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

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

On 10/01/2024 10:42, 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 ;-)