Date:   Tue, 9 Jun 2020 19:06:10 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
CC:     Matthew Wilcox <willy@...radead.org>, <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 9 Jun 2020, at 7:35, Anshuman Khandual wrote:

> On 06/05/2020 07:54 PM, Zi Yan wrote:
>> On 4 Jun 2020, at 23:35, Anshuman Khandual wrote:
>>
>>> On 06/04/2020 10:19 PM, Zi Yan wrote:
>>>> 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?
>>>
>>> Thanks Zi, for such a detailed explanation. Ideally, we should separate THP
>>> migration from base page migration in terms of statistics. PGMIGRATE_SUCCESS
>>> and PGMIGRATE_FAIL should continue to track statistics when migration starts
>>> with base pages. But for THP, we should track the following events.
>>
>> You mean PGMIGRATE_SUCCESS and PGMIGRATE_FAIL will not track the number of migrated subpages
>> from split THPs? Will it cause userspace issues since their semantics are changed?
>
> Yeah, basic idea is to carve out all THP migration related statistics from
> the normal page migration. Not sure if that might cause any issue for user
> space as you have mentioned. Does /proc/vmstat indicate some sort of an ABI
> whose semantics can not really change ? Some more opinions here from others
> would be helpful.
>
> The current situation is definitely bit problematic where PGMIGRATE_SUCCESS
> increments (+1) both for normal and successful THP migration. Same situation
> for PGMIGRATE_FAILURE as well. Hence, there are two clear choices available.
>
> 1. THP and normal page migration stats are separate and mutually exclusive
>
> OR
>
> 2. THP migration has specific counters but normal page migration counters
>    still account for everything in THP migration in terms of normal pages
>
> But IIUC, either way the current PGMIGRATE_SUCCESS or PGMIGRATE_FAIL stats
> will change for the user as visible from /proc/vmstat.

Why? In the case 2, PGMIGRATE_SUCCESS and PGMIGRATE_FAIL would remain the same
as today, right?

>
>>
>>>
>>> 1. THP_MIGRATION_SUCCESS 	- THP migration is successful, without split
>>> 2. THP_MIGRATION_FAILURE 	- THP could neither be migrated, nor be split
>>
>> They make sense to me.>
>>> 3. THP_MIGRATION_SPLIT_SUCCESS  - THP got split and all sub pages migrated
>>> 4. THP_MIGRATION_SPLIT_FAILURE  - THP got split but all sub pages could not be migrated
>>>
>>> THP_MIGRATION_SPLIT_FAILURE could either increment once for a single THP or
>>> number of subpages that did not get migrated after split. As you mentioned,
>>> this will need some extra code in the core migration. Nonetheless, if these
>>> new events look good, will be happy to make required changes.
>>
>> Maybe THP_MIGRATION_SPLIT would be simpler? My concern is that whether we need such
>
> Also, it will not require a new migration queue tracking the split THP sub pages.
>
>> detailed information or not. Maybe trace points would be good enough for 3 and 4.
>
> But without a separate queue for split THP subpages, will it be possible to track
> (3) and (4) through trace events ? Just wondering, where are the intercept points.

Not easily. You could get a trace of the migration result and the physical address
of individual page, whether each is a THP or not, and the split of a THP.
You then can see which THP is split and what is the migration result of its subpages.
It is more analysis effort but less kernel code.

>
>> But if you think it is useful to you, feel free to implement them.
>
> Original idea was that, all stats here should give high level view but new upcoming
> trace events should help in details like which particular subpages could not be
> migrated resulting in a THP_MIGRATION_SPLIT_FAILURE increment etc.
>
>>
>> BTW, in terms of stats tracking, what do you think of my patch below? I am trying to
>> aggregate all stats counting in one place. Feel free to use it if you think it works
>> for you.
>
> Assume that, I could take the liberty to add your 'Signed-off-by' in case end up
> using this code chunk below. This seems to be going with option (2) as mentioned
> before.

Feel free to do so.

