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

Barry Song
2024-01-10T22:14:56Z

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.

Thanks
Barry