(cpu_to_node(), __GFP_THISNODE) to only allocate
>>> memory from local node.
>>> 2) alloc_pages_node(cpu_to_mem(), __GFP_THISNODE) to allocate memory
>>> from local node or from nearest node if local node is memoryless.
>>>
>>
>> Right, so do you think it would be better to make the default zonelists be 
>> setup so that cpu_to_node()->zonelists == cpu_to_mem()->zonelists and then 
>> individual callers that want to fail for memoryless nodes check 
>> populated_zone() themselves?
> Hi David,
> 	Great idea:) I think that means we are going to kill the
> concept of memoryless node, and we only need to specially handle
> a few callers who really care about whether there is memory on
> local node.
> 	Then I need some time to audit all usages of __GFP_THISNODE
> and update you whether it's doable.
Hi David,
	It seems that I'm too optimistic:(. After auditing all usages
of __GFP_THISNODE and reading Documentation/vm/numa again, I feel it
would be better to keep cpu_to_mem()/numa_mem_id(). It makes things
more clear if we follow rules:
1) cpu_to_node()/numa_node_id() for schedule domain
2) cpu_to_mem()/numa_mem_id() for memory management domain
3) alloc_pages_node(cpu_to_node(cpu), __GFP_THIS_NODE) for special
   usage cases.
   And it would be easier for maintenance than open-coded checking of
populated_zone() by using alloc_pages_node(cpu_to_node(cpu),
__GFP_THIS_NODE).
Thanks!
Gerry
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