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imf22.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf22.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1739779556; a=rsa-sha256; cv=none; b=yTzlosBBTqKhFMJ076x4FhszakgI7GJwAItTVNDU+5EHQfavPJ6lPRNc0l1w4gLcp4ttY6 C1ZhJk9ULp4hRNt5l+jNk3hgorYnlBCxn/jxLc0g4+pd44mhrjPPXUNO24jzONqZNGtEdM WL4y4uC3mZopFAfc1YgMwq6MfmazGAk= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 153CF1007; Mon, 17 Feb 2025 00:06:14 -0800 (PST) Received: from [10.162.41.27] (K4MQJ0H1H2.blr.arm.com [10.162.41.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CF2D73F5A1; Mon, 17 Feb 2025 00:05:41 -0800 (PST) Message-ID: <7c2b0248-1153-4ce1-9170-56eeb0511ff1@arm.com> Date: Mon, 17 Feb 2025 13:35:38 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 0/9] khugepaged: mTHP support From: Dev Jain To: Nico Pache Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, jack@suse.cz, srivatsa@csail.mit.edu, haowenchao22@gmail.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, ioworker0@gmail.com, wangkefeng.wang@huawei.com, ziy@nvidia.com, jglisse@google.com, surenb@google.com, vishal.moola@gmail.com, zokeefe@google.com, zhengqi.arch@bytedance.com, jhubbard@nvidia.com, 21cnbao@gmail.com, willy@infradead.org, kirill.shutemov@linux.intel.com, david@redhat.com, aarcange@redhat.com, raquini@redhat.com, sunnanyong@huawei.com, usamaarif642@gmail.com, audra@redhat.com, akpm@linux-foundation.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, tiwai@suse.de References: <20250211003028.213461-1-npache@redhat.com> <5a995dc9-fee7-442f-b439-c484d9de1750@arm.com> <4ba52062-1bd3-4d53-aa28-fcbbd4913801@arm.com> <71490f8c-f234-4032-bc2a-f6cffa491fcb@arm.com> <5445bc55-6bd2-46fd-8107-99eb31aee172@arm.com> Content-Language: en-US In-Reply-To: <5445bc55-6bd2-46fd-8107-99eb31aee172@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 9qoq6bpr6i3bdjsoi53trq5fuezsacd8 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 9F502C0018 X-HE-Tag: 1739779555-328593 X-HE-Meta: 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 pQV33Fp7 Lv9eeleyigKss4aZlJPjhRE+Bz6T4FmRj6R5L/xBwXDivQHnbjh5fa/ciQejgkX8RC66RZaCsaQGHUT53jYfwO3b+xXYmaLvCq/lwJELCiy4tB2emYXdkQ3Ylv7F96V7e9uZNwPP4UborUEKtfvBh1DM6xkxAYNvTbCcVaK/XRbtL4asnD9rP+GNGGKioNIEYQE6FqXZ8tACzGmOwt0jBGeOehoJetoqcyf+rhtgJ4W9HNEuBSSfI/MzpkLFXZY5BpmG7Pnt+XnEQaDHmABRGFDim93NSUqantJ1e48K5glGXlCjL1j9UY8hWgAn3UzuWPRvf6phreLg1Wf1mDk84XImMDsuztj7iFTe2kwChdcWIO/z2TVdaTHHIR7/pHRe320+Z8if4ZdtvNRS9hL/3O7gTq+DhB39q+3M8Gn6fXtnEbPolq/DXE1tQx94kprOx7/rfxFmSqqZTZDA= 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 15/02/25 12:08 pm, Dev Jain wrote: > > > On 15/02/25 6:22 am, Nico Pache wrote: >> On Thu, Feb 13, 2025 at 7:02 PM Dev Jain wrote: >>> >>> >>> >>> On 14/02/25 1:09 am, Nico Pache wrote: >>>> On Thu, Feb 13, 2025 at 1:26 AM Dev Jain wrote: >>>>> >>>>> >>>>> >>>>> On 12/02/25 10:19 pm, Nico Pache wrote: >>>>>> On Tue, Feb 11, 2025 at 5:50 AM Dev Jain wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>> On 11/02/25 6:00 am, Nico Pache wrote: >>>>>>>> The following series provides khugepaged and madvise collapse >>>>>>>> with the >>>>>>>> capability to collapse regions to mTHPs. >>>>>>>> >>>>>>>> To achieve this we generalize the khugepaged functions to no >>>>>>>> longer depend >>>>>>>> on PMD_ORDER. Then during the PMD scan, we keep track of chunks >>>>>>>> of pages >>>>>>>> (defined by MTHP_MIN_ORDER) that are utilized. This info is tracked >>>>>>>> using a bitmap. After the PMD scan is done, we do binary >>>>>>>> recursion on the >>>>>>>> bitmap to find the optimal mTHP sizes for the PMD range. The >>>>>>>> restriction >>>>>>>> on max_ptes_none is removed during the scan, to make sure we >>>>>>>> account for >>>>>>>> the whole PMD range. max_ptes_none will be scaled by the >>>>>>>> attempted collapse >>>>>>>> order to determine how full a THP must be to be eligible. If a >>>>>>>> mTHP collapse >>>>>>>> is attempted, but contains swapped out, or shared pages, we dont >>>>>>>> perform the >>>>>>>> collapse. >>>>>>>> >>>>>>>> With the default