Re: [RFC PATCH v1] tools/mm: Add thpmaps script to dump THP usage info

David Hildenbrand
2024-01-11T13:18:40Z

On 11.01.24 13:25, Ryan Roberts wrote:
> On 10/01/2024 22:14, Barry Song wrote:
>> On Wed, Jan 10, 2024 at 7:59 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>
>>> On 10/01/2024 11:38, Barry Song wrote:
>>>> On Wed, Jan 10, 2024 at 7:21 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>
>>>>> On 10/01/2024 11:00, David Hildenbrand wrote:
>>>>>> On 10.01.24 11:55, Ryan Roberts wrote:
>>>>>>> On 10/01/2024 10:42, David Hildenbrand wrote:
>>>>>>>> On 10.01.24 11:38, Ryan Roberts wrote:
>>>>>>>>> On 10/01/2024 10:30, Barry Song wrote:
>>>>>>>>>> On Wed, Jan 10, 2024 at 6:23 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>> On 10/01/2024 09:09, Barry Song wrote:
>>>>>>>>>>>> On Wed, Jan 10, 2024 at 4:58 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> On 10/01/2024 08:02, Barry Song wrote:
>>>>>>>>>>>>>> On Wed, Jan 10, 2024 at 12:16 PM John Hubbard <jhubbard@nvidia.com> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On 1/9/24 19:51, Barry Song wrote:
>>>>>>>>>>>>>>>> On Wed, Jan 10, 2024 at 11:35 AM John Hubbard <jhubbard@nvidia.com>
>>>>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>> ...
>>>>>>>>>>>>>>>>> Hi Ryan,
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> One thing that immediately came up during some recent testing of mTHP
>>>>>>>>>>>>>>>>> on arm64: the pid requirement is sometimes a little awkward. I'm
>>>>>>>>>>>>>>>>> running
>>>>>>>>>>>>>>>>> tests on a machine at a time for now, inside various containers and
>>>>>>>>>>>>>>>>> such, and it would be nice if there were an easy way to get some
>>>>>>>>>>>>>>>>> numbers
>>>>>>>>>>>>>>>>> for the mTHPs across the whole machine.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Just to confirm, you're expecting these "global" stats be truely global
>>>>>>>>>>>>> and not
>>>>>>>>>>>>> per-container? (asking because you exploicitly mentioned being in a
>>>>>>>>>>>>> container).
>>>>>>>>>>>>> If you want per-container, then you can probably just create the container
>>>>>>>>>>>>> in a
>>>>>>>>>>>>> cgroup?
>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I'm not sure if that changes anything about thpmaps here. Probably
>>>>>>>>>>>>>>>>> this is fine as-is. But I wanted to give some initial reactions from
>>>>>>>>>>>>>>>>> just some quick runs: the global state would be convenient.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thanks for taking this for a spin! Appreciate the feedback.
>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> +1. but this seems to be impossible by scanning pagemap?
>>>>>>>>>>>>>>>> so may we add this statistics information in kernel just like
>>>>>>>>>>>>>>>> /proc/meminfo or a separate /proc/mthp_info?
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Yes. From my perspective, it looks like the global stats are more useful
>>>>>>>>>>>>>>> initially, and the more detailed per-pid or per-cgroup stats are the
>>>>>>>>>>>>>>> next level of investigation. So feels odd to start with the more
>>>>>>>>>>>>>>> detailed stats.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> probably because this can be done without the modification of the kernel.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Yes indeed, as John said in an earlier thread, my previous attempts to add
>>>>>>>>>>>>> stats
>>>>>>>>>>>>> directly in the kernel got pushback; DavidH was concerned that we don't
>>>>>>>>>>>>> really
>>>>>>>>>>>>> know exectly how to account mTHPs yet
>>>>>>>>>>>>> (whole/partial/aligned/unaligned/per-size/etc) so didn't want to end up
>>>>>>>>>>>>> adding
>>>>>>>>>>>>> the wrong ABI and having to maintain it forever. There has also been some
>>>>>>>>>>>>> pushback regarding adding more values to multi-value files in sysfs, so
>>>>>>>>>>>>> David
>>>>>>>>>>>>> was suggesting coming up with a whole new scheme at some point (I know
>>>>>>>>>>>>> /proc/meminfo isn't sysfs, but the equivalent files for NUMA nodes and
>>>>>>>>>>>>> cgroups
>>>>>>>>>>>>> do live in sysfs).
>>>>>>>>>>>>>
>>>>>>>>>>>>> Anyway, this script was my attempt to 1) provide a short term solution
>>>>>>>>>>>>> to the
>>>>>>>>>>>>> "we need some stats" request and 2) provide a context in which to explore
>>>>>>>>>>>>> what
>>>>>>>>>>>>> the right stats are - this script can evolve without the ABI problem.
>>>>>>>>>>>>>
>>>>>>>>>>>>>> The detailed per-pid or per-cgroup is still quite useful to my case in
>>>>>>>>>>>>>> which
>>>>>>>>>>>>>> we set mTHP enabled/disabled and allowed sizes according to vma types,
>>>>>>>>>>>>>> eg. libc_malloc, java heaps etc.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Different vma types can have different anon_name. So I can use the
>>>>>>>>>>>>>> detailed
>>>>>>>>>>>>>> info to find out if specific VMAs have gotten mTHP properly and how many
>>>>>>>>>>>>>> they have gotten.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> However, Ryan did clearly say, above, "In future we may wish to
