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Date: Wed, 02 Jun 2021 11:56:29 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>,
Michal Hocko <mhocko@...e.com>
Cc: Oscar Salvador <osalvador@...e.de>,
Michael Ellerman <mpe@...erman.id.au>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
Andy Lutomirski <luto@...nel.org>,
"Rafael J . Wysocki" <rafael@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Mike Rapoport <rppt@...nel.org>,
Anshuman Khandual <anshuman.khandual@....com>,
Dan Williams <dan.j.williams@...el.com>,
Wei Yang <richard.weiyang@...ux.alibaba.com>,
linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size
On 10 May 2021, at 10:36, Zi Yan wrote:
> On 7 May 2021, at 10:00, David Hildenbrand wrote:
>
>> On 07.05.21 13:55, Michal Hocko wrote:
>>> [I haven't read through respective patches due to lack of time but let
>>> me comment on the general idea and the underlying justification]
>>>
>>> On Thu 06-05-21 17:31:09, David Hildenbrand wrote:
>>>> On 06.05.21 17:26, Zi Yan wrote:
>>>>> From: Zi Yan <ziy@...dia.com>
>>>>>
>>>>> Hi all,
>>>>>
>>>>> This patchset tries to remove the restriction on memory hotplug/hotremove
>>>>> granularity, which is always greater or equal to memory section size[1].
>>>>> With the patchset, kernel is able to online/offline memory at a size independent
>>>>> of memory section size, as small as 2MB (the subsection size).
>>>>
>>>> ... which doesn't make any sense as we can only online/offline whole memory
>>>> block devices.
>>>
>>> Agreed. The subsection thingy is just a hack to workaround pmem
>>> alignement problems. For the real memory hotplug it is quite hard to
>>> argue for reasonable hotplug scenarios for very small physical memory
>>> ranges wrt. to the existing sparsemem memory model.
>>>
>>>>> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
>>>>> size without increasing memory hotplug/hotremove granularity at the same time,
>>>>
>>>> Gah, no. Please no. No.
>>>
>>> Agreed. Those are completely independent concepts. MAX_ORDER is can be
>>> really arbitrary irrespective of the section size with vmemmap sparse
>>> model. The existing restriction is due to old sparse model not being
>>> able to do page pointer arithmetic across memory sections. Is there any
>>> reason to stick with that memory model for an advance feature you are
>>> working on?
>
> No. I just want to increase MAX_ORDER. If the existing restriction can
> be removed, that will be great.
>
>>
>> I gave it some more thought yesterday. I guess the first thing we should look into is increasing MAX_ORDER and leaving pageblock_order and section size as is -- finding out what we have to tweak to get that up and running. Once we have that in place, we can actually look into better fragmentation avoidance etc. One step at a time.
>
> It makes sense to me.
>
>>
>> Because that change itself might require some thought. Requiring that bigger MAX_ORDER depends on SPARSE_VMEMMAP is something reasonable to do.
>
> OK, if with SPARSE_VMEMMAP MAX_ORDER can be set to be bigger than
> SECTION_SIZE, it is perfectly OK to me. Since 1GB THP support, which I
> want to add ultimately, will require SPARSE_VMEMMAP too (otherwise,
> all page++ will need to be changed to nth_page(page,1)).
>
>>
>> As stated somewhere here already, we'll have to look into making alloc_contig_range() (and main users CMA and virtio-mem) independent of MAX_ORDER and mainly rely on pageblock_order. The current handling in alloc_contig_range() is far from optimal as we have to isolate a whole MAX_ORDER - 1 page -- and on ZONE_NORMAL we'll fail easily if any part contains something unmovable although we don't even want to allocate that part. I actually have that on my list (to be able to fully support pageblock_order instead of MAX_ORDER -1 chunks in virtio-mem), however didn't have time to look into it.
>
> So in your mind, for gigantic page allocation (> MAX_ORDER), alloc_contig_range()
> should be used instead of buddy allocator while pageblock_order is kept at a small
> granularity like 2MB. Is that the case? Isn’t it going to have high fail rate
> when any of the pageblocks within a gigantic page range (like 1GB) becomes unmovable?
