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

Ryan Roberts
2024-01-10T10:39:12Z

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.

>
>>
>> So depending on what global stats you actually need, the route to getting them
>> cheaply may not be easy. (My previous attempt to add stats cheated and didn't
>> try to track "fully mapped" vs "partially mapped" - instead it just counted the
>> number of pages belonging to a THP (of any size) that were mapped.
>>
>> If you need the global mapping state, then the short term way to do this would
>> be to provide the root cgroup, then have the script recurse through all child
>> cgroups; That would pick up all the processes and iterate through them:
>>
>> $ thpmaps --cgroup /sys/fs/cgroup --summary ...
>>
>> This won't quite work with the current version because it doesn't recurse
>> through the cgroup children currently, but that would be easy to add.
>>
>>
>>>
>>> for debug purposes, it should be good. imaging there is a health
>>> monitor which needs
>>> to sample the stats of large folios online and periodically, this
>>> might be too expensive.
>>>
>>>>
>>>> If we can possibly avoid sysfs/debugfs I would prefer to keep it all in a script
>>>> for now.
>>>>
>>>>>
>>>>> +1.
>>>>>
>>>>>>
>>>>>>
>>>>>> thanks,
>>>>>> --
>>>>>> John Hubbard
>>>>>> NVIDIA
>>>>>>
>>>>>
>>>
>
> Thanks
> Barry