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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Cheers,

David / dhildenb