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. > > We want simple stats that tell us which folio sizes are actually allocated. For > everything else, just scan the process to figure out what exactly is going on. > Certainly that's much easier to do. But is it valuable? It might be if we also keep stats for the number of failures to allocate the various sizes - then we can see what percentage of high order allocation attempts are successful, which is probably useful.
