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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