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Date:   Fri, 3 Jul 2020 11:24:14 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Michal =?iso-8859-1?Q?Such=E1nek?= <msuchanek@suse.de>
Cc:     David Hildenbrand <david@redhat.com>,
        Gautham R Shenoy <ego@linux.vnet.ibm.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>,
        Mel Gorman <mgorman@suse.de>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        linuxppc-dev@lists.ozlabs.org, Christopher Lameter <cl@linux.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0
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[Cc Andi]

On Fri 03-07-20 11:10:01, Michal Suchanek wrote:
> On Wed, Jul 01, 2020 at 02:21:10PM +0200, Michal Hocko wrote:
> > On Wed 01-07-20 13:30:57, David Hildenbrand wrote:
[...]
> > > Yep, looks like it.
> > > 
> > > [    0.009726] SRAT: PXM 1 -> APIC 0x00 -> Node 0
> > > [    0.009727] SRAT: PXM 1 -> APIC 0x01 -> Node 0
> > > [    0.009727] SRAT: PXM 1 -> APIC 0x02 -> Node 0
> > > [    0.009728] SRAT: PXM 1 -> APIC 0x03 -> Node 0
> > > [    0.009731] ACPI: SRAT: Node 0 PXM 1 [mem 0x00000000-0x0009ffff]
> > > [    0.009732] ACPI: SRAT: Node 0 PXM 1 [mem 0x00100000-0xbfffffff]
> > > [    0.009733] ACPI: SRAT: Node 0 PXM 1 [mem 0x100000000-0x13fffffff]
> > 
> > This begs a question whether ppc can do the same thing?
> Or x86 stop doing it so that you can see on what node you are running?
> 
> What's the point of this indirection other than another way of avoiding
> empty node 0?

Honestly, I do not have any idea. I've traced it down to
Author: Andi Kleen <ak@suse.de>
Date:   Tue Jan 11 15:35:48 2005 -0800

    [PATCH] x86_64: Fix ACPI SRAT NUMA parsing

    Fix fallout from the recent nodemask_t changes. The node ids assigned
    in the SRAT parser were off by one.

    I added a new first_unset_node() function to nodemask.h to allocate
    IDs sanely.

    Signed-off-by: Andi Kleen <ak@suse.de>
    Signed-off-by: Linus Torvalds <torvalds@osdl.org>

which doesn't really tell all that much. The historical baggage and a
long term behavior which is not really trivial to fix I suspect.
-- 
Michal Hocko
SUSE Labs