From: David Hildenbrand <firstname.lastname@example.org>
To: Anshuman Khandual <email@example.com>,
Sudarshan Rajagopalan <firstname.lastname@example.org>,
Subject: Re: [PATCH 1/1] arm64: make section size configurable for memory hotplug
Date: Mon, 11 Jan 2021 11:13:12 +0100 [thread overview]
Message-ID: <email@example.com> (raw)
On 11.01.21 05:17, Anshuman Khandual wrote:
> On 1/8/21 9:00 PM, David Hildenbrand wrote:
>>> 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.
> Okay sure, will send a RFC to begin with.
Note that there is
(Sudarshan missed to cc linux-mm)
David / dhildenb
next prev parent reply other threads:[~2021-01-11 10:16 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
2021-01-11 4:17 ` Anshuman Khandual
2021-01-11 10:13 ` David Hildenbrand [this message]
2021-01-11 10:26 ` Anshuman Khandual
2021-01-08 1:01 ` Sudarshan Rajagopalan
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