> Are you thinking additional mechanism/policy to prevent such thing happening as
> an additional step for gigantic page allocation? Like your ZONE_PREFER_MOVABLE idea?
>
>>
>> Further, page onlining / offlining code and early init code most probably also needs care if MAX_ORDER - 1 crosses sections. Memory holes we might suddenly have in MAX_ORDER - 1 pages might become a problem and will have to be handled. Not sure which other code has to be tweaked (compaction? page isolation?).
>
> Can you elaborate it a little more? From what I understand, memory holes mean valid
> PFNs are not contiguous before and after a hole, so pfn++ will not work, but
> struct pages are still virtually contiguous assuming SPARSE_VMEMMAP, meaning page++
> would still work. So when MAX_ORDER - 1 crosses sections, additional code would be
> needed instead of simple pfn++. Is there anything I am missing?
>
> BTW, to test a system with memory holes, do you know is there an easy of adding
> random memory holes to an x86_64 VM, which can help reveal potential missing pieces
> in the code? Changing BIOS-e820 table might be one way, but I have no idea on
> how to do it on QEMU.
>
>>
>> Figuring out what needs care itself might take quite some effort.
>>
>> One thing I was thinking about as well: The bigger our MAX_ORDER, the slower it could be to allocate smaller pages. If we have 1G pages, splitting them down to 4k then takes 8 additional steps if I'm, not wrong. Of course, that's the worst case. Would be interesting to evaluate.
>
> Sure. I am planning to check it too. As a simple start, I am going to run will it scale
> benchmarks to see if there is any performance difference between different MAX_ORDERs.
I ran vm-scalablity and memory-related will-it-scale on a server with 256GB memory to
see the impact of increasing MAX_ORDER and didn’t see much difference for most of
the workloads like page_fault1, page_fault2, and page_fault3 from will-it-scale.
But feel free to check the attached complete results and let me know what should be
looked into. Thanks.
# Environment
Dell R630 with 2x 12-core E5-2650 v4 and 256GB memory.
# Kernel changes
On top of v5.13-rc1-mmotm-2021-05-13-17-18, with SECTION_SIZE_BITS set to 31 and
MAX_ORDER set to 11 and 20 respectively.
# Results of page_fault1, page_fault2, and page_fault3
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/50%/debian/dellr630/page_fault3/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
3199850 ± 2% +6.0% 3390866 ± 3% will-it-scale.24.threads
54.94 +1.7% 55.85 will-it-scale.24.threads_idle
133326 ± 2% +6.0% 141285 ± 3% will-it-scale.per_thread_ops
3199850 ± 2% +6.0% 3390866 ± 3% will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/50%/debian/dellr630/page_fault2/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2016984 -6.6% 1883075 ± 2% will-it-scale.24.threads
69.69 -4.4% 66.64 will-it-scale.24.threads_idle
84040 -6.6% 78461 ± 2% will-it-scale.per_thread_ops
2016984 -6.6% 1883075 ± 2% will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/50%/debian/dellr630/page_fault1/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2138067 -1.3% 2109865 will-it-scale.24.threads
63.34 +1.1% 64.06 will-it-scale.24.threads_idle
89085 -1.3% 87910 will-it-scale.per_thread_ops
2138067 -1.3% 2109865 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/16/debian/dellr630/page_fault3/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
3216287 ± 3% +4.8% 3369356 ± 10% will-it-scale.16.threads
69.18 +0.5% 69.51 will-it-scale.16.threads_idle
201017 ± 3% +4.8% 210584 ± 10% will-it-scale.per_thread_ops
3216287 ± 3% +4.8% 3369356 ± 10% will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/16/debian/dellr630/page_fault2/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2005510 -2.7% 1950620 ± 2% will-it-scale.16.threads
78.77 -0.2% 78.64 will-it-scale.16.threads_idle
125344 -2.7% 121913 ± 2% will-it-scale.per_thread_ops
2005510 -2.7% 1950620 ± 2% will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/16/debian/dellr630/page_fault1/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2332446 -6.5% 2179823 ± 2% will-it-scale.16.threads
