On Wed, Jan 10, 2024 at 6:38 PM Ryan Roberts <ryan.roberts@xxxxxxx> 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. 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
