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    SubjectRe: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline
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    On 01.07.20 13:01, Srikar Dronamraju wrote:
    > * David Hildenbrand <david@redhat.com> [2020-07-01 12:15:54]:
    >
    >> 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.
    >>
    >
    > 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

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