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Subject: Re: [PATCH 1/3] powerpc/numa: Set numa_node for all possible cpus
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Sachin Sant <sachinp@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        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>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>
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From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <65b99db6-3bdf-6caa-74e5-6d6b681f16b5@suse.cz>
Date:   Tue, 17 Mar 2020 14:44:45 +0100
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On 3/16/20 10:06 AM, Michal Hocko wrote:
> On Thu 12-03-20 17:41:58, Vlastimil Babka wrote:
> [...]
>> with nid present in:
>> N_POSSIBLE - pgdat might not exist, node_to_mem_node() must return some online
> 
> I would rather have a dummy pgdat for those. Have a look at 
> $ git grep "NODE_DATA.*->" | wc -l
> 63
> 
> Who knows how many else we have there. I haven't looked more closely.
> Besides that what is a real reason to not have pgdat ther and force all
> users of a $random node from those that the platform considers possible
> for special casing? Is that a memory overhead? Is that really a thing?

I guess we can ignore memory overhead. I guess there only might be some concern
that for nodes that are initially offline, we will allocate the pgdat on a
different node, and after they are online, it will stay on a different node with
more access latency from local cpus. If we only allocate for online nodes, it
can always be local? But I guess it doesn't matter that much.

> Somebody has suggested to tweak some of the low level routines to do the
> special casing but I really have to say I do not like that. We shouldn't
> use the first online node or anything like that. We should simply always
> follow the topology presented by FW and of that we need to have a pgdat.
>