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

Barry Song
2024-01-12T06:04:16Z

On Fri, Jan 12, 2024 at 9:28 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 11.01.24 21:21, Barry Song wrote:
> > On Fri, Jan 12, 2024 at 2:18 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 11.01.24 13:25, Ryan Roberts wrote:
> >>> On 10/01/2024 22:14, Barry Song wrote:
> >>>> On Wed, Jan 10, 2024 at 7:59 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>>>>
> >>>>> On 10/01/2024 11:38, Barry Song wrote:
> >>>>>> On Wed, Jan 10, 2024 at 7:21 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>>>>>>
> >>>>>>> On 10/01/2024 11:00, David Hildenbrand wrote:
> >>>>>>>> On 10.01.24 11:55, Ryan Roberts wrote:
> >>>>>>>>> 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 ;-)
> >>>>>>>>
> >>>>>>>> Unlikely :) As you said, once you have multiple folio sizes, it stops really
> >>>>>>>> making sense.
> >>>>>>>>
> >>>>>>>> Assume you have a 128 kiB pageache folio, and half of that is mapped. You can
> >>>>>>>> set cont-pte bits on that half and all is fine. Or AMD can benefit from it's
> >>>>>>>> optimizations without the cont-pte bit and everything is fine.
> >>>>>>>
> >>>>>>> Yes, but for debug and optimization, its useful to know when THPs are
> >>>>>>> fully/partially mapped, when they are unaligned etc. Anyway, the script does
> >>>>>>> that for us, and I think we are tending towards agreement that there are
> >>>>>>> unlikely to be any cost benefits by moving it into the kernel.
> >>>>>>
> >>>>>> frequent partial unmap can defeat all purpose for us to use large folios.
> >>>>>> just imagine a large folio can soon be splitted after it is formed. we lose
> >>>>>> the performance gain and might get regression instead.
> >>>>>
> >>>>> nit: just because a THP gets partially unmapped in a process doesn't mean it
> >>>>> gets split into order-0 pages. If the folio still has all its pages mapped at
> >>>>> least once then no further action is taken. If the page being unmapped was the
> >>>>> last mapping of that page, then the THP is put on the deferred split queue, so
> >>>>> that it can be split in future if needed.
> >>>>>>
> >>>>>> and this can be very frequent, for example, one userspace heap management
> >>>>>> is releasing memory page by page.
> >>>>>>
> >>>>>> In our real product deployment, we might not care about the second partial
> >>>>>> unmapped, we do care about the first partial unmapped as we can use this
> >>>>>> to know if split has ever happened on this large folios. an partial unmapped
> >>>>>> subpage can be unlikely re-mapped back.
> >>>>>>
> >>>>>> so i guess 1st unmap is probably enough, at least for my product. I mean we
> >>>>>> care about if partial unmap has ever happened on a large folio more than how
> >>>>>> they are exactly partially unmapped :-)
> >>>>>
> >>>>> I'm not sure what you are suggesting here? A global boolean that tells you if
> >>>>> any folio in the system has ever been partially unmapped? That will almost
> >>>>> certainly always be true, even for a very well tuned system.
> >>>>
> >>>> not a global boolean but a per-folio boolean. in case userspace maps a region
> >>>> and has no userspace management, then we are fine as it is unlikely to have
> >>>> partial unmap/map things; in case userspace maps a region, but manages it
> >>>> by itself, such as heap things, we might result in lots of partial map/unmap,
> >>>> which can lead to 3 problems:
> >>>> 1. potential memory footprint increase, for example, while userspace releases
> >>>> some pages in a folio, we might still keep it as frequent splitting folio into
> >>>> basepages and releasing the unmapped subpage might be too expensive.
> >>>> 2. if cont-pte is involved, frequent dropping cont-pte/tlb shootdown
> >>>> might happen.
> >>>> 3. other maintenance overhead such as splitting large folios etc.
> >>>>
> >>>> We'd like to know how serious partial map things are happening. so either
> >>>> we will disable mTHP in this kind of VMAs, or optimize userspace to do
> >>>> some alignment according to the size of large folios.
