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. > > 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
