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

[Wei Yang]

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@xxxxxxxxxxxxxxxx>
>> > ---
>> > 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.

>
>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.

>> 
>> > +			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

-- 
Wei Yang
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