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

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Date:   Mon, 5 Oct 2020 08:05:42 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, Daniel Jordan <daniel.m.jordan@...cle.com>,
        Zi Yan <ziy@...dia.com>, John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC V2] mm/vmstat: Add events for HugeTLB migration

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?

> 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.

-- 
Michal Hocko
SUSE Labs