linux-kernel - Re: [PATCH] mm/vmstat: Add events for PMD based THP migration without split

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Message-ID: <274be3bf-2206-5e98-8114-1d91fac472c9@nvidia.com>
Date:   Mon, 1 Jun 2020 21:48:09 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
CC:     <linux-mm@...ck.org>, <hughd@...gle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <ziy@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/vmstat: Add events for PMD based THP migration without
 split

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.

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 ?

thanks,
-- 
John Hubbard
NVIDIA