Re: [PATCH v2 00/12] mm: page migration enhancement for thp

[Balbir Singh]

On 09/11/16 15:59, Naoya Horiguchi wrote:
> On Wed, Nov 09, 2016 at 01:32:04PM +1100, Balbir Singh wrote:
>> On 08/11/16 10:31, Naoya Horiguchi wrote:
>>> Hi everyone,
>>>
>>> I've updated thp migration patches for v4.9-rc2-mmotm-2016-10-27-18-27
>>> with feedbacks for ver.1.
>>>
>>> General description (no change since ver.1)
>>> ===========================================
>>>
>>> This patchset enhances page migration functionality to handle thp migration
>>> for various page migration's callers:
>>>  - mbind(2)
>>>  - move_pages(2)
>>>  - migrate_pages(2)
>>>  - cgroup/cpuset migration
>>>  - memory hotremove
>>>  - soft offline
>>>
>>> The main benefit is that we can avoid unnecessary thp splits, which helps us
>>> avoid performance decrease when your applications handles NUMA optimization on
>>> their own.
>>>
>>> The implementation is similar to that of normal page migration, the key point
>>> is that we modify a pmd to a pmd migration entry in swap-entry like format.
>>>
>>> Changes / Notes
>>> ===============
>>>
>>> - pmd_present() in x86 checks _PAGE_PRESENT, _PAGE_PROTNONE and _PAGE_PSE
>>>   bits together, which makes implementing thp migration a bit hard because
>>>   _PAGE_PSE bit is currently used by soft-dirty in swap-entry format.
>>>   I was advised to dropping _PAGE_PSE in pmd_present(), but I don't think
>>>   of the justification, so I keep it in this version. Instead, my approach
>>>   is to move _PAGE_SWP_SOFT_DIRTY to bit 6 (unused) and reserve bit 7 for
>>>   pmd non-present cases.
>>
>> Thanks, IIRC
>>
>> pmd_present = _PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE
>>
>> AutoNUMA balancing would change it to
>>
>> pmd_present = _PAGE_PROTNONE | _PAGE_PSE
>>
>> and PMD_SWP_SOFT_DIRTY would make it
>>
>> pmd_present = _PAGE_PSE
>>
>> What you seem to be suggesting in your comment is that
>>
>> pmd_present should be _PAGE_PRESENT | _PAGE_PROTNONE
> 
> This (no _PAGE_PSE) was a possibile solution, and as I described I gave up
> this solution, because I noticed that what I actually wanted was that
> pmd_present() certainly returns false during thp migration and that's done
> by moving _PAGE_SWP_SOFT_DIRTY. So
> 
>   pmd_present = _PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE
> 
> is still correct in this patchset.
> 

Thanks, I was wondering if there is any advantage or you felt its
safer not to change pmd_present().

>>
>> Isn't that good enough?
>>
>> For THP migration I guess we use
>>
>> _PAGE_PRESENT | _PAGE_PROTNONE | is_migration_entry(pmd)
> 
> Though I might misread your notations, I hope that the following code
> seems describe itself well.
> 
>   static inline int is_pmd_migration_entry(pmd_t pmd)                            
>   {                                                                              
>           return !pmd_present(pmd) && is_migration_entry(pmd_to_swp_entry(pmd)); 
>   }                                                                              
> 

Thanks, yes my notation is not the best.



Balbir Singh.