linux-kernel - Re: [PATCH v2 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER

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Date:   Fri, 12 Jul 2019 17:00:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Hoan Tran OS <hoan@...amperecomputing.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Paul Mackerras <paulus@...ba.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "x86@...nel.org" <x86@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Open Source Submission <patches@...erecomputing.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Will Deacon <will.deacon@....com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Oscar Salvador <osalvador@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "David S . Miller" <davem@...emloft.net>, willy@...radead.org
Subject: Re: [PATCH v2 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by
 default for NUMA

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