Subject: Re: [PATCH] aarch64/mm: speedup memory initialisation

From: Anshuman Khandual <[email-masked]>
Date: Thu, 12 Sep 2019 14:29:49 +0530

On 09/12/2019 02:10 PM, Hubert Ralf wrote:
> Am Donnerstag, den 12.09.2019, 13:42 +0530 schrieb Anshuman Khandual:
>>
>> On 09/10/2019 02:29 PM, Hubert Ralf wrote:
>>> On ARM64 memmap_init_zone is used during bootmem_init, which iterates over
>>> all pages in the memory starting at the lowest address until the highest
>>> address is reached. On arm64 this ends up in searching a memmory region
>>> containing for each single page between lowest and highest available
>>> physicall address.
>>>
>>> Having a sparse memory system there may be some big holes in the
>>> memory map. For each page in this holes a lookup is done, which is
>>> implemented as a binary search on the available memory blocks.
>>>
>>> Adding a memmap_init for aarch64 to do the init only for the available
>>> memory areas reduces the time needed for initialising memory on startup.
>>> On a Renesas R-CAR M3 based system with a total hole of 20GB bootmem_init
>>> execution time is reduced from 378ms to 84ms.
>>>
>>> Signed-off-by: Ralf Hubert <[email-masked]>
>>> ---
>>>  arch/arm64/include/asm/pgtable.h |  7 +++++++
>>>  arch/arm64/mm/init.c             | 24 ++++++++++++++++++++++++
>>>  2 files changed, 31 insertions(+)
>>>
>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>> index e09760ece844..8c6eefc08b0b 100644
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -298,6 +298,13 @@ static inline int pte_same(pte_t pte_a, pte_t pte_b)
>>>  	return (lhs == rhs);
>>>  }
>>>  
>>> +#ifdef CONFIG_SPARSEMEM
>>> +/* arch mem_map init routine is needed due to holes in a memmap */
>>> +#   define __HAVE_ARCH_MEMMAP_INIT
>>
>> This is not required any more. Its gone with the following commit which
>> also made generic memmap_init() an weak function currently overridden
>> only on ia64.
>>
>> dfb3ccd00a0 ("mm: make memmap_init a proper function")
>>
>>> +	void memmap_init(unsigned long size, int nid, unsigned long zone,
>>> +			 unsigned long start_pfn);
>>> +#endif /* CONFIG_SPARSEMEM */
>>> +
>>>  /*
>>>   * Huge pte definitions.
>>>   */
>>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>>> index f3c795278def..206b28310872 100644
>>> --- a/arch/arm64/mm/init.c
>>> +++ b/arch/arm64/mm/init.c
>>> @@ -250,6 +250,30 @@ int pfn_valid(unsigned long pfn)
>>>  }
>>>  EXPORT_SYMBOL(pfn_valid);
>>>  
>>> +#ifdef CONFIG_SPARSEMEM
>>> +void __meminit
>>> +memmap_init(unsigned long size, int nid, unsigned long zone,
>>> +	    unsigned long start_pfn)
>>> +{
>>> +	struct memblock_region *reg;
>>> +
>>> +	for_each_memblock(memory, reg) {
>>> +		unsigned long start = memblock_region_memory_base_pfn(reg);
>>> +		unsigned long end = memblock_region_memory_end_pfn(reg);
>>> +
>>> +		if (start < start_pfn)
>>> +			start = start_pfn;
>>> +		if (end > start_pfn + size)
>>> +			end = start_pfn + size;
>>> +
>>> +		if (start < end) {
>>> +			memmap_init_zone(end - start, nid, zone, start,
>>> +					 MEMMAP_EARLY, NULL);
>>> +		}
>>> +	}
>>> +}
>>> +#endif /* CONFIG_SPARSEMEM */
>>
>> In generic mmap_init(), the current high cost comes from early_pfn_valid()
>> check for each pfn in memmap_init_zone() given that early_pfn_valid() is
>> pfn_valid() when CONFIG_SPARSEMEM which is known to be expensive on arm64.
>>
>> Though we cannot do anything about pfns which are really present but the
>> high cost for non present pfns should be eliminated. The following check
>> in the above for_each_memblock() loop can achieve that.
>>
>> if (reg->flags & MEMBLOCK_NOMAP)
>> 	continue;
>>
>> MEMBLOCK_NOMAP universally should not be initialized into a zone and holes
>> if any should also be universally skipped across platforms. So these changes
>> can be moved into generic memmap_init() which will benefit other platforms.
> Not sure if I got this. This is a additional short path for memblocks with
> the MEMBLOCK_NOMAP flag set, right? For memblocks without these flag the remaining
> code still needs to be executed?

Right, they should be initialized in the zone.

> 
> In my case I have 4 memblocks with 1.5GB RAM at 0x4000 0000, 0x4 8000 0000,
> 0x6 0000 0000 and 0x6 8000 0000. None of them has the MEMBLOCK_NOMAP flag set.

All of them should be added into the zone with memmap_init_zone() because none of
them has the flag.

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