> >>>>
> >>>> in android phones, we detect lots of apps, and also found some apps might
> >>>> do things like
> >>>> 1. mprotect on some pages within a large folio
> >>>> 2. mlock on some pages within a large folio
> >>>> 3. madv_free on some pages within a large folio
> >>>> 4. madv_pageout on some pages within a large folio.
> >>>>
> >>>> it would be good if we have a per-folio boolean to know how serious userspace
> >>>> is breaking the large folios. for example, if more than 50% folios in a vma has
> >>>> this problem, we can find it out and take some action.
> >>>
> >>> The high level value of these stats seems clear - I agree we need to be able to
> >>> get these insights. I think the issues are more around the implementation
> >>> though. I'm struggling to understand exactly how we could implement a lot of
> >>> these things cheaply (either in the kernel or in user space).
> >>>
> >>> Let me try to work though what I think you are suggesting:
> >>>
> >>> - every THP is initially fully mapped
> >>
> >> Not for pagecache folios.
> >>
> >>> - when an operation causes a partial unmap, mark the folio as having at least
> >>> one partial mapping
> >>> - on transition from "no partial mappings" to "at least one partial mapping"
> >>> increment a "anon-partial-<size>kB" (one for each supported folio size)
> >>> counter by the folio size
> >>> - on transition from "at least one partial mapping" to "fully unampped
> >>> everywhere" decrement the counter by the folio size
> >>>
> >>> I think the issue with this is that a folio that is fully mapped in a process
> >>> that gets forked, then is partially unmapped in 1 process, will be accounted as
> >>> partially mapped even after the process that partially unmapped it exits, even
> >>> though that folio is now fully mapped in all processes that map it. Is that a
> >>> problem, perhaps not? I'm not sure.
> >>
> >> What I can offer with my total mapcount I am working on (+ entire/pmd
> >> mapcount, but let's put that aside):
> >>
> >> 1) total_mapcount not multiples of folio_nr_page -> at least one process
> >> currently maps the folio partially
> >>
> >> 2) total_mapcount is less than folio_nr_page -> surely partially mapped
> >>
> >> I think for most of anon memory (note that most folios are always
> >> exclusive in our system, not cow-shared) 2) would already be sufficient.
> >
> > if we can improve Ryan's "mm: Batch-copy PTE ranges during fork()" to
> > add nr_pages in copy_pte_range for rmap.
> > copy_pte_range()
> > {
> > folio_try_dup_anon_rmap_ptes(...nr_pages....)
> > }
> > and at the same time, in zap_pte_range(), we remove the whole anon_rmap
> > if the zapped-range covers the whole folio.
> >
> > Replace the for-loop
> > for (i = 0; i < nr; i++, page++) {
> > add_rmap(1);
> > }
> > for (i = 0; i < nr; i++, page++) {
> > remove_rmap(1);
> > }
> > by always using add_rmap(nr_pages) and remove_rmap(nr_pages) if we
> > are doing the entire mapping/unmapping
>
> That's precisely what I have already running as protoypes :) And I
> promised Ryan to get to this soon, clean it up and sent it out.

Cool. Glad we'll have it soon.

>
> .
> >
> > then we might be able to TestAndSetPartialMapped flag for this folio anywhile
> > 1. someone is adding rmap with a number not equal nr_pages
> > 2. someone is removing rmap with a number not equal nr_pages
> > That means we are doing partial mapping or unmapping.
> > and we increment partialmap_count by 1, let debugfs or somewhere present
> > this count.
>
> Yes. The only "ugly" corner case if you have a split VMA. We're not
> batching rmap exceeding that.

I am sorry I don't quite get what the problem is. Do you mean splitting
vma is crossing a PTE-mapped mTHP or a PMD-mapped THP?

for the latter, I see __split_huge_pmd_locked() does have some mapcount
operation but it is batched by
folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
vma, haddr, rmap_flags);

for the former, I don't find any special mapcount thing is needed.
Do I miss something?

>
> --
> Cheers,
>
> David / dhildenb

Thanks
Barry