On 1/21/21 7:06 PM, Will Deacon wrote: > 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 <sudar...@codeaurora.org> >> Suggested-by: Anshuman Khandual <anshuman.khand...@arm.com> >> Suggested-by: David Hildenbrand <da...@redhat.com> >> Cc: Catalin Marinas <catalin.mari...@arm.com> >> Cc: Will Deacon <w...@kernel.org> >> Cc: Anshuman Khandual <anshuman.khand...@arm.com> >> Cc: David Hildenbrand <da...@redhat.com> >> Cc: Mike Rapoport <r...@linux.ibm.com> >> Cc: Mark Rutland <mark.rutl...@arm.com> >> Cc: Logan Gunthorpe <log...@deltatee.com> >> Cc: Andrew Morton <a...@linux-foundation.org> >> Cc: Steven Price <steven.pr...@arm.com> >> Cc: Suren Baghdasaryan <sur...@google.com> >> --- >> arch/arm64/include/asm/sparsemem.h | 23 +++++++++++++++++++++-- >> 1 file changed, 21 insertions(+), 2 deletions(-) > > Anshuman -- are you happy with this now? Yes. A small nit. There are couple of extra lines in the patch which can be dropped, probably while merging. Reviewed-by: Anshuman Khandual <anshuman.khand...@arm.com>
