Re: [PATCH 1/1] arm64: make section size configurable for memory hotplug

[David Hildenbrand]

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
>>
>> Agreed.
>>
>>>
>>> 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

https://lkml.kernel.org/r/15cf9a2359197fee0168f820c5c904650d07939e.1610146597.git.sudaraja@xxxxxxxxxxxxxx

(Sudarshan missed to cc linux-mm)

-- 
Thanks,

David / dhildenb