Re: [PATCH 1/1] arm64/sparsemem: reduce SECTION_SIZE_BITS
From: Mike Rapoport
Date: Thu Jan 21 2021 - 09:18:53 EST
On Wed, Jan 20, 2021 at 09:29:13PM -0800, Sudarshan Rajagopalan wrote:
> memory_block_size_bytes() determines the memory hotplug granularity i.e the
> amount of memory which can be hot added or hot removed from the kernel. The
> generic value here being MIN_MEMORY_BLOCK_SIZE (1UL << SECTION_SIZE_BITS)
> for memory_block_size_bytes() on platforms like arm64 that does not override.
> Current SECTION_SIZE_BITS is 30 i.e 1GB which is large and a reduction here
> increases memory hotplug granularity, thus improving its agility. A reduced
> section size also reduces memory wastage in vmemmmap mapping for sections
> with large memory holes. So we try to set the least section size as possible.
> A section size bits selection must follow:
> (MAX_ORDER - 1 + PAGE_SHIFT) <= SECTION_SIZE_BITS
> CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 and so just following it
> would help achieve the smallest section size.
> SECTION_SIZE_BITS = (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)
> SECTION_SIZE_BITS = 22 (11 - 1 + 12) i.e 4MB for 4K pages
> SECTION_SIZE_BITS = 24 (11 - 1 + 14) i.e 16MB for 16K pages without THP
> SECTION_SIZE_BITS = 25 (12 - 1 + 14) i.e 32MB for 16K pages with THP
> SECTION_SIZE_BITS = 26 (11 - 1 + 16) i.e 64MB for 64K pages without THP
> SECTION_SIZE_BITS = 29 (14 - 1 + 16) i.e 512MB for 64K pages with THP
> But there are other problems in reducing SECTION_SIZE_BIT. Reducing it by too
> much would over populate /sys/devices/system/memory/ and also consume too many
> page->flags bits in the !vmemmap case. Also section size needs to be multiple
> of 128MB to have PMD based vmemmap mapping with CONFIG_ARM64_4K_PAGES.
> Given these constraints, lets just reduce the section size to 128MB for 4K
> and 16K base page size configs, and to 512MB for 64K base page size config.
> Signed-off-by: Sudarshan Rajagopalan <sudaraja@xxxxxxxxxxxxxx>
> Suggested-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Steven Price <steven.price@xxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
BTW, after reduction of the section size maybe arm64 should consider opting
out of freeing unused memory map.
This will make David even more happy as this will allow dropping custom
> arch/arm64/include/asm/sparsemem.h | 23 +++++++++++++++++++++--
> 1 file changed, 21 insertions(+), 2 deletions(-)
> diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
> index 1f43fcc79738..eb4a75d720ed 100644
> --- a/arch/arm64/include/asm/sparsemem.h
> +++ b/arch/arm64/include/asm/sparsemem.h
> @@ -7,7 +7,26 @@
> #ifdef CONFIG_SPARSEMEM
> #define MAX_PHYSMEM_BITS CONFIG_ARM64_PA_BITS
> -#define SECTION_SIZE_BITS 30
> + * Section size must be at least 512MB for 64K base
> + * page size config. Otherwise it will be less than
> + * (MAX_ORDER - 1) and the build process will fail.
> + */
> +#ifdef CONFIG_ARM64_64K_PAGES
> +#define SECTION_SIZE_BITS 29
> + * Section size must be at least 128MB for 4K base
> + * page size config. Otherwise PMD based huge page
> + * entries could not be created for vmemmap mappings.
> + * 16K follows 4K for simplicity.
> + */
> +#define SECTION_SIZE_BITS 27
> +#endif /* CONFIG_ARM64_64K_PAGES */
> +#endif /* CONFIG_SPARSEMEM*/
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