Message-ID: <50B6A66E.8030406@linux.vnet.ibm.com>
Date: Wed, 28 Nov 2012 16:03:58 -0800
From: Dave Hansen <dave@linux.vnet.ibm.com>
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To: Tejun Heo <tj@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        linux-mm@kvack.org, Cody P Schafer <cody@linux.vnet.ibm.com>
Subject: 32/64-bit NUMA consolidation behavior regresion
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Hi Tejun,

I was bisecting a boot problem on a 32-bit NUMA kernel and it bisected
down to commit 8db78cc4.  It turns out that, with this patch,
pcpu_need_numa() changed its return value on my system from 1 to 0.
What that basically meant was that we stopped using the remapped lowmem
areas for percpu data.

My system is just qemu booted with:

-smp 8 -m 8192 -numa node,nodeid=0,cpus=0-3 -numa node,nodeid=1,cpus=4-7

Watch the "PERCPU:" line early in boot, and you can see the "Embedded"
come and go with or without your patch:

[    0.000000] PERCPU: Embedded 11 pages/cpu @f3000000 s30592 r0 d14464
vs
[    0.000000] PERCPU: 11 4K pages/cpu @f83fe000 s30592 r0 d14464

I believe this has to do with the hunks in your patch that do:

-#ifdef CONFIG_X86_64
        init_cpu_to_node();
-#endif
...
-#ifdef CONFIG_X86_32
-DEFINE_EARLY_PER_CPU(int, x86_cpu_to_node_map, 0);
-#else
 DEFINE_EARLY_PER_CPU(int, x86_cpu_to_node_map, NUMA_NO_NODE);
-#endif
 EXPORT_EARLY_PER_CPU_SYMBOL(x86_cpu_to_node_map);

I don't have a fix handy because I'm working on the original problem,
but I just happened to run across this during a bisect.

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