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[2003:cb:c747:f900:6099:1901:b863:450f]) by smtp.gmail.com with ESMTPSA id b14-20020a05600c4e0e00b0040d3276ba19sm1989007wmq.25.2024.01.11.05.18.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Jan 2024 05:18:19 -0800 (PST) Message-ID: <40945967-3f16-4574-8f5d-80e0fcb0bcb4@redhat.com> Date: Thu, 11 Jan 2024 14:18:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1] tools/mm: Add thpmaps script to dump THP usage info To: Ryan Roberts , Barry Song <21cnbao@gmail.com> Cc: John Hubbard , Andrew Morton , Zenghui Yu , Matthew Wilcox , Kefeng Wang , Zi Yan , Alistair Popple , linux-mm@kvack.org References: <20240102153828.1002295-1-ryan.roberts@arm.com> <3bd5e4a3-9f67-4483-9a0e-9abb5eb783cd@arm.com> <94ebe62b-5f55-4be9-b464-4105b4692496@arm.com> <68d5ce7e-6587-47c6-bd0f-988adf5d92a4@arm.com> <974a2670-7fa9-425e-921e-8d54a596e6cf@arm.com> <6c77f143-9c2c-4d17-9a2a-d69d9adf2eea@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 2F0CB40013 X-Rspam-User: X-Stat-Signature: e8zh7q3bkqfgzhhx51igxx95izgcxpmr X-Rspamd-Server: rspam01 X-HE-Tag: 1704979104-439303 X-HE-Meta: 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 2ipsEzBR 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11.01.24 13:25, Ryan Roberts wrote: > On 10/01/2024 22:14, Barry Song wrote: >> On Wed, Jan 10, 2024 at 7:59 PM Ryan Roberts wrote: >>> >>> On 10/01/2024 11:38, Barry Song wrote: >>>> On Wed, Jan 10, 2024 at 7:21 PM Ryan Roberts wrote: >>>>> >>>>> 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 wrote: >>>>>>>>>>> >>>>>>>>>>> On 10/01/2024 09:09, Barry Song wrote: >>>>>>>>>>>> On Wed, Jan 10, 2024 at 4:58 PM Ryan Roberts wrote: >>>>>>>>>>>>> >>>>>>>>>>>>> On 10/01/2024 08:02, Barry Song wrote: >>>>>>>>>>>>>> On Wed, Jan 10, 2024 at 12:16 PM John Hubbard wrote: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On 1/9/24 19:51, Barry Song wrote: >>>>>>>>>>>>>>>> On Wed, Jan 10, 2024 at 11:35 AM John Hubbard >>>>>>>>>>>>>>>> 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. >>>> >>>> frequent partial unmap can defeat all purpose for us to use large folios. >>>> just imagine a large folio can soon be splitted after it is formed. we lose >>>> the performance gain and might get regression instead. >>> >>> nit: just because a THP gets partially unmapped in a process doesn't mean it >>> gets split into order-0 pages. If the folio still has all its pages mapped at >>> least once then no further action is taken. If the page being unmapped was the >>> last mapping of that page, then the THP is put on the deferred split queue, so >>> that it can be split in future if needed. >>>> >>>> and this can be very frequent, for example, one userspace heap management >>>> is releasing memory page by page. >>>> >>>> In our real product deployment, we might not care about the second partial >>>> unmapped, we do care about the first partial unmapped as we can use this >>>> to know if split has ever happened on this large folios. an partial unmapped >>>> subpage can be unlikely re-mapped back. >>>> >>>> so i guess 1st unmap is probably enough, at least for my product. I mean we >>>> care about if partial unmap has ever happened on a large folio more than how >>>> they are exactly partially unmapped :-) >>> >>> I'm not sure what you are suggesting here? A global boolean that tells you if >>> any folio in the system has ever been partially unmapped? That will almost >>> certainly always be true, even for a very well tuned system. >> >> not a global boolean but a per-folio boolean. in case userspace maps a region >> and has no userspace management, then we are fine as it is unlikely to have >> partial unmap/map things; in case userspace maps a region, but manages it >> by itself, such as heap things, we might result in lots of partial map/unmap, >> which can lead to 3 problems: >> 1. potential memory footprint increase, for example, while userspace releases >> some pages in a folio, we might still keep it as frequent splitting folio into >> basepages and releasing the unmapped subpage might be too expensive. >> 2. if cont-pte is involved, frequent dropping cont-pte/tlb shootdown >> might happen. >> 3. other maintenance overhead such as splitting large folios etc. >> >> We'd like to know how serious partial map things are happening. so either >> we will disable mTHP in this kind of VMAs, or optimize userspace to do >> some alignment according to the size of large folios. >> >> in android phones, we detect lots of apps, and also found some apps might >> do things like >> 1. mprotect on some pages within a large folio >> 2. mlock on some pages within a large folio >> 3. madv_free on some pages within a large folio >> 4. madv_pageout on some pages within a large folio. >> >> it would be good if we have a per-folio boolean to know how serious userspace >> is breaking the large folios. for example, if more than 50% folios in a vma has >> this problem, we can find it out and take some action. > > The high level value of these stats seems clear - I agree we need to be able to > get these insights. I think the issues are more around the implementation > though. I'm struggling to understand exactly how we could implement a lot of > these things cheaply (either in the kernel or in user space). > > Let me try to work though what I think you are suggesting: > > - every THP is initially fully mapped Not for pagecache folios. > - when an operation causes a partial unmap, mark the folio as having at least > one partial mapping > - on transition from "no partial mappings" to "at least one partial mapping" > increment a "anon-partial-kB" (one for each supported folio size) > counter by the folio size > - on transition from "at least one partial mapping" to "fully unampped > everywhere" decrement the counter by the folio size > > I think the issue with this is that a folio that is fully mapped in a process > that gets forked, then is partially unmapped in 1 process, will be accounted as > partially mapped even after the process that partially unmapped it exits, even > though that folio is now fully mapped in all processes that map it. Is that a > problem, perhaps not? I'm not sure. What I can offer with my total mapcount I am working on (+ entire/pmd mapcount, but let's put that aside): 1) total_mapcount not multiples of folio_nr_page -> at least one process currently maps the folio partially 2) total_mapcount is less than folio_nr_page -> surely partially mapped I think for most of anon memory (note that most folios are always exclusive in our system, not cow-shared) 2) would already be sufficient. -- Cheers, David / dhildenb