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Date:   Tue, 12 May 2020 16:12:26 +0530
From:   Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        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>,
        Christopher Lameter <cl@linux.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline
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Reply-To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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* David Hildenbrand <david@redhat.com> [2020-05-12 09:49:05]:

> On 11.05.20 19:47, Srikar Dronamraju wrote:
> > * David Hildenbrand <david@redhat.com> [2020-05-08 15:42:12]:
> > 
> > 
> > [root@localhost ~]# cat /sys/devices/system/node/online
> > 0
> > [root@localhost ~]# cat /sys/devices/system/node/possible
> > 0-1
> > 
> > Even without my patch, both the combinations, I am still unable to see a
> > cpuless, memoryless node being online. And the interesting part being even
> 
> Yeah, I think on x86, all memory-less and cpu-less nodes are offline as
> default. Especially when hotunplugging cpus/memory, we set them offline
> as well.

I also came to the same conclusion that we may not have a cpuless,memoryless
node on x86.

> 
> But as Michal mentioned, the node handling code is complicated and
> differs between various architectures.
> 

I do agree that node handling code differs across various architectures and
quite complicated.

> > if I mark node 0 as cpuless,memoryless and node 1 as actual node, the system
> > somewhere marks node 0 as the actual node.
> 
> Is the kernel maybe mapping PXM 1 to node 0 in that case, because it
> always requires node 0 to be online/contain memory? Would be interesting
> what happens if you hotplug a DIMM to (QEMU )node 0 - if PXM 0 will be
> mapped to node 1 then as well.
> 

Satheesh Rajendra had tried with cpu hotplug on a similar setup and we found
that it crashes the x86 system.
reference: https://bugzilla.kernel.org/show_bug.cgi?id=202187

Even if we were able to hotplug 1 DIMM memory into node 1, that would no
more be a memoryless node.

-- 
Thanks and Regards
Srikar Dronamraju