linux-kernel - Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline

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Date:   Tue, 30 Jun 2020 09:31:25 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline

* Christopher Lameter <cl@...ux.com> [2020-06-29 14:58:40]:

> On Wed, 24 Jun 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.
> 
> Maybe add something to explain why you are not simply mapping the
> existing memory to NUMA node 0 which is after all just a numbering scheme
> used by the kernel and can be used arbitrarily?
> 

I thought Michal Hocko already gave a clear picture on why mapping is a bad
idea. https://lore.kernel.org/lkml/20200316085425.GB11482@dhcp22.suse.cz/t/#u
Are you suggesting that we add that as part of the changelog?

> This could be seen more as a bug in the arch code during the setup of NUMA
> nodes. The two nodes are created by the firmwware / bootstrap code after
> all. Just do not do it?
> 

- The arch/setup code in powerpc is not onlining these nodes. 
- Later on cpus/memory in node 0 can be onlined.
- Firmware in this case Phyp is an independent code by itself.

-- 
Thanks and Regards
Srikar Dronamraju