77.57 -2.0% 76.03 will-it-scale.16.threads_idle
145777 -6.5% 136238 ± 2% will-it-scale.per_thread_ops
2332446 -6.5% 2179823 ± 2% will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/100%/debian/dellr630/page_fault3/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
3236057 ± 2% -4.5% 3089222 ± 4% will-it-scale.48.threads
24.64 ± 7% -3.3% 23.83 ± 2% will-it-scale.48.threads_idle
67417 ± 2% -4.5% 64358 ± 4% will-it-scale.per_thread_ops
3236057 ± 2% -4.5% 3089222 ± 4% will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/100%/debian/dellr630/page_fault2/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
1611363 -0.1% 1609891 will-it-scale.48.threads
47.42 ± 2% +1.2% 48.01 will-it-scale.48.threads_idle
33569 -0.1% 33539 will-it-scale.per_thread_ops
1611363 -0.1% 1609891 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/100%/debian/dellr630/page_fault1/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
1776494 ± 3% -2.6% 1730693 will-it-scale.48.threads
43.36 ± 4% +0.5% 43.57 ± 2% will-it-scale.48.threads_idle
37010 ± 3% -2.6% 36055 will-it-scale.per_thread_ops
1776494 ± 3% -2.6% 1730693 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/50%/debian/dellr630/page_fault3/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
15235214 -0.3% 15185167 will-it-scale.24.processes
49.63 -0.4% 49.45 will-it-scale.24.processes_idle
634800 -0.3% 632715 will-it-scale.per_process_ops
15235214 -0.3% 15185167 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/50%/debian/dellr630/page_fault2/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
6700813 -0.6% 6662570 will-it-scale.24.processes
49.17 +0.0% 49.18 will-it-scale.24.processes_idle
279200 -0.6% 277606 will-it-scale.per_process_ops
6700813 -0.6% 6662570 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/50%/debian/dellr630/page_fault1/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
8052059 -1.2% 7952172 will-it-scale.24.processes
49.48 -0.4% 49.29 will-it-scale.24.processes_idle
335502 -1.2% 331340 will-it-scale.per_process_ops
8052059 -1.2% 7952172 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/16/debian/dellr630/page_fault3/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
10152559 +0.7% 10221240 will-it-scale.16.processes
66.10 -0.0% 66.09 will-it-scale.16.processes_idle
634534 +0.7% 638827 will-it-scale.per_process_ops
10152559 +0.7% 10221240 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/16/debian/dellr630/page_fault2/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
4621434 -1.0% 4576959 will-it-scale.16.processes
66.14 -0.2% 65.98 will-it-scale.16.processes_idle
288839 -1.0% 286059 will-it-scale.per_process_ops
4621434 -1.0% 4576959 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/16/debian/dellr630/page_fault1/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
5546153 -1.3% 5474778 will-it-scale.16.processes
66.02 -0.1% 65.98 will-it-scale.16.processes_idle
346634 -1.3% 342173 will-it-scale.per_process_ops
5546153 -1.3% 5474778 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/100%/debian/dellr630/page_fault3/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
20575719 +0.4% 20651992 will-it-scale.48.processes
0.06 +5.6% 0.06 ± 7% will-it-scale.48.processes_idle
428660 +0.4% 430249 will-it-scale.per_process_ops
20575719 +0.4% 20651992 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/100%/debian/dellr630/page_fault2/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
6984071 -1.1% 6906022 will-it-scale.48.processes
0.07 +4.8% 0.07 ± 6% will-it-scale.48.processes_idle
145501 -1.1% 143875 will-it-scale.per_process_ops
6984071 -1.1% 6906022 will-it-scale.workload
=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/100%/debian/dellr630/page_fault1/will-it-scale
commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+
5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
7527654 -1.7% 7399284 will-it-scale.48.processes
0.07 +0.0% 0.07 will-it-scale.48.processes_idle
156826 -1.7% 154151 will-it-scale.per_process_ops
7527654 -1.7% 7399284 will-it-scale.workload
—
Best Regards,
Yan, Zi
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