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

Barry Song
2024-01-10T10:48:36Z

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.

>
> >
> >>
> >> 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