[PATCH 0/1] arm64: make section size configurable for memory hotplug

[PATCH 0/1] arm64: make section size configurable for memory hotplug

[Sudarshan Rajagopalan]

The section size defines the granularity of memory hotplug. This is currently
hard coded to 1GB on arm64 linux, which defines that the least size of memblock
that can be hotplugged out is 1GB. Some DDR configurations (especially low RAM
and dual-rank DDRs) may have section sizes that are less than 1GB (ex. 512MB, 256MB etc.).
Having an option to reduce the memblock size to section size or lower gives more
granularity of memory hotplug. For example, a system with DDR section size of 512MB
and kernel memblock size of 1GB, we would have to remove two segments of DDR sections
in order to hotplug out atleast 1 memblock from kernel POV. 

Section sizes of DDRs vary based on specs (number of ranks, channels, regions etc.)
Making this section size configurable helps users to assign based on the DDR being used.
The default is set to 1GB which is the current memblock size.

Sudarshan Rajagopalan (1):
  arm64: Make section size configurable for memory hotplug

 arch/arm64/Kconfig                 | 11 +++++++++++
 arch/arm64/include/asm/sparsemem.h |  4 ++++
 2 files changed, 15 insertions(+)

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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

[Sudarshan Rajagopalan]

Currently on arm64, memory section size is hard-coded to 1GB.
Make this configurable if memory-hotplug is enabled, to support
more finer granularity for hotplug-able memory.

Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
---
 arch/arm64/Kconfig                 | 11 +++++++++++
 arch/arm64/include/asm/sparsemem.h |  4 ++++
 2 files changed, 15 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 6d232837cbee..34124eee65da 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -294,6 +294,17 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
 config SMP
 	def_bool y
 
+config HOTPLUG_SIZE_BITS
+	int "Memory hotplug block size(29 => 512MB 30 => 1GB)"
+	depends on SPARSEMEM
+	depends on MEMORY_HOTPLUG
+	range 28 30
+	default 30
+	help
+	 Selects granularity of hotplug memory. Block size for
+	 memory hotplug is represent as a power of 2.
+	 If unsure, stick with default value.
+
 config KERNEL_MODE_NEON
 	def_bool y
 
diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
index 1f43fcc79738..3d5310f3aad5 100644
--- a/arch/arm64/include/asm/sparsemem.h
+++ b/arch/arm64/include/asm/sparsemem.h
@@ -7,7 +7,11 @@
 
 #ifdef CONFIG_SPARSEMEM
 #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
+#ifndef CONFIG_MEMORY_HOTPLUG
 #define SECTION_SIZE_BITS	30
+#else
+#define SECTION_SIZE_BITS	CONFIG_HOTPLUG_SIZE_BITS
+#endif
 #endif
 
 #endif
-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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

[Randy Dunlap]

On 1/5/21 5:28 PM, Sudarshan Rajagopalan wrote:
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 6d232837cbee..34124eee65da 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -294,6 +294,17 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
>  config SMP
>  	def_bool y
>  
> +config HOTPLUG_SIZE_BITS
> +	int "Memory hotplug block size(29 => 512MB 30 => 1GB)"

	                    block size (29 => 512MB, 30 => 1GB)"

> +	depends on SPARSEMEM
> +	depends on MEMORY_HOTPLUG
> +	range 28 30
> +	default 30
> +	help
> +	 Selects granularity of hotplug memory. Block size for
> +	 memory hotplug is represent as a power of 2.

	                   represented

> +	 If unsure, stick with default value.

and all of the help text should be indented with 1 tab + 2 spaces.


thanks.
-- 
~Randy

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

[Anshuman Khandual]

Hi Sudershan,

This patch (and the cover letter) does not copy LAKML even though the
entire change here is arm64 specific. Please do copy all applicable
mailing lists for a given patch.

On 1/6/21 6:58 AM, Sudarshan Rajagopalan wrote:
> Currently on arm64, memory section size is hard-coded to 1GB.
> Make this configurable if memory-hotplug is enabled, to support
> more finer granularity for hotplug-able memory.

Section size has always been decided by the platform. It cannot be a
configurable option because the user would not know the constraints
for memory representation on the platform and besides it also cannot
be trusted.

