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 ;-)
