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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 09/29/2020 03:34 AM, Mike Kravetz wrote:
> On 9/25/20 2:12 AM, 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.
> 
> Thanks.  This makes sense with recent THP changes.
> 
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 3ab965f83029..d53dd101ffff 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1415,13 +1415,17 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>  {
>>  	int retry = 1;
>>  	int thp_retry = 1;
>> +	int hugetlb_retry = 1;
>>  	int nr_failed = 0;
>>  	int nr_succeeded = 0;
>>  	int nr_thp_succeeded = 0;
>>  	int nr_thp_failed = 0;
>>  	int nr_thp_split = 0;
>> +	int nr_hugetlb_succeeded = 0;
>> +	int nr_hugetlb_failed = 0;
>>  	int pass = 0;
>>  	bool is_thp = false;
>> +	bool is_hugetlb = false;
>>  	struct page *page;
>>  	struct page *page2;
>>  	int swapwrite = current->flags & PF_SWAPWRITE;
>> @@ -1433,6 +1437,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>  	for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
>>  		retry = 0;
>>  		thp_retry = 0;
>> +		hugetlb_retry = 0;
>>  
>>  		list_for_each_entry_safe(page, page2, from, lru) {
>>  retry:
>> @@ -1442,7 +1447,12 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>  			 * during migration.
>>  			 */
>>  			is_thp = PageTransHuge(page) && !PageHuge(page);
>> +			is_hugetlb = PageTransHuge(page) && PageHuge(page);
> 
> PageHuge does not depend on PageTransHuge.  So, this could just be
> 			is_hugetlb = PageHuge(page);

Sure.

> 
> Actually, the current version of PageHuge is more expensive than PageTransHuge.
> So, the most optimal way to set these would be something like.
> 			if (PageTransHuge(page))
> 				if (PageHuge(page))
> 					is_hugetlb = true;
> 				else
> 					is_thp = true;
> 
> Although, the compiler may be able to optimize.  I did not check.

Both is_hugetlb and is_thp need to have either a true or false value
during each iteration as they are not getting reset otherwise. Hence
basically it should either be

is_thp = PageTransHuge(page) && !PageHuge(page);
is_hugetlb = PageHuge(page);

OR

is_hugetlb = false;
is_thp = false;
if (PageTransHuge(page))
	if (PageHuge(page))
		is_hugetlb = true;
	else
		is_thp = true;
}

> 
>> +
>>  			nr_subpages = thp_nr_pages(page);
>> +			if (is_hugetlb)
>> +				nr_subpages = pages_per_huge_page(page_hstate(page));
> 
> Can we just use compound_order() here for all cases?

Sure but we could also directly use compound_nr().