Re: [PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 6/13/2023 5:56 PM, David Hildenbrand wrote:
> On 13.06.23 10:55, Baolin Wang wrote:
>> 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>
>> ---
>> Changes from v1:
>>   - Fix building errors if CONFIG_SPARSEMEM is not selected.
>>   - Use NR_MEM_SECTIONS instead of '-1' per Huang Ying.
>> ---
>>   include/linux/mmzone.h | 10 ++++++++++
>>   mm/compaction.c        | 30 +++++++++++++++++++++++++++++-
>>   2 files changed, 39 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 5a7ada0413da..5ff1fa2efe28 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 NR_MEM_SECTIONS;
>> +}
>> +
>>   /*
>>    * 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..c31ff6123891 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -229,6 +229,28 @@ static void reset_cached_positions(struct zone 
>> *zone)
>>                   pageblock_start_pfn(zone_end_pfn(zone) - 1);
>>   }
>> +#ifdef CONFIG_SPARSEMEM
>> +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);
>> +
> 
> I would simply inline next_online_section_nr and simplify (and add a 
> comment):
> 
> /*
>   * If the PFN falls into an offline section, return the start PFN of the
>   * next online section. If the PFN falls into an online section or if
>   * there is no next online section, return 0.
>   */
> static unsigned long skip_hole_pageblock(unsigned long start_pfn)
> {
>      unsigned long nr = pfn_to_section_nr(start_pfn);
> 
>      if (online_section_nr(nr))
>          return 0;
> 
>      while (++nr <= __highest_present_section_nr) {
>          if (online_section_nr(nr))
>              return section_nr_to_pfn(nr);
>      }
>      return 0
> }
> 
> Easier, no?

Originally I want to add a common helper like next_present_section_nr(), 
which can be used by other users. But yes, your suggestion is easier, 
and I am fine with that.

> And maybe just call that function "skip_offline_sections()" then? 
> Because we're not operating on pageblocks.

OK. Thanks.