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Date:   Wed, 1 Jul 2020 14:21:10 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        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
Message-ID: <20200701122110.GT2369@dhcp22.suse.cz>
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 <20200701100442.GB17918@linux.vnet.ibm.com>
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On Wed 01-07-20 13:30:57, David Hildenbrand wrote:
> On 01.07.20 13:06, David Hildenbrand wrote:
> > 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.

I would really appreciate a feedback to these two as well.

> >>>>> 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.
> > 
> 
> 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]

This begs a question whether ppc can do the same thing?

I would swear that we've had x86 system with node 0 but I cannot really
find it and it is possible that it was not x86 after all...
-- 
Michal Hocko
SUSE Labs