> 
> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
> ---
>  arch/arm64/Kconfig                 | 11 +++++++++++
>  arch/arm64/include/asm/sparsemem.h |  4 ++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 6d232837cbee..34124eee65da 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -294,6 +294,17 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
>  config SMP
>  	def_bool y
>  
> +config HOTPLUG_SIZE_BITS
> +	int "Memory hotplug block size(29 => 512MB 30 => 1GB)"
> +	depends on SPARSEMEM
> +	depends on MEMORY_HOTPLUG
> +	range 28 30

28 would not work for 64K pages.

> +	default 30
> +	help
> +	 Selects granularity of hotplug memory. Block size for
> +	 memory hotplug is represent as a power of 2.
> +	 If unsure, stick with default value.
> +
>  config KERNEL_MODE_NEON
>  	def_bool y
>  
> diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
> index 1f43fcc79738..3d5310f3aad5 100644
> --- a/arch/arm64/include/asm/sparsemem.h
> +++ b/arch/arm64/include/asm/sparsemem.h
> @@ -7,7 +7,11 @@
>  
>  #ifdef CONFIG_SPARSEMEM
>  #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
> +#ifndef CONFIG_MEMORY_HOTPLUG
>  #define SECTION_SIZE_BITS	30
> +#else
> +#define SECTION_SIZE_BITS	CONFIG_HOTPLUG_SIZE_BITS
> +#endif
>  #endif
>  
>  #endif
> 

There was an inconclusive discussion regarding this last month.

https://lore.kernel.org/linux-arm-kernel/20201204014443.43329-1-liwei213@huawei.com/

I have been wondering if this would solve the problem for 4K page size
config which requires PMD mapping for the vmemmap mapping while making
section size bits dependent on max order. But this has not been tested
properly.

diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
index 1f43fcc79738..fe4353cb1dce 100644
--- a/arch/arm64/include/asm/sparsemem.h
+++ b/arch/arm64/include/asm/sparsemem.h
@@ -7,7 +7,18 @@
 
 #ifdef CONFIG_SPARSEMEM
 #define MAX_PHYSMEM_BITS       CONFIG_ARM64_PA_BITS
-#define SECTION_SIZE_BITS      30
-#endif
+
+#ifdef CONFIG_ARM64_4K_PAGES
+#define SECTION_SIZE_BITS 27
+#else
+#ifdef CONFIG_FORCE_MAX_ZONEORDER
+#define SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)
+#else
+#define SECTION_SIZE_BITS 30
+#endif /* CONFIG_FORCE_MAX_ZONEORDER */
+
+#endif /* CONFIG_ARM64_4K_PAGES */
+
+#endif /* CONFIG_SPARSEMEM*/
 
 #endif

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

[David Hildenbrand]

On 06.01.21 07:11, Anshuman Khandual wrote:
> Hi Sudershan,
> 
> This patch (and the cover letter) does not copy LAKML even though the
> entire change here is arm64 specific. Please do copy all applicable
> mailing lists for a given patch.
> 
> On 1/6/21 6:58 AM, Sudarshan Rajagopalan wrote:
>> Currently on arm64, memory section size is hard-coded to 1GB.
>> Make this configurable if memory-hotplug is enabled, to support
>> more finer granularity for hotplug-able memory.
> 
> Section size has always been decided by the platform. It cannot be a
> configurable option because the user would not know the constraints
> for memory representation on the platform and besides it also cannot
> be trusted.
> 
>>
>> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
>> ---
>>  arch/arm64/Kconfig                 | 11 +++++++++++
>>  arch/arm64/include/asm/sparsemem.h |  4 ++++
>>  2 files changed, 15 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 6d232837cbee..34124eee65da 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -294,6 +294,17 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
>>  config SMP
>>  	def_bool y
>>  
>> +config HOTPLUG_SIZE_BITS
>> +	int "Memory hotplug block size(29 => 512MB 30 => 1GB)"

"Memory hotplug block size" and "HOTPLUG_SIZE_BITS" is confusing. It's
the section size. Please use that terminology.

(if at all, it would have to be "minimum memory hot(un)plug
granularity", but even that is somewhat misleading)

"SECTION_SIZE_BITS"

But I agree that letting the user configure it is sub-optimal.

>> +	depends on SPARSEMEM
>> +	depends on MEMORY_HOTPLUG
>> +	range 28 30
> 
> 28 would not work for 64K pages.

@Anshuman, what's your feeling about changing this to 128 MB for 4k/16k
base pages (as we have on x86-64 right now) and 512 MB for 64k as
default for now?

