Re: [PATCH v3 4/5] powerpc/mm: restore top-down allocation when using movable_node

[Balbir Singh]

On 27/09/16 10:14, Reza Arbab wrote:
> On Tue, Sep 27, 2016 at 07:12:31AM +1000, Benjamin Herrenschmidt wrote:
>> In any case, if the memory hasn't been hotplug, this shouldn't be necessary 
>> as we shouldn't be considering it for allocation.
> 
> Right. To be clear, the background info I put in the commit log refers to 
> x86, where the SRAT can describe movable nodes which exist at boot.  They're 
> trying to avoid allocations from those nodes before they've been identified.
> 
> On power, movable nodes can only exist via hotplug, so that scenario can't 
> happen. We can immediately go back to top-down allocation. That is the 
> missing call being added in the patch.
> 

Can we fix cmdline_parse_movable_node() to do the right thing? I suspect that
code is heavily x86 only in the sense that no other arch needs it.

Balbir Singh.
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Re: [PATCH v3 4/5] powerpc/mm: restore top-down allocation when using movable_node

[Reza Arbab]

On Tue, Sep 27, 2016 at 07:12:31AM +1000, Benjamin Herrenschmidt wrote:
In any case, if the memory hasn't been hotplug, this shouldn't be 
necessary as we shouldn't be considering it for allocation.


Right. To be clear, the background info I put in the commit log refers 
to x86, where the SRAT can describe movable nodes which exist at boot.  
They're trying to avoid allocations from those nodes before they've been 
identified.


On power, movable nodes can only exist via hotplug, so that scenario 
can't happen. We can immediately go back to top-down allocation. That is 
the missing call being added in the patch.


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Reza Arbab

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Re: [PATCH v3 4/5] powerpc/mm: restore top-down allocation when using movable_node

[Reza Arbab]

On Mon, Sep 26, 2016 at 09:17:43PM +0530, Aneesh Kumar K.V wrote:

+   /* bottom-up allocation may have been set by movable_node */
+   memblock_set_bottom_up(false);
+


By then we have done few memblock allocation right ?


Yes, some allocations do occur while bottom-up is set.

IMHO, we should do this early enough in prom.c after we do 
parse_early_param, with a comment there explaining that, we don't 
really support hotplug memblock and when we do that, this should be 
moved to a place where we can handle memblock allocation such that we 
avoid spreading memblock allocation to movable node.


Sure, we can do it earlier. The only consideration is that any potential 
calls to memblock_mark_hotplug() happen before we reset to top-down.  
Since we don't do that at all on power, the call can go anywhere.


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Reza Arbab

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[PATCH v3 4/5] powerpc/mm: restore top-down allocation when using movable_node

[Reza Arbab]

At boot, the movable_node option sets bottom-up memblock allocation.

This reduces the chance that, in the window before movable memory has
been identified, an allocation for the kernel might come from a movable
node. By going bottom-up, early allocations will most likely come from
the same node as the kernel image, which is necessarily in a nonmovable
node.

Then, once any known hotplug memory has been marked, allocation can be
reset back to top-down. On x86, this is done in numa_init(). This patch
does the same on power, in numa initmem_init().

Signed-off-by: Reza Arbab 
---
 arch/powerpc/mm/numa.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index d7ac419..fdf1e69 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c
@@ -945,6 +945,9 @@ void __init initmem_init(void)
max_low_pfn = memblock_end_of_DRAM() >> PAGE_SHIFT;
max_pfn = max_low_pfn;
 
+   /* bottom-up allocation may have been set by movable_node */
+   memblock_set_bottom_up(false);
+
if (parse_numa_properties())
setup_nonnuma();
else
-- 
1.8.3.1

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