On Wed, Jan 10, 2024 at 6:58 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: > > On 10/01/2024 10:54, David Hildenbrand wrote: > > On 10.01.24 11:48, Barry Song wrote: > >> 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. > > > > That doesn't work as soon as you unmap a second subpage. Not to mention that > > people ( :) ) are working on removing the subpage mapcounts. > > Yes, that was my point - Oppo's implementation relies on the bit in the PTE to > tell the difference between unmapping the first subpage and unmapping the > others. We don't have that luxury here. right. The devil is in the details :-) > > > > > I'm going propose that as a topic for LSF/MM soon, once I get to it. > > >