>
>>
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 7bfd0962149e..0f3c60470489 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1429,9 +1429,14 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>                 enum migrate_mode mode, int reason)
>>  {
>>         int retry = 1;
>> +       int thp_retry = 1;
>>         int nr_failed = 0;
>> +       int nr_thp_failed = 0;
>> +       int nr_thp_split = 0;
>>         int nr_succeeded = 0;
>> +       int nr_thp_succeeded = 0;
>>         int pass = 0;
>> +       bool is_thp = false;
>>         struct page *page;
>>         struct page *page2;
>>         int swapwrite = current->flags & PF_SWAPWRITE;
>> @@ -1440,11 +1445,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>         if (!swapwrite)
>>                 current->flags |= PF_SWAPWRITE;
>>
>> -       for(pass = 0; pass < 10 && retry; pass++) {
>> +       for(pass = 0; pass < 10 && (retry || thp_retry); pass++) {
>
> 'thp_retry' check might not be necessary here as 'retry' already
> contains 'thp_retry'.

Right.
>
>>                 retry = 0;
>> +               thp_retry = 0;
>>
>>                 list_for_each_entry_safe(page, page2, from, lru) {
>>  retry:
>> +                       is_thp = PageTransHuge(page);
>>                         cond_resched();
>>
>>                         if (PageHuge(page))
>> @@ -1475,15 +1482,20 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>                                         unlock_page(page);
>>                                         if (!rc) {
>>                                                 list_safe_reset_next(page, page2, lru);
>> +                                               nr_thp_split++;
>>                                                 goto retry;
>>                                         }
>>                                 }
>
> Check 'if_thp' and increment 'nr_thp_failed' like 'default' case and
> also increment nr_failed by (1 << order) for the THP being migrated.

Right. hpage_nr_pages() would be better.

>
>
>>                                 nr_failed++;
>>                                 goto out;
>>                         case -EAGAIN:
>> +                               if (is_thp)
>> +                                       thp_retry++;
>>                                 retry++;
>>                                 break;
>>                         case MIGRATEPAGE_SUCCESS:
>> +                               if (is_thp)
>> +                                       nr_thp_succeeded++;
>
> Increment nr_succeeded by (1 << order) for the THP being migrated.
Ditto.

>
>>                                 nr_succeeded++;
>>                                 break;
>>                         default:
>> @@ -1493,18 +1505,27 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>                                  * removed from migration page list and not
>>                                  * retried in the next outer loop.
>>                                  */
>> +                               if (is_thp)
>> +                                       nr_thp_failed++;
>
> Increment nr_failed by (1 << order) for the THP being migrated.
Ditto.
>
>>                                 nr_failed++;
>>                                 break;
>>                         }
>>                 }
>>         }
>>         nr_failed += retry;
>> +       nr_thp_failed += thp_retry;
>>         rc = nr_failed;
>
> Right, nr_failed already contains nr_thp_failed. Hence need not change.
>
>>         if (nr_succeeded)
>>                 count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
>>         if (nr_failed)
>>                 count_vm_events(PGMIGRATE_FAIL, nr_failed);
>> +       if (nr_thp_succeeded)
>> +               count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
>> +       if (nr_thp_failed)
>> +               count_vm_events(THP_MIGRATION_FAIL, nr_thp_failed);
>> +       if (nr_thp_split)
>> +               count_vm_events(THP_MIGRATION_SPLIT, nr_thp_split);
>>         trace_mm_migrate_pages(nr_succeeded, nr_failed, mode, reason);
>
> This existing trace event should add stats for THP migration as well.

Sure.
>
>>
>>         if (!swapwrite)
>>
>
> Regardless, this change set (may be with some more modifications), makes sense if
> we decide to go with option (2), where existing normal page migration stats will
> cover THP related stats as well. But it will be really great, if we get some more
> opinions on this. Meanwhile, will continue looking into it further.

Thank you for working on this. :)

—
Best Regards,
Yan Zi

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