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

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Date:   Wed, 1 Jul 2020 13:06:01 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Christopher Lameter <cl@...ux.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline

On 01.07.20 13:01, Srikar Dronamraju wrote:
> * David Hildenbrand <david@...hat.com> [2020-07-01 12:15:54]:
> 
>> On 01.07.20 12:04, Srikar Dronamraju wrote:
>>> * Michal Hocko <mhocko@...nel.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.
>>
> 
> 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.

-- 
Thanks,

David / dhildenb