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

[Ryan Roberts <ryan.roberts@arm.com>]

On 10/01/2024 10:54, David Hildenbrand wrote:
> On 10.01.24 11:48, Barry Song wrote:
>> On Wed, Jan 10, 2024 at 6:38 PM Ryan Roberts <ryan.roberts@arm.com> 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.
>>
>> My current implementation does use cont_pte but i don't think it is a must-have.
>> we don't need a bit in PTE to know if we are partially unmapping a large folio
>> at all.
>>
>> as long as we are unmapping a part of a large folio, we do know what we are
>> doing. if a large folio is mapped entirely in a process, we get only
>> entire_map +1,
>> if we are unmapping a subpage of it, we get entire_map -1 and remained subpage's
>> mapcount + 1. if we are only mapping a part of this large folio, we
>> only increase
>> its subpages' mapcount.
> 
> That doesn't work as soon as you unmap a second subpage. Not to mention that
> people ( :) ) are working on removing the subpage mapcounts.

Yes, that was my point - Oppo's implementation relies on the bit in the PTE to
tell the difference between unmapping the first subpage and unmapping the
others. We don't have that luxury here.

> 
> I'm going propose that as a topic for LSF/MM soon, once I get to it.
>