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

From: David Hildenbrand <[email-masked]>
Date: Wed, 1 Jul 2020 13:30:57 +0200

On 01.07.20 13:06, David Hildenbrand wrote:
> On 01.07.20 13:01, Srikar Dronamraju wrote:
>> * David Hildenbrand <[email-masked]> [2020-07-01 12:15:54]:
>>
>>> On 01.07.20 12:04, Srikar Dronamraju wrote:
>>>> * Michal Hocko <[email-masked]> [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/[email-masked]/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.
>>>
>>
>> Let me clarify, we are looking for a node 0 which is cpuless/memoryless at
>> boot.  The code in question tries to handle a cpuless/memoryless node 0 at
>> boot.
> 
> I was just correcting your statement, because it was wrong.
> 
> Could be that x86 code maps PXM 1 to node 0 because PXM 1 does neither
> have CPUs nor memory. That would imply that we can, in fact, never have
> node 0 offline during boot.
> 

Yep, looks like it.

[    0.009726] SRAT: PXM 1 -> APIC 0x00 -> Node 0
[    0.009727] SRAT: PXM 1 -> APIC 0x01 -> Node 0
[    0.009727] SRAT: PXM 1 -> APIC 0x02 -> Node 0
[    0.009728] SRAT: PXM 1 -> APIC 0x03 -> Node 0
[    0.009731] ACPI: SRAT: Node 0 PXM 1 [mem 0x00000000-0x0009ffff]
[    0.009732] ACPI: SRAT: Node 0 PXM 1 [mem 0x00100000-0xbfffffff]
[    0.009733] ACPI: SRAT: Node 0 PXM 1 [mem 0x100000000-0x13fffffff]

-- 
Thanks,

David / dhildenb