(If we worry about the number of section bits in page->flags, we could
glue it to vmemmap support where that does not matter)


-- 
Thanks,

David / dhildenb

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

[Anshuman Khandual]

On 1/7/21 6:00 PM, David Hildenbrand wrote:
> On 06.01.21 07:11, Anshuman Khandual wrote:
>> Hi Sudershan,
>>
>> This patch (and the cover letter) does not copy LAKML even though the
>> entire change here is arm64 specific. Please do copy all applicable
>> mailing lists for a given patch.
>>
>> On 1/6/21 6:58 AM, Sudarshan Rajagopalan wrote:
>>> Currently on arm64, memory section size is hard-coded to 1GB.
>>> Make this configurable if memory-hotplug is enabled, to support
>>> more finer granularity for hotplug-able memory.
>>
>> Section size has always been decided by the platform. It cannot be a
>> configurable option because the user would not know the constraints
>> for memory representation on the platform and besides it also cannot
>> be trusted.
>>
>>>
>>> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
>>> ---
>>>  arch/arm64/Kconfig                 | 11 +++++++++++
>>>  arch/arm64/include/asm/sparsemem.h |  4 ++++
>>>  2 files changed, 15 insertions(+)
>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 6d232837cbee..34124eee65da 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -294,6 +294,17 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
>>>  config SMP
>>>  	def_bool y
>>>  
>>> +config HOTPLUG_SIZE_BITS
>>> +	int "Memory hotplug block size(29 => 512MB 30 => 1GB)"
> 
> "Memory hotplug block size" and "HOTPLUG_SIZE_BITS" is confusing. It's
> the section size. Please use that terminology.
> 
> (if at all, it would have to be "minimum memory hot(un)plug
> granularity", but even that is somewhat misleading)
> 
> "SECTION_SIZE_BITS"
> 
> But I agree that letting the user configure it is sub-optimal.
> 
>>> +	depends on SPARSEMEM
>>> +	depends on MEMORY_HOTPLUG
>>> +	range 28 30
>>
>> 28 would not work for 64K pages.
> 
> @Anshuman, what's your feeling about changing this to 128 MB for 4k/16k
> base pages (as we have on x86-64 right now) and 512 MB for 64k as
> default for now?

To summarize, the section size bits for each base page size config
should always

a. Avoid (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS

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.

c. Allow 4K base page configs to have PMD based vmemmap mappings

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.

> 
> (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
i.e 26 bits in section size, fails to boot.

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.

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

[David Hildenbrand]

> 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.

-- 
Thanks,

David / dhildenb

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

[Anshuman Khandual]

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.

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@codeaurora.org

(Sudarshan missed to cc linux-mm)

-- 
Thanks,

David / dhildenb

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

[Anshuman Khandual]

On 1/11/21 3:43 PM, David Hildenbrand wrote:
> 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@codeaurora.org
> 
> (Sudarshan missed to cc linux-mm)
> 

Right, some how missed that. Anyways, ended up spending some time testing
the change for various configs.

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

[Sudarshan Rajagopalan]

On 2021-01-05 22:11, Anshuman Khandual wrote:

Hello Anshuman, thanks for your response.


(+ Will)

> Hi Sudershan,
> 
> This patch (and the cover letter) does not copy LAKML even though the
> entire change here is arm64 specific. Please do copy all applicable
> mailing lists for a given patch.

I used ./scripts/get_maintainer.pl patch.patch to get the maintainers 
list. It somehow didn't mention LAKML. I've added the mailing list to 
this thread.

