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imf29.hostedemail.com; dkim=none; spf=pass (imf29.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1739498516; a=rsa-sha256; cv=none; b=yLAnCWdRpTz73Wex/R9udgKYrh6qLeiVDKid44FG5uRRITn1ONcecoBRdKDeIYXn00QiXW NsleqwWlcobfwgb/fRW//O3vU2rbxH0ghKSUiBoaehw28HVvPA10g2u51xsPB3v0ttrt8f JxR1LsJE+Kfxxq8/ygOCiJn6byEQCMs= 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 178991682; Thu, 13 Feb 2025 18:02:13 -0800 (PST) Received: from [10.163.92.186] (unknown [10.163.92.186]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 47AC03F58B; Thu, 13 Feb 2025 18:01:39 -0800 (PST) Message-ID: <71490f8c-f234-4032-bc2a-f6cffa491fcb@arm.com> Date: Fri, 14 Feb 2025 07:31:36 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 0/9] khugepaged: mTHP support 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> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 7777D120016 X-Stat-Signature: o4yqc69kkx1wnoinoqndosf31setaiot X-Rspamd-Server: rspam03 X-HE-Tag: 1739498514-69706 X-HE-Meta: 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 BhcvjggO Bne/80C1tIjWtitfuOszt7xk7w6ddoEHfHyHnBoccRArqx3QyxVDyZ1D/f08JXrmN8Mcg7tZIMAgC0nPPs+F9ZPFIoMiakRgcxOzQPseIdhTEdTE/6gquuhvKGMzsPpHP5skm2tKos/Dwhih5Cf7JOIJ/zli/+WCVTuDboG6t9WS8wxVU4KE3PIlowSNngYw/OPHyJT5qQsyxMxAQbbJQFxYgSMnYn1NqAKhDUb0v2TZ5iD7wg+fT3/jAqEXdCGG4bk8hkXS+If5EOgKWWD7+Z8/yRWZAUXftcy6G9nIajBp29ES00TBTR3XKYSDHPxuEW3YDa1o5o6twJJp2XQ/PAwoD+ZqdlQzWT3380lUTt6pp2LH82iQIlDNxgUDexIdungOugpH2Pn7lFuGzl6D1lXXdbPaILiT/5m8/Y9F4xmiKTKY/WDB8rw9RzzV5/vbPC1aRtsZivzbjQqaECPma5opjSQ== 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 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? >> >> [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 >>> >>> >>>> >>> >> >