Re: [PATCH] mm: compaction: skip memory hole rapidly when isolating migratable pages

["Huang, Ying" <ying.huang@intel.com>]

Baolin Wang <baolin.wang@linux.alibaba.com> writes:

> On 6/12/2023 2:39 PM, Huang, Ying wrote:
>> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
>> 
>>> On some machines, the normal zone can have a large memory hole like
>>> below memory layout, and we can see the range from 0x100000000 to
>>> 0x1800000000 is a hole. So when isolating some migratable pages, the
>>> scanner can meet the hole and it will take more time to skip the large
>>> hole. From my measurement, I can see the isolation scanner will take
>>> 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000].
>>>
>>> So adding a new helper to fast search next online memory section
>>> to skip the large hole can help to find next suitable pageblock
>>> efficiently. With this patch, I can see the large hole scanning only
>>> takes < 1us.
>>>
>>> [    0.000000] Zone ranges:
>>> [    0.000000]   DMA      [mem 0x0000000040000000-0x00000000ffffffff]
>>> [    0.000000]   DMA32    empty
>>> [    0.000000]   Normal   [mem 0x0000000100000000-0x0000001fa7ffffff]
>>> [    0.000000] Movable zone start for each node
>>> [    0.000000] Early memory node ranges
>>> [    0.000000]   node   0: [mem 0x0000000040000000-0x0000000fffffffff]
>>> [    0.000000]   node   0: [mem 0x0000001800000000-0x0000001fa3c7ffff]
>>> [    0.000000]   node   0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff]
>>> [    0.000000]   node   0: [mem 0x0000001fa4000000-0x0000001fa402ffff]
>>> [    0.000000]   node   0: [mem 0x0000001fa4030000-0x0000001fa40effff]
>>> [    0.000000]   node   0: [mem 0x0000001fa40f0000-0x0000001fa73cffff]
>>> [    0.000000]   node   0: [mem 0x0000001fa73d0000-0x0000001fa745ffff]
>>> [    0.000000]   node   0: [mem 0x0000001fa7460000-0x0000001fa746ffff]
>>> [    0.000000]   node   0: [mem 0x0000001fa7470000-0x0000001fa758ffff]
>>> [    0.000000]   node   0: [mem 0x0000001fa7590000-0x0000001fa7ffffff]
>>>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>>   include/linux/mmzone.h | 10 ++++++++++
>>>   mm/compaction.c        | 23 ++++++++++++++++++++++-
>>>   2 files changed, 32 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index 5a7ada0413da..87e6c535d895 100644
>>> --- a/include/linux/mmzone.h
>>> +++ b/include/linux/mmzone.h
>>> @@ -2000,6 +2000,16 @@ static inline unsigned long next_present_section_nr(unsigned long section_nr)
>>>   	return -1;
>>>   }
>>>   +static inline unsigned long next_online_section_nr(unsigned long
>>> section_nr)
>>> +{
>>> +	while (++section_nr <= __highest_present_section_nr) {
>>> +		if (online_section_nr(section_nr))
>>> +			return section_nr;
>>> +	}
>>> +
>>> +	return -1UL;
>>> +}
>>> +
>>>   /*
>>>    * These are _only_ used during initialisation, therefore they
>>>    * can use __initdata ...  They could have names to indicate
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 3398ef3a55fe..3a55fdd20c49 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -229,6 +229,21 @@ static void reset_cached_positions(struct zone *zone)
>>>   				pageblock_start_pfn(zone_end_pfn(zone) - 1);
>>>   }
>>>   +static unsigned long skip_hole_pageblock(unsigned long
>>> start_pfn)
>>> +{
>>> +	unsigned long next_online_nr;
>>> +	unsigned long start_nr = pfn_to_section_nr(start_pfn);
>>> +
>>> +	if (online_section_nr(start_nr))
>>> +		return -1UL;
>> Define a macro for the maigic "-1UL"?  Which is used for multiple
>> times
>> in the patch.
>
> I am struggling to find a readable macro for these '-1UL', since the
> '-1UL' in next_online_section_nr() indicates that it can not find an 
> online section. However the '-1' in skip_hole_pageblock() indicates
> that it can not find an online pfn.
>
> So after more thinking, I will change to return 'NR_MEM_SECTIONS' if
> can not find next online section in next_online_section_nr(). And in 
> skip_hole_pageblock(), I will change to return 0 if can not find next
> online pfn. What do you think?
>
> static unsigned long skip_hole_pageblock(unsigned long start_pfn)
> {
>         unsigned long next_online_nr;
>         unsigned long start_nr = pfn_to_section_nr(start_pfn);
>
>         if (online_section_nr(start_nr))
>                 return 0;
>
>         next_online_nr = next_online_section_nr(start_nr);
>         if (next_online_nr < NR_MEM_SECTIONS)
>                 return section_nr_to_pfn(next_online_nr);
>
>         return 0;
> }

Sounds good to me.

Best Regards,
Huang, Ying

>>> +
>>> +	next_online_nr = next_online_section_nr(start_nr);
>>> +	if (next_online_nr != -1UL)
>>> +		return section_nr_to_pfn(next_online_nr);
>>> +
>>> +	return -1UL;
>>> +}
>>> +
>>>   /*
>>>    * Compound pages of >= pageblock_order should consistently be skipped until
>>>    * released. It is always pointless to compact pages of such order (if they are
>>> @@ -1991,8 +2006,14 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
>>>     		page = pageblock_pfn_to_page(block_start_pfn,
>>>   						block_end_pfn, cc->zone);
>>> -		if (!page)
>>> +		if (!page) {
>>> +			unsigned long next_pfn;
>>> +
>>> +			next_pfn = skip_hole_pageblock(block_start_pfn);
>>> +			if (next_pfn != -1UL)
>>> +				block_end_pfn = next_pfn;
>>>   			continue;
>>> +		}
>>>     		/*
>>>   		 * If isolation recently failed, do not retry. Only check the
>> Do we need to do similar change in isolate_freepages()?
>
> Yes, it's in my todo list with some measurement data.
>
> Thanks for your comments.