From: David Hildenbrand <email@example.com>
To: Anshuman Khandual <firstname.lastname@example.org>,
Sudarshan Rajagopalan <email@example.com>,
Subject: Re: [PATCH 1/1] arm64: make section size configurable for memory hotplug
Date: Fri, 8 Jan 2021 16:30:31 +0100 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
> To summarize, the section size bits for each base page size config
> should always
> a. Avoid (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS
Pageblocks must also always fall completely into a section.
> b. Provide minimum possible section size for a given base page config to
> have increased agility during memory hotplug operations and reduced
> vmemmap wastage for sections with holes.
OTOH, making the section size too small (e.g., 16MB) creates way to many
memory block devices in /sys/devices/system/memory/, and might consume
too many page->flags bits in the !vmemmap case.
For bigger setups, we might, similar to x86-64 (e.g., >= 64 GiB),
determine the memory_block_size_bytes() at runtime (spanning multiple
sections then), once it becomes relevant.
> c. Allow 4K base page configs to have PMD based vmemmap mappings
> Because CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 platform,
> the following would always avoid the condition (a)
> SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)
> - 22 (11 - 1 + 12) for 4K pages
> - 24 (11 - 1 + 14) for 16K pages without THP
> - 25 (12 - 1 + 14) for 16K pages with THP
> - 26 (11 - 1 + 16) for 64K pages without THP
> - 29 (14 - 1 + 16) for 64K pages with THP
> Apart from overriding 4K base page size config to have 27 as section size
> bits, should not all other values be okay here ? But then wondering what
> benefit 128MB (27 bits) section size for 16K config would have ? OR the
> objective here is to match 16K page size config with default x86-64.
We don't want to have sections that are too small. We don't want to have
sections that are too big :)
Not sure if we really want to allow setting e.g., a section size of 4
MB. That's just going to hurt. IMHO, something in the range of 64..256
MB is quite a good choice, where possible.
>> (If we worry about the number of section bits in page->flags, we could
>> glue it to vmemmap support where that does not matter)
> Could you please elaborate ? Could smaller section size bits numbers like
> 22 or 24 create problems in page->flags without changing other parameters
> like NR_CPUS or NODES_SHIFT ? A quick test with 64K base page without THP
Yes, in the !vmemmap case, we have to store the section_nr in there.
IIRC, it's less of an issue with section sizes like 128 MB.
> i.e 26 bits in section size, fails to boot.
26 bits would mean 64 MB, no? Not sure if that's possible even without
THP (MAX_ORDER - 1, pageblock_order ...) on 64k pages. I'd assume 512 MB
is the lowest we can go. I'd assume this would crash :)
> As you have suggested, probably constant defaults (128MB for 4K/16K, 512MB
> for 64K) might be better than depending on the CONFIG_FORCE_MAX_ZONEORDER,
> at least for now.
That's also what I would prefer, keeping it simple.
David / dhildenb
next prev parent reply other threads:[~2021-01-08 15:32 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-06 1:28 [PATCH 0/1] arm64: make section size configurable for memory hotplug Sudarshan Rajagopalan
2021-01-06 1:28 ` [PATCH 1/1] " Sudarshan Rajagopalan
2021-01-06 2:20 ` Randy Dunlap
2021-01-06 6:11 ` Anshuman Khandual
2021-01-07 12:30 ` David Hildenbrand
2021-01-08 7:02 ` Anshuman Khandual
2021-01-08 15:30 ` David Hildenbrand [this message]
2021-01-11 4:17 ` Anshuman Khandual
2021-01-11 10:13 ` David Hildenbrand
2021-01-11 10:26 ` Anshuman Khandual
2021-01-08 1:01 ` Sudarshan Rajagopalan
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