On 11/01/2024 20:45, Barry Song wrote: > On Fri, Jan 12, 2024 at 1:25 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >> >> On 10/01/2024 22:14, Barry Song wrote: >>> On Wed, Jan 10, 2024 at 7:59 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >>>> >>>> On 10/01/2024 11:38, Barry Song wrote: >>>>> On Wed, Jan 10, 2024 at 7:21 PM Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxx> wrote: >>>>>>>>>>>> >>>>>>>>>>>> On 10/01/2024 09:09, Barry Song wrote: >>>>>>>>>>>>> On Wed, Jan 10, 2024 at 4:58 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>> On 10/01/2024 08:02, Barry Song wrote: >>>>>>>>>>>>>>> On Wed, Jan 10, 2024 at 12:16 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> On 1/9/24 19:51, Barry Song wrote: >>>>>>>>>>>>>>>>> On Wed, Jan 10, 2024 at 11:35 AM John Hubbard <jhubbard@xxxxxxxxxx> >>>>>>>>>>>>>>>>> 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 >> - 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. > > I don't think this is a problem as what we really care about is if some "bad" > behaviour has ever happened in userspace. Though the "bad" guy has exited > or been killed, we still need the record to find out. > > except the global count, if we can reflect the partially mapped count in vma > such as smaps, this will help even more to locate the problematic userspace > code. Right. Although note that smaps is already scanning the page table, so for the smaps case we could do it precisely - it's already slow. The thpmaps script already gives a precise account of partially mapped THPs, FYI. > > Thanks > Barry
