Re: [PATCH v3 2/5] mm: page_alloc: reduce unnecessary binary search in memblock_next_valid_pfn()

[Jia He <hejianet@gmail.com>]

On 3/30/2018 9:43 AM, Wei Yang Wrote:
> On Thu, Mar 29, 2018 at 04:06:38PM +0800, Jia He wrote:
>>
>> On 3/28/2018 5:26 PM, Wei Yang Wrote:
>>> On Sun, Mar 25, 2018 at 08:02:16PM -0700, Jia He wrote:
>>>> Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
>>>> where possible") optimized the loop in memmap_init_zone(). But there is
>>>> still some room for improvement. E.g. if pfn and pfn+1 are in the same
>>>> memblock region, we can simply pfn++ instead of doing the binary search
>>>> in memblock_next_valid_pfn. This patch only works when
>>>> CONFIG_HAVE_ARCH_PFN_VALID is enable.
>>>>
>>>> Signed-off-by: Jia He <jia.he@hxt-semitech.com>
>>>> ---
>>>> include/linux/memblock.h |  2 +-
>>>> mm/memblock.c            | 73 +++++++++++++++++++++++++++++-------------------
>>>> mm/page_alloc.c          |  3 +-
>>>> 3 files changed, 47 insertions(+), 31 deletions(-)
>>>>
>>>> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
>>>> index efbbe4b..a8fb2ab 100644
>>>> --- a/include/linux/memblock.h
>>>> +++ b/include/linux/memblock.h
>>>> @@ -204,7 +204,7 @@ void __next_mem_pfn_range(int *idx, int nid, unsigned long *out_start_pfn,
>>>> #endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
>>>>
>>>> #ifdef CONFIG_HAVE_ARCH_PFN_VALID
>>>> -unsigned long memblock_next_valid_pfn(unsigned long pfn);
>>>> +unsigned long memblock_next_valid_pfn(unsigned long pfn, int *idx);
>>>> #endif
>>>>
>>>> /**
>>>> diff --git a/mm/memblock.c b/mm/memblock.c
>>>> index bea5a9c..06c1a08 100644
>>>> --- a/mm/memblock.c
>>>> +++ b/mm/memblock.c
>>>> @@ -1102,35 +1102,6 @@ void __init_memblock __next_mem_pfn_range(int *idx, int nid,
>>>> 		*out_nid = r->nid;
>>>> }
>>>>
>>>> -#ifdef CONFIG_HAVE_ARCH_PFN_VALID
>>>> -unsigned long __init_memblock memblock_next_valid_pfn(unsigned long pfn)
>>>> -{
>>>> -	struct memblock_type *type = &memblock.memory;
>>>> -	unsigned int right = type->cnt;
>>>> -	unsigned int mid, left = 0;
>>>> -	phys_addr_t addr = PFN_PHYS(++pfn);
>>>> -
>>>> -	do {
>>>> -		mid = (right + left) / 2;
>>>> -
>>>> -		if (addr < type->regions[mid].base)
>>>> -			right = mid;
>>>> -		else if (addr >= (type->regions[mid].base +
>>>> -				  type->regions[mid].size))
>>>> -			left = mid + 1;
>>>> -		else {
>>>> -			/* addr is within the region, so pfn is valid */
>>>> -			return pfn;
>>>> -		}
>>>> -	} while (left < right);
>>>> -
>>>> -	if (right == type->cnt)
>>>> -		return -1UL;
>>>> -	else
>>>> -		return PHYS_PFN(type->regions[right].base);
>>>> -}
>>>> -#endif /*CONFIG_HAVE_ARCH_PFN_VALID*/
>>>> -
>>>> /**
>>>>    * memblock_set_node - set node ID on memblock regions
>>>>    * @base: base of area to set node ID for
>>>> @@ -1162,6 +1133,50 @@ int __init_memblock memblock_set_node(phys_addr_t base, phys_addr_t size,
>>>> }
>>>> #endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
>>>>
>>>> +#ifdef CONFIG_HAVE_ARCH_PFN_VALID
>>>> +unsigned long __init_memblock memblock_next_valid_pfn(unsigned long pfn,
>>>> +							int *last_idx)
>>>> +{
>>>> +	struct memblock_type *type = &memblock.memory;
>>>> +	unsigned int right = type->cnt;
>>>> +	unsigned int mid, left = 0;
>>>> +	unsigned long start_pfn, end_pfn;
>>>> +	phys_addr_t addr = PFN_PHYS(++pfn);
>>>> +
>>>> +	/* fast path, return pfh+1 if next pfn is in the same region */
>>>                                ^^^  pfn
>> Thanks
>>>> +	if (*last_idx != -1) {
>>>> +		start_pfn = PFN_DOWN(type->regions[*last_idx].base);
>>> To me, it should be PFN_UP().
>> hmm.., seems all the base of memory region is pfn aligned (0x10000 aligned).
>> So
>>
>> PFN_UP is the same as PFN_DOWN here?
>> I got this logic from memblock_search_pfn_nid()
> Ok, I guess here hide some buggy code.
>
> When you look at __next_mem_pfn_range(), it uses PFN_UP() for base. The reason
> is try to clip some un-page-aligned memory. While PFN_DOWN() will introduce
> some unavailable memory to system.
>
> Even mostly those address are page-aligned, we need to be careful for this.
>
> Let me drop a patch to fix the original one.
Ok, please cc me, I will change the related code when your patch is 
accepted. ;-)
>> Cheers,
>> Jia
>>
>>>> +		end_pfn = PFN_DOWN(type->regions[*last_idx].base +
>>>> +				type->regions[*last_idx].size);
>>>> +
>>>> +		if (pfn < end_pfn && pfn > start_pfn)
>>> Could be (pfn < end_pfn && pfn >= start_pfn)?
>>>
>>> pfn == start_pfn is also a valid address.
>> No, pfn=pfn+1 at the beginning, so pfn != start_pfn
> This is a little bit tricky.
>
> There is no requirement to pass a valid pfn to memblock_next_valid_pfn(). So
> suppose we have memory layout like this:
>
>      [0x100, 0x1ff]
>      [0x300, 0x3ff]
>
> And I call memblock_next_valid_pfn(0x2ff, 1), would this fits the fast path
> logic?
>
> Well, since memblock_next_valid_pfn() only used memmap_init_zone(), the
> situation as I mentioned seems will not happen.
>
> Even though, I suggest to chagne this, otherwise your logic in slow path and
> fast path differs. In the case above, your slow path returns 0x300 at last.
ok. looks like it does no harm, even I thought the code will guarantee 
it to skip from 0x1ff
to 0x300 directly.
I will change it after fuctional test.

