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

David Hildenbrand
2024-01-10T11:24:23Z

On 10.01.24 12:20, Ryan Roberts 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.

Agreed. And just adding: while one process might map a folio
unaligned/partial/ ... another one might map it aligned/fully. So this
per-process scanning is really required (because per process stats per
folio are pretty much out of scope :) ).

>
>>
>> We want simple stats that tell us which folio sizes are actually allocated. For
>> everything else, just scan the process to figure out what exactly is going on.
>>
>
> Certainly that's much easier to do. But is it valuable? It might be if we also
> keep stats for the number of failures to allocate the various sizes - then we
> can see what percentage of high order allocation attempts are successful, which
> is probably useful.

Agreed.

--
Cheers,

David / dhildenb