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Subject: Re: [PATCH] mm/vmstat: Add events for PMD based THP migration without
 split
To:     John Hubbard <jhubbard@nvidia.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>
Cc:     linux-mm@kvack.org, hughd@google.com,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Zi Yan <ziy@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
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From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <f88603c5-f9ce-115d-1976-3b0e7558777e@arm.com>
Date:   Tue, 2 Jun 2020 11:00:37 +0530
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On 06/02/2020 10:18 AM, John Hubbard wrote:
> On 2020-06-01 21:20, Anshuman Khandual wrote:
> ...
>>> also important: maybe this patch should also be tracking other causes
>>> of THP PMD migration failure, in order to get a truer accounting of the
>>> situation.
> 
> I hope one of the experts here can weigh in on that...
> 
>> Is there any other failure reasons which are only specific to THP migration.
>> Else, adding stats about generic migration failure reasons will just blur
>> the overall understanding about THP migration successes and failure cases
>> that results in splitting.
>>
> 
> Thinking about that: we do have PGMIGRATE_SUCCESS and PGMIGRATE_FAIL, so I
> suppose comparing those numbers with the new THP_PMD_MIGRATION_SUCCESS and
> THP_PMD_MIGRATION_FAILURE events should cover it.

That is right.

> 
> However, the fact that this is under discussion hints at the need for a
> bit of documentation help. What do you think about adding some notes about
> all of this to, say, Documentation/vm/page_migration.rst ?

Sure but probably bit later. Because I am also planning to add couple of
trace events for THP migration, hence will update the documentation part
for both VM stat and trace events together.