max_ptes_none=511, the code should keep its >>>>>>>> most of its >>>>>>>> original behavior. To exercise mTHP collapse we need to set >>>>>>>> max_ptes_none<=255. >>>>>>>> With max_ptes_none > HPAGE_PMD_NR/2 you will experience collapse >>>>>>>> "creep" and >>>>>>>> constantly promote mTHPs to the next available size. >>>>>>>> >>>>>>>> Patch 1:     Some refactoring to combine madvise_collapse and >>>>>>>> khugepaged >>>>>>>> Patch 2:     Refactor/rename hpage_collapse >>>>>>>> Patch 3-5:   Generalize khugepaged functions for arbitrary orders >>>>>>>> Patch 6-9:   The mTHP patches >>>>>>>> >>>>>>>> --------- >>>>>>>>      Testing >>>>>>>> --------- >>>>>>>> - Built for x86_64, aarch64, ppc64le, and s390x >>>>>>>> - selftests mm >>>>>>>> - I created a test script that I used to push khugepaged to its >>>>>>>> limits while >>>>>>>>        monitoring a number of stats and tracepoints. The code is >>>>>>>> available >>>>>>>>        here[1] (Run in legacy mode for these changes and set >>>>>>>> mthp sizes to inherit) >>>>>>>>        The summary from my testings was that there was no >>>>>>>> significant regression >>>>>>>>        noticed through this test. In some cases my changes had >>>>>>>> better collapse >>>>>>>>        latencies, and was able to scan more pages in the same >>>>>>>> amount of time/work, >>>>>>>>        but for the most part the results were consistant. >>>>>>>> - redis testing. I tested these changes along with my defer changes >>>>>>>>       (see followup post for more details). >>>>>>>> - some basic testing on 64k page size. >>>>>>>> - lots of general use. These changes have been running in my VM >>>>>>>> for some time. >>>>>>>> >>>>>>>> Changes since V1 [2]: >>>>>>>> - Minor bug fixes discovered during review and testing >>>>>>>> - removed dynamic allocations for bitmaps, and made them stack >>>>>>>> based >>>>>>>> - Adjusted bitmap offset from u8 to u16 to support 64k pagesize. >>>>>>>> - Updated trace events to include collapsing order info. >>>>>>>> - Scaled max_ptes_none by order rather than scaling to a 0-100 >>>>>>>> scale. >>>>>>>> - No longer require a chunk to be fully utilized before setting >>>>>>>> the bit. Use >>>>>>>>        the same max_ptes_none scaling principle to achieve this. >>>>>>>> - Skip mTHP collapse that requires swapin or shared handling. >>>>>>>> This helps prevent >>>>>>>>        some of the "creep" that was discovered in v1. >>>>>>>> >>>>>>>> [1] - https://gitlab.com/npache/khugepaged_mthp_test >>>>>>>> [2] - https://lore.kernel.org/lkml/20250108233128.14484-1- >>>>>>>> npache@redhat.com/ >>>>>>>> >>>>>>>> Nico Pache (9): >>>>>>>>       introduce khugepaged_collapse_single_pmd to unify >>>>>>>> khugepaged and >>>>>>>>         madvise_collapse >>>>>>>>       khugepaged: rename hpage_collapse_* to khugepaged_* >>>>>>>>       khugepaged: generalize hugepage_vma_revalidate for mTHP >>>>>>>> support >>>>>>>>       khugepaged: generalize alloc_charge_folio for mTHP support >>>>>>>>       khugepaged: generalize __collapse_huge_page_* for mTHP >>>>>>>> support >>>>>>>>       khugepaged: introduce khugepaged_scan_bitmap for mTHP support >>>>>>>>       khugepaged: add mTHP support >>>>>>>>       khugepaged: improve tracepoints for mTHP orders >>>>>>>>       khugepaged: skip collapsing mTHP to smaller orders >>>>>>>> >>>>>>>>      include/linux/khugepaged.h         |   4 + >>>>>>>>      include/trace/events/huge_memory.h |  34 ++- >>>>>>>>      mm/khugepaged.c                    | 422 ++++++++++++++++++ >>>>>>>> +---------- >>>>>>>>      3 files changed, 306 insertions(+), 154 deletions(-) >>>>>>>> >>>>>>> >>>>>>> Does this patchset suffer from the problem described here: >>>>>>> https://lore.kernel.org/all/8abd99d5-329f-4f8d-8680- >>>>>>> c2d48d4963b6@arm.com/ >>>>>> Hi Dev, >>>>>> >>>>>> Sorry I meant to get back to you about that. >>>>>> >>>>>> I understand your concern, but like I've mentioned before, the scan >>>>>> with the read lock was done so we dont have to do the more expensive >>>>>> locking, and could still gain insight into the state. You are right >>>>>> that this info could become stale if the state changes dramatically, >>>>>> but the collapse_isolate function will verify it and not collapse. >>>>> >>>>> If the state changes dramatically, the _isolate function will >>>>> verify it, >>>>> and fallback. And this fallback happens after following this costly >>>>> path: retrieve a large folio from the buddy allocator -> swapin pages >>>>> from the disk -> mmap_write_lock() -> anon_vma_lock_write() -> TLB >>>>> flush >>>>> on all CPUs -> fallback in _isolate(). >>>>> If you do fail in _isolate(), doesn't it make sense to get the updated >>>>> state for the next fallback order immediately, because we have prior >>>>> information that we failed because of PTE state? What your algorithm >>>>> will do is *still* follow the costly path described above, and again >>>>> fail in _isolate(), instead of failing in hpage_collapse_scan_pmd() >>>>> like >>>>> mine would. >>>> >>>> You do raise a valid point here, I can optimize my solution by >>>> detecting certain collapse failure types and jump to the next scan. >>>> I'll add that to my solution, thanks! >>>> >>>> As for the disagreement around the bitmap, we'll leave that up to the >>>> community to decide since we have differing opinions/solutions. >>>> >>>>> >>>>> The verification of the PTE state by the _isolate() function is the >>>>> "no >>>>> turning back" point of the algorithm. The verification by >>>>> hpage_collapse_scan_pmd() is the "let us see if proceeding is even >>>>> worth >>>>> it, before we do costly operations" point of the algorithm. >>>>> >>>>>>    From my testing I found this to rarely happen. >>>>> >>>>> Unfortunately, I am not very familiar with performance testing/load >>>>> testing, I am fairly new to kernel programming, so I am getting there. >>>>> But it really depends on the type of test you are running, what >>>>> actually >>>>> runs on memory-intensive systems, etc etc. In fact, on loaded >>>>> systems I >>>>> would expect the PTE state to dramatically change. But still, no >>>>> opinion >>>>> here. >>>> >>>> Yeah there are probably some cases where it happens more often. >>>> Probably in cases of short lived allocations, but khugepaged doesn't >>>> run that frequently so those won't be that big of an issue. >>>> >>>> Our performance team is currently testing my implementation so I >>>> should have more real workload test results soon. The redis testing >>>> had some gains and didn't show any signs of obvious regressions. >>>> >>>> As for the testing, check out >>>> https://gitlab.com/npache/khugepaged_mthp_test/-/blob/master/record- >>>> khuge-performance.sh?ref_type=heads >>>> this does the tracing for my testing script. It can help you get >>>> started. There are 3 different traces being applied there: the >>>> bpftrace for collapse latencies, the perf record for the flamegraph >>>> (not actually that useful, but may be useful to visualize any >>>> weird/long paths that you may not have noticed), and the trace-cmd >>>> which records the tracepoint of the scan and the collapse functions >>>> then processes the data using the awk script-- the output being the >>>> scan rate, the pages collapsed, and their result status (grouped by >>>> order). >>>> >>>> You can also look into https://github.com/gormanm/mmtests for >>>> testing/comparing kernels. I was running the >>>> config-memdb-redis-benchmark-medium workload. >>> >>> Thanks. I'll take a look. >>> >>>> >>>>> >>>>>> >>>>>> Also, khugepaged, my changes, and your changes are all a victim of >>>>>> this. Once we drop the read lock (to either allocate the folio, or >>>>>> right before acquiring the write_lock), the state can change. In your >>>>>> case, yes, you are gathering more up to date information, but is it >>>>>> really that important/worth it to retake locks and rescan for each >>>>>> instance if we are about to reverify with the write lock taken? >>>>> >>>>> You said "reverify": You are removing the verification, so this step >>>>> won't be reverification, it will be verification. We do not want to >>>>> verify *after* we