>>>>>>>>>>>>>>> introduce stats directly into the kernel (e.g. smaps or similar)". And
>>>>>>>>>>>>>>> earlier he ran into some pushback on trying to set up /proc or /sys
>>>>>>>>>>>>>>> values because this is still such an early feature.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I wonder if we could put the global stats in debugfs for now? That's
>>>>>>>>>>>>>>> specifically supposed to be a "we promise *not* to keep this ABI stable"
>>>>>>>>>>>>>>> location.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Now that I think about it, I wonder if we can add a --global mode to the
>>>>>>>>>>>>> script
>>>>>>>>>>>>> (or just infer global when neither --pid nor --cgroup are provided). I
>>>>>>>>>>>>> think I
>>>>>>>>>>>>> should be able to determine all the physical memory ranges from
>>>>>>>>>>>>> /proc/iomem,
>>>>>>>>>>>>> then grab all the info we need from /proc/kpageflags. We should then be
>>>>>>>>>>>>> able to
>>>>>>>>>>>>> process it all in much the same way as for --pid/--cgroup and provide the
>>>>>>>>>>>>> same
>>>>>>>>>>>>> stats, but it will apply globally. What do you think?
>>>>>>>>>>>
>>>>>>>>>>> Having now thought about this for a few mins (in the shower, if anyone wants
>>>>>>>>>>> the
>>>>>>>>>>> complete picture :) ), this won't quite work. This approach doesn't have the
>>>>>>>>>>> virtual mapping information so the best it can do is tell us "how many of
>>>>>>>>>>> each
>>>>>>>>>>> size of THP are allocated?" - it doesn't tell us anything about whether they
>>>>>>>>>>> are
>>>>>>>>>>> fully or partially mapped or what their alignment is (all necessary if we
>>>>>>>>>>> want
>>>>>>>>>>> to know if they are contpte-mapped). So I don't think this approach is
>>>>>>>>>>> going to
>>>>>>>>>>> be particularly useful.
>>>>>>>>>>>
>>>>>>>>>>> And this is also the big problem if we want to gather stats inside the
>>>>>>>>>>> kernel;
>>>>>>>>>>> if we want something equivalant to /proc/meminfo's
>>>>>>>>>>> AnonHugePages/ShmemPmdMapped/FilePmdMapped, we need to consider not just the
>>>>>>>>>>> allocation of the THP but also whether it is mapped. That's easy for
>>>>>>>>>>> PMD-mappings, because there is only one entry to consider - when you set it,
>>>>>>>>>>> you
>>>>>>>>>>> increment the number of PMD-mapped THPs, when you clear it, you decrement.
>>>>>>>>>>> But
>>>>>>>>>>> for PTE-mappings it's harder; you know the size when you are mapping so its
>>>>>>>>>>> easy
>>>>>>>>>>> to increment, but you can do a partial unmap, so you would need to scan the
>>>>>>>>>>> PTEs
>>>>>>>>>>> to figure out if we are unmapping the first page of a previously
>>>>>>>>>>> fully-PTE-mapped THP, which is expensive. We would need a cheap mechanism to
>>>>>>>>>>> determine "is this folio fully and contiguously mapped in at least one
>>>>>>>>>>> process?".
>>>>>>>>>>
>>>>>>>>>> as OPPO's approach I shared to you before is maintaining two mapcount
>>>>>>>>>> 1. entire map
>>>>>>>>>> 2. subpage's map
>>>>>>>>>> 3. if 1 and 2 both exist, it is DoubleMapped.
>>>>>>>>>>
>>>>>>>>>> This isn't a problem for us. and everytime if we do a partial unmap,
>>>>>>>>>> we have an explicit
>>>>>>>>>> cont_pte split which will decrease the entire map and increase the
>>>>>>>>>> subpage's mapcount.
>>>>>>>>>>
>>>>>>>>>> but its downside is that we expose this info to mm-core.
>>>>>>>>>
>>>>>>>>> OK, but I think we have a slightly more generic situation going on with the
>>>>>>>>> upstream; If I've understood correctly, you are using the PTE_CONT bit in the
>>>>>>>>> PTE to determne if its fully mapped? That works for your case where you only
>>>>>>>>> have 1 size of THP that you care about (contpte-size). But for the upstream, we
>>>>>>>>> have multi-size THP so we can't use the PTE_CONT bit to determine if its fully
>>>>>>>>> mapped because we can only use that bit if the THP is at least 64K and aligned,
>>>>>>>>> and only on arm64. We would need a SW bit for this purpose, and the mm would
>>>>>>>>> need to update that SW bit for every PTE one the full -> partial map
>>>>>>>>> transition.
>>>>>>>>
>>>>>>>> Oh no. Let's not make everything more complicated for the purpose of some stats.
>>>>>>>>
>>>>>>>
>>>>>>> Indeed, I was intending to argue *against* doing it this way. Fundamentally, if
>>>>>>> we want to know what's fully mapped and what's not, then I don't see any way
>>>>>>> other than by scanning the page tables and we might as well do that in user
>>>>>>> space with this script.
>>>>>>>
>>>>>>> Although, I expect you will shortly make a proposal that is simple to implement
>>>>>>> and prove me wrong ;-)
>>>>>>
>>>>>> Unlikely :) As you said, once you have multiple folio sizes, it stops really
>>>>>> making sense.
>>>>>>
>>>>>> Assume you have a 128 kiB pageache folio, and half of that is mapped. You can
>>>>>> set cont-pte bits on that half and all is fine. Or AMD can benefit from it's
