Re: [RFC V2] mm/vmstat: Add events for HugeTLB migration

[Anshuman Khandual]

On 10/05/2020 11:35 AM, Michal Hocko wrote:
> On Mon 05-10-20 07:59:12, Anshuman Khandual wrote:
>>
>>
>> On 10/02/2020 05:34 PM, Michal Hocko wrote:
>>> On Wed 30-09-20 11:30:49, Anshuman Khandual wrote:
>>>> Add following new vmstat events which will track HugeTLB page migration.
>>>>
>>>> 1. HUGETLB_MIGRATION_SUCCESS
>>>> 2. HUGETLB_MIGRATION_FAILURE
>>>>
>>>> It follows the existing semantics to accommodate HugeTLB subpages in total
>>>> page migration statistics. While here, this updates current trace event
>>>> 'mm_migrate_pages' to accommodate now available HugeTLB based statistics.
>>> What is the actual usecase? And how do you deal with the complexity
>>> introduced by many different hugetlb page sizes. Really, what is the
>>> point to having such a detailed view on hugetlb migration?
>>>
>>
>> It helps differentiate various page migration events i.e normal, THP and
>> HugeTLB and gives us more reliable and accurate measurement. Current stats
>> as per PGMIGRATE_SUCCESS and PGMIGRATE_FAIL are misleading, as they contain
>> both normal and HugeTLB pages as single entities, which is not accurate.
> 
> Yes this is true. But why does it really matter? Do you have a specific
> example?

An example which demonstrates that mixing and misrepresentation in these
stats create some problem ? Well, we could just create one scenario via
an application with different VMA types and triggering some migrations.
But the fact remains, that these stats are inaccurate and misleading
which is very clear and apparent.

> 
>> After this change, PGMIGRATE_SUCCESS and PGMIGRATE_FAIL will contain page
>> migration statistics in terms of normal pages irrespective of whether any
>> previous migrations until that point involved normal pages, THP or HugeTLB
>> (any size) pages. At the least, this fixes existing misleading stats with
>> PGMIGRATE_SUCCESS and PGMIGRATE_FAIL.
>>
>> Besides, it helps us understand HugeTLB migrations in more detail. Even
>> though HugeTLB can be of various sizes on a given platform, these new
>> stats HUGETLB_MIGRATION_SUCCESS and HUGETLB_MIGRATION_FAILURE give enough
>> overall insight into HugeTLB migration events.
> 
> While true this all is way too vague to add yet another imprecise
> counter.

Given that user knows about all HugeTLB mappings it has got, these counters
are not really vague and could easily be related back. Moreover this change
completes the migration stats restructuring which was started with adding
THP counters. Otherwise it remains incomplete.