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Date:   Thu, 21 Jan 2021 13:36:52 +0000
From:   Will Deacon <will@kernel.org>
To:     Sudarshan Rajagopalan <sudaraja@codeaurora.org>
Cc:     linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Hildenbrand <david@redhat.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Price <steven.price@arm.com>,
        Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH 1/1] arm64/sparsemem: reduce SECTION_SIZE_BITS
Message-ID: <20210121133652.GA22341@willie-the-truck>
References: <cover.1611206601.git.sudaraja@codeaurora.org>
 <43843c5e092bfe3ec4c41e3c8c78a7ee35b69bb0.1611206601.git.sudaraja@codeaurora.org>
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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@codeaurora.org>
> Suggested-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> ---
>  arch/arm64/include/asm/sparsemem.h | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)

Anshuman -- are you happy with this now?

Will