> 
> On 1/6/21 6:58 AM, Sudarshan Rajagopalan wrote:
>> Currently on arm64, memory section size is hard-coded to 1GB.
>> Make this configurable if memory-hotplug is enabled, to support
>> more finer granularity for hotplug-able memory.
> 
> Section size has always been decided by the platform. It cannot be a
> configurable option because the user would not know the constraints
> for memory representation on the platform and besides it also cannot
> be trusted.
> 
>> 
>> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
>> ---
>>  arch/arm64/Kconfig                 | 11 +++++++++++
>>  arch/arm64/include/asm/sparsemem.h |  4 ++++
>>  2 files changed, 15 insertions(+)
>> 
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 6d232837cbee..34124eee65da 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -294,6 +294,17 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
>>  config SMP
>>  	def_bool y
>> 
>> +config HOTPLUG_SIZE_BITS
>> +	int "Memory hotplug block size(29 => 512MB 30 => 1GB)"
>> +	depends on SPARSEMEM
>> +	depends on MEMORY_HOTPLUG
>> +	range 28 30
> 
> 28 would not work for 64K pages.
> 
>> +	default 30
>> +	help
>> +	 Selects granularity of hotplug memory. Block size for
>> +	 memory hotplug is represent as a power of 2.
>> +	 If unsure, stick with default value.
>> +
>>  config KERNEL_MODE_NEON
>>  	def_bool y
>> 
>> diff --git a/arch/arm64/include/asm/sparsemem.h 
>> b/arch/arm64/include/asm/sparsemem.h
>> index 1f43fcc79738..3d5310f3aad5 100644
>> --- a/arch/arm64/include/asm/sparsemem.h
>> +++ b/arch/arm64/include/asm/sparsemem.h
>> @@ -7,7 +7,11 @@
>> 
>>  #ifdef CONFIG_SPARSEMEM
>>  #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
>> +#ifndef CONFIG_MEMORY_HOTPLUG
>>  #define SECTION_SIZE_BITS	30
>> +#else
>> +#define SECTION_SIZE_BITS	CONFIG_HOTPLUG_SIZE_BITS
>> +#endif
>>  #endif
>> 
>>  #endif
>> 
> 
> There was an inconclusive discussion regarding this last month.
> 
> https://lore.kernel.org/linux-arm-kernel/20201204014443.43329-1-liwei213@huawei.com/

Thanks for pointing out this thread. Looking into all the comments, 
major concern with reducing the section size seems to be risk of running 
out of bits in the page flags. And while SECTION_SIZE must be greater or 
equal to highest order page in the buddy, it must also satisfy cases for 
4K page size where it doesn't break PMD mapping for vmemmap - and hence 
SECTION_SIZE_BITS of 27 could be set for 4K page size that could allow 
2MB PMD mappings for each 128M(2^27) block size.

While this is the least value that can be set (27 for 4K_PAGE, 
MAX_ZONEORDER - 1 + PAGE_SHIFT for 16K or 64K_PAGE), are there any 
concerns with setting higher values (but <= 30bits). It seems like any 
arbitrary number between this range could be applied that wouldn't break 
vmemmaps. That's why we were thinking of letting the user configure it 
since this directly impacts memory hotplug about the granularity or 
least size that can be hot (up)plugged. The current setting of 1GB for 
arm64 does poses a lot of challenges in utilizing memory hotplug via a 
driver, esp. for low RAM targets. I agree its sub-optimal in some sense 
but wanted to know the maintainers opinion on this.

Also, the patch introduced in that thread does seem to help reduce 
vmemmap memory if there are large holes. So there is some merit in 
reducing the section size along with memory hotplug leveraging it.

> 
> I have been wondering if this would solve the problem for 4K page size
> config which requires PMD mapping for the vmemmap mapping while making
> section size bits dependent on max order. But this has not been tested
> properly.
> 
> diff --git a/arch/arm64/include/asm/sparsemem.h
> b/arch/arm64/include/asm/sparsemem.h
> index 1f43fcc79738..fe4353cb1dce 100644
> --- a/arch/arm64/include/asm/sparsemem.h
> +++ b/arch/arm64/include/asm/sparsemem.h
> @@ -7,7 +7,18 @@
> 
>  #ifdef CONFIG_SPARSEMEM
>  #define MAX_PHYSMEM_BITS       CONFIG_ARM64_PA_BITS
> -#define SECTION_SIZE_BITS      30
> -#endif
> +
> +#ifdef CONFIG_ARM64_4K_PAGES
> +#define SECTION_SIZE_BITS 27
> +#else
> +#ifdef CONFIG_FORCE_MAX_ZONEORDER
> +#define SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + 
> PAGE_SHIFT)
> +#else
> +#define SECTION_SIZE_BITS 30
> +#endif /* CONFIG_FORCE_MAX_ZONEORDER */
> +
> +#endif /* CONFIG_ARM64_4K_PAGES */
> +
> +#endif /* CONFIG_SPARSEMEM*/
> 
>  #endif

SECTION_SIZE_BITS of 27 for 4K_PAGES should be fine for us. Would you 
know if there's possibility of this patch above being applied in 
upstream anytime soon? This is in regards with Generic Kernel Image 
(GKI) that we are working with Google. If this patch would positively 
end up in upstream, we could apply it in aosp kernel.


Sudarshan

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