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From:   Zi Yan <ziy@nvidia.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     Anshuman Khandual <anshuman.khandual@arm.com>,
        <linux-mm@kvack.org>, <hughd@google.com>,
        <daniel.m.jordan@oracle.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        John Hubbard <jhubbard@nvidia.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V2] mm/vmstat: Add events for THP migration without split
Date:   Thu, 4 Jun 2020 09:51:10 -0400
Message-ID: <CBF71911-6BB7-4AA7-AC0F-95AADBB45569@nvidia.com>
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On 4 Jun 2020, at 7:34, Matthew Wilcox wrote:

> On Thu, Jun 04, 2020 at 09:30:45AM +0530, Anshuman Khandual wrote:
>> Add the following new VM events which will help in validating THP migr=
ation
>> without split. Statistics reported through these new events will help =
in
>> performance debugging.
>>
>> 1. THP_MIGRATION_SUCCESS
>> 2. THP_MIGRATION_FAILURE
>>
>> THP_MIGRATION_FAILURE in particular represents an event when a THP cou=
ld
>> not be migrated as a single entity following an allocation failure and=

>> ended up getting split into constituent normal pages before being retr=
ied.
>> This event, along with PGMIGRATE_SUCCESS and PGMIGRATE_FAIL will help =
in
>> quantifying and analyzing THP migration events including both success =
and
>> failure cases.
>
>> +Quantifying Migration
>> +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>> +Following events can be used to quantify page migration.
>> +
>> +- PGMIGRATE_SUCCESS
>> +- PGMIGRATE_FAIL
>> +- THP_MIGRATION_SUCCESS
>> +- THP_MIGRATION_FAILURE
>> +
>> +THP_MIGRATION_FAILURE in particular represents an event when a THP co=
uld not be
>> +migrated as a single entity following an allocation failure and ended=
 up getting
>> +split into constituent normal pages before being retried. This event,=
 along with
>> +PGMIGRATE_SUCCESS and PGMIGRATE_FAIL will help in quantifying and ana=
lyzing THP
>> +migration events including both success and failure cases.
>
> First, I'd suggest running this paragraph through 'fmt'.  That way you
> don't have to care about line lengths.
>
> Second, this paragraph doesn't really explain what I need to know to
> understand the meaning of these numbers.  When Linux attempts to migrat=
e
> a THP, one of three things can happen:
>
>  - It is migrated as a single THP
>  - It is migrated, but had to be split
>  - Migration fails
>
> How do I turn these four numbers into an understanding of how often eac=
h
> of those three situations happen?  And why do we need four numbers to
> report three situations?
>
> Or is there something else that can happen?  If so, I'd like that expla=
ined
> here too ;-)

PGMIGRATE_SUCCESS and PGMIGRATE_FAIL record a combination of different ev=
ents,
so it is not easy to interpret them. Let me try to explain them.

1. migrating only base pages: PGMIGRATE_SUCCESS and PGMIGRATE_FAIL just m=
ean
these base pages are migrated and fail to migrate respectively.
THP_MIGRATION_SUCCESS and THP_MIGRATION_FAILURE should be 0 in this case.=

Simple.

2. migrating only THPs:
	- PGMIGRATE_SUCCESS means THPs that are migrated and base pages
	(from the split of THPs) that are migrated,

	- PGMIGRATE_FAIL means THPs that fail to migrate and base pages that fai=
l to migrated.

	- THP_MIGRATION_SUCCESS means THPs that are migrated.

	- THP_MIGRATION_FAILURE means THPs that are split.

So PGMIGRATE_SUCCESS - THP_MIGRATION_SUCCESS means the number of migrated=
 base pages,
which are from the split of THPs.

When it comes to analyze failed migration, PGMIGRATE_FAIL - THP_MIGRATION=
_FAILURE
means the number of pages that are failed to migrate, but we cannot tell =
how many
are base pages and how many are THPs.

3. migrating base pages and THP:

The math should be very similar to the second case, except that
a) from PGMIGRATE_SUCCESS - THP_MIGRATION_SUCCESS, we cannot tell how man=
y are pages begin
as base pages and how many are pages begin as THPs but become base pages =
after split;
b) from PGMIGRATE_FAIL - THP_MIGRATION_FAILURE, an additional case,
base pages that begin as base pages fail to migrate, is mixed into the nu=
mber and we
cannot tell three cases apart.


=E2=80=94
Best Regards,
Yan Zi

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