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List:       linux-hotplug
Subject:    Re: [Patch V3 2/9] kernel/profile.c: Replace cpu_to_mem() with cpu_to_node()
From:       David Rientjes <rientjes () google ! com>
Date:       2015-08-20 0:00:22
Message-ID: alpine.DEB.2.10.1508191657330.30666 () chino ! kir ! corp ! google ! com
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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.

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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