linux-kernel - Re: [PATCH V2] mm/vmstat: Add events for THP migration without split

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Date:   Thu, 4 Jun 2020 12:49:11 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Anshuman Khandual <anshuman.khandual@....com>,
        <linux-mm@...ck.org>, <hughd@...gle.com>,
        <daniel.m.jordan@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] mm/vmstat: Add events for THP migration without split

On 4 Jun 2020, at 12:36, Matthew Wilcox wrote:

> On Thu, Jun 04, 2020 at 09:51:10AM -0400, Zi Yan wrote:
>> On 4 Jun 2020, at 7:34, Matthew Wilcox wrote:
>>> On Thu, Jun 04, 2020 at 09:30:45AM +0530, Anshuman Khandual wrote:
>>>> +Quantifying Migration
>>>> +=====================
>>>> +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 could 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 analyzing 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 migrate
>>> 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 each
>>> 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 explained
>>> here too ;-)
>>
>> PGMIGRATE_SUCCESS and PGMIGRATE_FAIL record a combination of different events,
>> so it is not easy to interpret them. Let me try to explain them.
>
> Thanks!  Very helpful explanation.
>
>> 1. migrating only base pages: PGMIGRATE_SUCCESS and PGMIGRATE_FAIL just mean
>> 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 fail 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.
>
> Are you sure about that?  If I split a THP and each of those subpages
> migrates, won't I then see PGMIGRATE_SUCCESS increase by 512?

That is what I mean. I guess my words did not work. I should have used subpages.

>
>> 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 many 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 number and we
>> cannot tell three cases apart.
>
> So why don't we just expose PGMIGRATE_SPLIT?  That would be defined as
> the number of times we succeeded in migrating a THP but had to split it
> to succeed.

It might need extra code to get that number. Currently, the subpages from split
THPs are appended to the end of the original page list, so we might need a separate
page list for these subpages to count PGMIGRATE_SPLIT. Also what if some of the
subpages fail to migrate? Do we increase PGMIGRATE_SPLIT or not?



--
Best Regards,
Yan Zi

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