linux-kernel - Re: [PATCH 1/3] powerpc/numa: Set numa

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Date:   Tue, 17 Mar 2020 14:44:45 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Christopher Lameter <cl@...ux.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 1/3] powerpc/numa: Set numa_node for all possible cpus

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.
>