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

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Date:   Wed, 31 Jul 2019 10:03:09 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Hoan Tran OS <hoan@...amperecomputing.com>,
        Will Deacon <will@...nel.org>,
        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>,
        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" <willy@...radead.org>
Subject: Re: [PATCH v2 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by
 default for NUMA

On Wed 31-07-19 09:24:21, Mike Rapoport wrote:
> [ sorry for a late reply too, somehow I missed this thread before ]
> 
> On Tue, Jul 30, 2019 at 10:14:15AM +0200, Michal Hocko wrote:
> > [Sorry for a late reply]
> > 
> > On Mon 15-07-19 17:55:07, Hoan Tran OS wrote:
> > > Hi,
> > > 
> > > On 7/12/19 10:00 PM, Michal Hocko wrote:
> > [...]
> > > > 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.
> > > 
> > > Yes, do you have any other comments about my patch?
> > 
> > Not really. Just make sure to explicitly state that
> > CONFIG_NODES_SPAN_OTHER_NODES is only about __early_pfn_to_nid and that
> > doesn't really deserve it's own config and can be pulled under NUMA.
> > 
> > > > 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?
> > > > 
> 
> HAVE_MEMBLOCK_NODE_MAP makes huge difference in node/zone initialization
> sequence so it's not only about a singe function.

The question is whether we want to have this a config option or enable
it unconditionally for each NUMA system.

> > > As I checked with arch Kconfig files, there are 2 architectures, riscv 
> > > and microblaze, do not support NUMA but enable this config.
> 
> My take would be that riscv will support NUMA some day.
>  
> > > And 1 architecture, alpha, supports NUMA but does not enable this config.
> 
> alpha's NUMA support is BROKEN for more than a decade now, I doubt it'll
> ever get fixed.

I can see Al has marked it BROKEN in 2005. Maybe time to rip it out?
Although it doesn't seem to be a lot of code in arch/alpha at first
glance so maybe not worth an effort.
-- 
Michal Hocko
SUSE Labs