>>>>>> optimizations without the cont-pte bit and everything is fine.
>>>>>
>>>>> Yes, but for debug and optimization, its useful to know when THPs are
>>>>> fully/partially mapped, when they are unaligned etc. Anyway, the script does
>>>>> that for us, and I think we are tending towards agreement that there are
>>>>> unlikely to be any cost benefits by moving it into the kernel.
>>>>
>>>> frequent partial unmap can defeat all purpose for us to use large folios.
>>>> just imagine a large folio can soon be splitted after it is formed. we lose
>>>> the performance gain and might get regression instead.
>>>
>>> nit: just because a THP gets partially unmapped in a process doesn't mean it
>>> gets split into order-0 pages. If the folio still has all its pages mapped at
>>> least once then no further action is taken. If the page being unmapped was the
>>> last mapping of that page, then the THP is put on the deferred split queue, so
>>> that it can be split in future if needed.
>>>>
>>>> and this can be very frequent, for example, one userspace heap management
>>>> is releasing memory page by page.
>>>>
>>>> In our real product deployment, we might not care about the second partial
>>>> unmapped, we do care about the first partial unmapped as we can use this
>>>> to know if split has ever happened on this large folios. an partial unmapped
>>>> subpage can be unlikely re-mapped back.
>>>>
>>>> so i guess 1st unmap is probably enough, at least for my product. I mean we
>>>> care about if partial unmap has ever happened on a large folio more than how
>>>> they are exactly partially unmapped :-)
>>>
>>> I'm not sure what you are suggesting here? A global boolean that tells you if
>>> any folio in the system has ever been partially unmapped? That will almost
>>> certainly always be true, even for a very well tuned system.
>>
>> not a global boolean but a per-folio boolean. in case userspace maps a region
>> and has no userspace management, then we are fine as it is unlikely to have
>> partial unmap/map things; in case userspace maps a region, but manages it
>> by itself, such as heap things, we might result in lots of partial map/unmap,
>> which can lead to 3 problems:
>> 1. potential memory footprint increase, for example, while userspace releases
>> some pages in a folio, we might still keep it as frequent splitting folio into
>> basepages and releasing the unmapped subpage might be too expensive.
>> 2. if cont-pte is involved, frequent dropping cont-pte/tlb shootdown
>> might happen.
>> 3. other maintenance overhead such as splitting large folios etc.
>>
>> We'd like to know how serious partial map things are happening. so either
>> we will disable mTHP in this kind of VMAs, or optimize userspace to do
>> some alignment according to the size of large folios.
>>
>> in android phones, we detect lots of apps, and also found some apps might
>> do things like
>> 1. mprotect on some pages within a large folio
>> 2. mlock on some pages within a large folio
>> 3. madv_free on some pages within a large folio
>> 4. madv_pageout on some pages within a large folio.
>>
>> it would be good if we have a per-folio boolean to know how serious userspace
>> is breaking the large folios. for example, if more than 50% folios in a vma has
>> this problem, we can find it out and take some action.
>
> The high level value of these stats seems clear - I agree we need to be able to
> get these insights. I think the issues are more around the implementation
> though. I'm struggling to understand exactly how we could implement a lot of
> these things cheaply (either in the kernel or in user space).
>
> Let me try to work though what I think you are suggesting:
>
> - every THP is initially fully mapped

Not for pagecache folios.

> - when an operation causes a partial unmap, mark the folio as having at least
> one partial mapping
> - on transition from "no partial mappings" to "at least one partial mapping"
> increment a "anon-partial-<size>kB" (one for each supported folio size)
> counter by the folio size
> - on transition from "at least one partial mapping" to "fully unampped
> everywhere" decrement the counter by the folio size
>
> I think the issue with this is that a folio that is fully mapped in a process
> that gets forked, then is partially unmapped in 1 process, will be accounted as
> partially mapped even after the process that partially unmapped it exits, even
> though that folio is now fully mapped in all processes that map it. Is that a
> problem, perhaps not? I'm not sure.

What I can offer with my total mapcount I am working on (+ entire/pmd
mapcount, but let's put that aside):

1) total_mapcount not multiples of folio_nr_page -> at least one process
currently maps the folio partially

2) total_mapcount is less than folio_nr_page -> surely partially mapped

I think for most of anon memory (note that most folios are always
exclusive in our system, not cow-shared) 2) would already be sufficient.

--
Cheers,

David / dhildenb