have already done 95% of latency-heavy stuff, >>>>> only to >>>>> know that we are going to fail. >>>>> >>>>> Algorithms in the kernel, in general, are of the following form: 1) >>>>> Verify if a condition is true, resulting in taking a control path - >>>>> > 2) >>>>> do a lot of stuff -> "no turning back" step, wherein before committing >>>>> (by taking locks, say), reverify if this is the control path we should >>>>> be in. You are eliminating step 1). >>>>> >>>>> Therefore, I will have to say that I disagree with your approach. >>>>> >>>>> On top of this, in the subjective analysis in [1], point number 7 >>>>> (along >>>>> with point number 1) remains. And, point number 4 remains. >>>> >>>> for 1) your worst case of 1024 is not the worst case. There are 8 >>>> possible orders in your implementation, if all are enabled, that is >>>> 4096 iterations in the worst case. >>> >>> Yes, that is exactly what I wrote in 1). I am still not convinced that >>> the overhead you produce + 512 iterations is going to beat 4096 >>> iterations. Anyways, that is hand-waving and we should test this. >>> >>>> This becomes WAY worse on 64k page size, ~45,000 iterations vs 4096 >>>> in my case. >>> >>> Sorry, I am missing something here; how does the number of iterations >>> change with page size? Am I not scanning the PTE table, which is >>> invariant to the page size? >> >> I got the calculation wrong the first time and it's actually worst. >> Lets hope I got this right this time >> on ARM64 64k kernel: >> PMD size = 512M >> PTE= 64k >> PTEs per PMD = 8192 > > *facepalm* my bad, thanks. I got thrown off thinking HPAGE_PMD_NR won't > depend on page size, but #pte entries = PAGE_SIZE / sizeof(pte) = > PAGE_SIZE / 8. So it does depend. You are correct, the PTEs per PMD is 1 > << 13. > >> log2(8192) = 13 - 2 = 11 number of (m)THP sizes including PMD (the >> first and second order are skipped) >> >> Assuming I understand your algorithm correctly, in the worst case you >> are scanning the whole PMD for each order. >> >> So you scan 8192 PTEs 11 times. 8192 * 11 = 90112. > > Yup. Now it seems that the bitmap overhead may just be worth it; for the > worst case the bitmap will give us an 11x saving...for the average case, > it will give us 2x, but still, 8192 is a large number. I'll think of Clearing on this: the saving is w.r.t the initial scan. That is, if time taken by NP is x + y + collapse_huge_page(), where x is the PMD scan and y is the bitmap overhead, then time taken by DJ is 2x + collapse_huge_page(). In collapse_huge_page(), both perform PTE scans in _isolate(). Anyhow, we differ in opinion as to where the max_ptes_* check should be placed; I recalled the following: https://lore.kernel.org/all/20240809103129.365029-2-dev.jain@arm.com/ https://lore.kernel.org/all/761ba58e-9d6f-4a14-a513-dcc098c2aa94@redhat.com/ One thing you can do to relieve one of my criticisms (not completely) is apply the following patch (this can be done in both methods): diff --git a/mm/khugepaged.c b/mm/khugepaged.c index b589f889bb5a..dc5cb602eaad 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1080,8 +1080,14 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm, } vmf.orig_pte = ptep_get_lockless(pte); - if (!is_swap_pte(vmf.orig_pte)) + if (!is_swap_pte(vmf.orig_pte)) { continue; + } else { + if (order != HPAGE_PMD_ORDER) { + result = SCAN_EXCEED_SWAP_PTE; + goto out; + } + } vmf.pte = pte; vmf.ptl = ptl; -- But this really is the same thing being done in the links above :) > ways to test this out. > > Btw, I was made aware that an LWN article just got posted on our work! > https://lwn.net/Articles/1009039/ > >> >> Please let me know if I'm missing something here. >>> >>>>> >>>>> [1] >>>>> https://lore.kernel.org/all/23023f48-95c6-4a24-ac8b- >>>>> aba4b1a441b4@arm.com/ >>>>> >>>>>> >>>>>> So in my eyes, this is not a "problem" >>>>> >>>>> Looks like the kernel scheduled us for a high-priority debate, I hope >>>>> there's no deadlock :) >>>>> >>>>>> >>>>>> Cheers, >>>>>> -- Nico >>>>>> >>>>>> >>>>>>> >>>>>> >>>>> >>>> >>> >> > >