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From: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
To: linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	David Hildenbrand <david@redhat.com>
Cc: Sudarshan Rajagopalan <sudaraja@codeaurora.org>,
	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: [PATCH 1/1] arm64/sparsemem: reduce SECTION_SIZE_BITS
Date: Wed, 20 Jan 2021 21:29:13 -0800	[thread overview]
Message-ID: <43843c5e092bfe3ec4c41e3c8c78a7ee35b69bb0.1611206601.git.sudaraja@codeaurora.org> (raw)
In-Reply-To: <cover.1611206601.git.sudaraja@codeaurora.org>

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(-)

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
-#endif
+
+/*
+ * 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
+
+#else
+
+/*
+ * 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*/
 
 #endif
-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project



  reply	other threads:[~2021-01-21  5:29 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-21  5:29 [PATCH 0/1] " Sudarshan Rajagopalan
2021-01-21  5:29 ` Sudarshan Rajagopalan [this message]
2021-01-21 10:08   ` [PATCH 1/1] " Christoph Lameter
2021-01-21 15:54     ` Catalin Marinas
2021-01-21 13:36   ` Will Deacon
2021-01-22  2:58     ` Anshuman Khandual
2021-01-21 13:45   ` David Hildenbrand
2021-01-21 14:16   ` Mike Rapoport
2021-01-21 16:04     ` David Hildenbrand
2021-01-21 15:51   ` Catalin Marinas
2021-01-21 18:26 ` [PATCH 0/1] " Will Deacon
2021-01-29  0:17   ` Pavel Tatashin

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