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

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Date:   Mon, 28 Sep 2020 15:04:39 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Zi Yan <ziy@...dia.com>, John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm/vmstat: Add events for HugeTLB migration

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);

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.

> +
>  			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?

-- 
Mike Kravetz