--
Cheers,
Jia

>>>> +			return pfn;
>>>> +	}
>>>> +
>>>> +	/* slow path, do the binary searching */
>>>> +	do {
>>>> +		mid = (right + left) / 2;
>>>> +
>>>> +		if (addr < type->regions[mid].base)
>>>> +			right = mid;
>>>> +		else if (addr >= (type->regions[mid].base +
>>>> +				  type->regions[mid].size))
>>>> +			left = mid + 1;
>>>> +		else {
>>>> +			*last_idx = mid;
>>>> +			return pfn;
>>>> +		}
>>>> +	} while (left < right);
>>>> +
>>>> +	if (right == type->cnt)
>>>> +		return -1UL;
>>>> +
>>>> +	*last_idx = right;
>>>> +
>>>> +	return PHYS_PFN(type->regions[*last_idx].base);
>>>> +}
>>>> +#endif /*CONFIG_HAVE_ARCH_PFN_VALID*/
>>> The same comment as Daniel, you are moving the function out of
>>> CONFIG_HAVE_MEMBLOCK_NODE_MAP.
>>>> +
>>>> static phys_addr_t __init memblock_alloc_range_nid(phys_addr_t size,
>>>> 					phys_addr_t align, phys_addr_t start,
>>>> 					phys_addr_t end, int nid, ulong flags)
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 2a967f7..0bb0274 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -5459,6 +5459,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>>> 	unsigned long end_pfn = start_pfn + size;
>>>> 	pg_data_t *pgdat = NODE_DATA(nid);
>>>> 	unsigned long pfn;
>>>> +	int idx = -1;
>>>> 	unsigned long nr_initialised = 0;
>>>> 	struct page *page;
>>>> #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
>>>> @@ -5490,7 +5491,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>>> 			 * end_pfn), such that we hit a valid pfn (or end_pfn)
>>>> 			 * on our next iteration of the loop.
>>>> 			 */
>>>> -			pfn = memblock_next_valid_pfn(pfn) - 1;
>>>> +			pfn = memblock_next_valid_pfn(pfn, &idx) - 1;
>>>> #endif
>>>> 			continue;
>>>> 		}
>>>> -- 
>>>> 2.7.4