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Date:   Mon, 16 Mar 2020 09:54:25 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Christopher Lameter <cl@linux.com>
Cc:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Mel Gorman <mgorman@suse.de>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline
Message-ID: <20200316085425.GB11482@dhcp22.suse.cz>
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 <alpine.DEB.2.21.2003151416230.14449@www.lameter.com>
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On Sun 15-03-20 14:20:05, Cristopher Lameter wrote:
> On Wed, 11 Mar 2020, Srikar Dronamraju wrote:
> 
> > Currently Linux kernel with CONFIG_NUMA on a system with multiple
> > possible nodes, marks node 0 as online at boot.  However in practice,
> > there are systems which have node 0 as memoryless and cpuless.
> 
> Would it not be better and simpler to require that node 0 always has
> memory (and processors)? A  mininum operational set?

I do not think you can simply ignore the reality. I cannot say that I am
a fan of memoryless/cpuless numa configurations but they are a sad
reality of different LPAR configurations. We have to deal with them.
Besides that I do not really see any strong technical arguments to lack
a support for those crippled configurations. We do have zonelists that
allow to do reasonable decisions on memoryless nodes. So no, I do not
think that this is a viable approach.

> We can dynamically number the nodes right? So just make sure that the
> firmware properly creates memory on node 0?

Are you suggesting that the OS would renumber NUMA nodes coming
from FW just to satisfy node 0 existence? If yes then I believe this is
really a bad idea because it would make HW/LPAR configuration matching
to the resulting memory layout really hard to follow.

-- 
Michal Hocko
SUSE Labs