Re: [PATCH v2 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by default for NUMA

[Michal Hocko]

On Fri 12-07-19 15:37:30, Will Deacon wrote:
> Hi all,
> 
> On Fri, Jul 12, 2019 at 02:12:23PM +0200, Michal Hocko wrote:
> > On Fri 12-07-19 10:56:47, Hoan Tran OS wrote:
> > [...]
> > > It would be good if we can enable it by-default. Otherwise, let arch 
> > > enables it by them-self. Do you have any suggestions?
> > 
> > I can hardly make any suggestions when it is not really clear _why_ you
> > want to remove this config option in the first place. Please explain
> > what motivated you to make this change.
> 
> Sorry, I think this confusion might actually be my fault and Hoan has just
> been implementing my vague suggestion here:
> 
> https://lore.kernel.org/linux-arm-kernel/20190625101245.s4vxfosoop52gl4e@willie-the-truck/
> 
> If the preference of the mm folks is to leave CONFIG_NODES_SPAN_OTHER_NODES
> as it is, then we can define it for arm64. I just find it a bit weird that
> the majority of NUMA-capable architectures have to add a symbol in the arch
> Kconfig file, for what appears to be a performance optimisation applicable
> only to ia64, mips and sh.
> 
> At the very least we could make the thing selectable.

Hmm, I thought this was selectable. But I am obviously wrong here.
Looking more closely, it seems that this is indeed only about
__early_pfn_to_nid and as such not something that should add a config
symbol. This should have been called out in the changelog though.

Also while at it, does HAVE_MEMBLOCK_NODE_MAP fall into a similar
bucket? Do we have any NUMA architecture that doesn't enable it?

Thanks!
-- 
Michal Hocko
SUSE Labs