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Subject: Re: [RFC] mm/vmstat: Add events for THP migration without split
To:     John Hubbard <jhubbard@nvidia.com>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Zi Yan <ziy@nvidia.com>, linux-kernel@vger.kernel.org
References: <1589257372-29576-1-git-send-email-anshuman.khandual@arm.com>
 <8be2c517-ba87-400b-dfbe-461bd6065c2d@nvidia.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <9cac0480-210c-28de-259e-c17c88e8c899@arm.com>
Date:   Fri, 15 May 2020 09:33:54 +0530
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On 05/14/2020 11:59 PM, John Hubbard wrote:
> On 2020-05-11 21:22, Anshuman Khandual wrote:
>> Add the following new trace events which will help in validating
>> migration events involving PMD based THP pages.
>> 
>> 1. THP_PMD_MIGRATION_ENTRY_SET 2. THP_PMD_MIGRATION_ENTRY_REMOVE
>> 
>> There are no clear method to confirm whether a THP migration
>> happened with out involving it's split. These trace events along
>> with PGMIGRATE_SUCCESS and PGMIGRATE_FAILURE will provide
>> additional insights. After this change,
>> 
> 
> 
> Hi Anshuman,
> 
> It's very nice to see this work, and I think that reporting a bit
> more about THP migration stats is going to make development and
> performance debugging a lot more efficient (and pleasant).

That is definitely one of the motivations for these events here.

> 
> 
>> A single 2M THP (2K base page) when migrated
>> 
>> 1. Without split
>> 
>> ................ pgmigrate_success 1 pgmigrate_fail 0 
>> ................ thp_pmd_migration_entry_set 1 
>> thp_pmd_migration_entry_remove 1 ................
>> 
> 
> I do think we should decouple the trace event name(s) just a *little*
> more, from the mechanisms used to migrate THPs. In other words, let's
> report the number of THP migration successes, and name it
> accordingly--rather than "set" and "remove", which are pretty
> low-level and furthermore depend on today's exact code.

Agreed, the events are low level and follows the implementation very
closely. Hence posted as a RFC instead, as I was not very sure about
these events.

> 
> Maybe Zi Yan's recommended name is exactly right, in fact:
> 
> THP_PMD_MIGRATION_SUCCESS

Will also add another THP_PMD_MIGRATION_FAILURE even in migrate_pages()
when a huge page could not be allocated and THP gets split.