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

From: David Hildenbrand <david@redhat.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	Michal Hocko <mhocko@kernel.org>
Cc: 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>,
	Gautham R Shenoy <ego@linux.vnet.ibm.com>,
	Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline
Date: Wed, 1 Jul 2020 12:15:54 +0200	[thread overview]
Message-ID: <184102af-ecf2-c834-db46-173ab2e66f51@redhat.com> (raw)
In-Reply-To: <20200701100442.GB17918@linux.vnet.ibm.com>

On 01.07.20 12:04, Srikar Dronamraju wrote:
> * Michal Hocko <mhocko@kernel.org> [2020-07-01 10:42:00]:
> 
>>
>>>
>>> 2. Also existence of dummy node also leads to inconsistent information. The
>>> number of online nodes is inconsistent with the information in the
>>> device-tree and resource-dump
>>>
>>> 3. When the dummy node is present, single node non-Numa systems end up showing
>>> up as NUMA systems and numa_balancing gets enabled. This will mean we take
>>> the hit from the unnecessary numa hinting faults.
>>
>> I have to say that I dislike the node online/offline state and directly
>> exporting that to the userspace. Users should only care whether the node
>> has memory/cpus. Numa nodes can be online without any memory. Just
>> offline all the present memory blocks but do not physically hot remove
>> them and you are in the same situation. If users are confused by an
>> output of tools like numactl -H then those could be updated and hide
>> nodes without any memory&cpus.
>>
>> The autonuma problem sounds interesting but again this patch doesn't
>> really solve the underlying problem because I strongly suspect that the
>> problem is still there when a numa node gets all its memory offline as
>> mentioned above.
>>
>> While I completely agree that making node 0 special is wrong, I have
>> still hard time to review this very simply looking patch because all the
>> numa initialization is so spread around that this might just blow up
>> at unexpected places. IIRC we have discussed testing in the previous
>> version and David has provided a way to emulate these configurations
>> on x86. Did you manage to use those instruction for additional testing
>> on other than ppc architectures?
>>
> 
> I have tried all the steps that David mentioned and reported back at
> https://lore.kernel.org/lkml/20200511174731.GD1961@linux.vnet.ibm.com/t/#u
> 
> As a summary, David's steps are still not creating a memoryless/cpuless on
> x86 VM.

Now, that is wrong. You get a memoryless/cpuless node, which is *not
online*. Once you hotplug some memory, it will switch online. Once you
remove memory, it will switch back offline.

-- 
Thanks,

David / dhildenb