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[2003:d8:2f49:bc00:12fa:1681:c754:1630]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b79c3c33fesm27989570f8f.29.2025.08.07.11.01.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Aug 2025 11:01:53 -0700 (PDT) Message-ID: Date: Thu, 7 Aug 2025 20:01:51 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [linus:master] [mm] f822a9a81a: stress-ng.bigheap.realloc_calls_per_sec 37.3% regression To: Lorenzo Stoakes , Jann Horn Cc: kernel test robot , Dev Jain , oe-lkp@lists.linux.dev, lkp@intel.com, linux-kernel@vger.kernel.org, Andrew Morton , Barry Song , Pedro Falcato , Anshuman Khandual , Bang Li , Baolin Wang , bibo mao , Hugh Dickins , Ingo Molnar , Lance Yang , Liam Howlett , Matthew Wilcox , Peter Xu , Qi Zheng , Ryan Roberts , Vlastimil Babka , Yang Shi , Zi Yan , linux-mm@kvack.org References: <202508071609.4e743d7c-lkp@intel.com> <9e3a59b2-11c0-43ca-aff3-414091f04aa4@lucifer.local> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 07.08.25 19:51, Lorenzo Stoakes wrote: > On Thu, Aug 07, 2025 at 07:46:39PM +0200, Jann Horn wrote: >> On Thu, Aug 7, 2025 at 7:41 PM Lorenzo Stoakes >> wrote: >>> On Thu, Aug 07, 2025 at 07:37:38PM +0200, Jann Horn wrote: >>>> On Thu, Aug 7, 2025 at 10:28 AM Lorenzo Stoakes >>>> wrote: >>>>> On Thu, Aug 07, 2025 at 04:17:09PM +0800, kernel test robot wrote: >>>>>> 94dab12d86cf77ff f822a9a81a31311d67f260aea96 >>>>>> ---------------- --------------------------- >>>>>> %stddev %change %stddev >>>>>> \ | \ >>>>>> 13777 ą 37% +45.0% 19979 ą 27% numa-vmstat.node1.nr_slab_reclaimable >>>>>> 367205 +2.3% 375703 vmstat.system.in >>>>>> 55106 ą 37% +45.1% 79971 ą 27% numa-meminfo.node1.KReclaimable >>>>>> 55106 ą 37% +45.1% 79971 ą 27% numa-meminfo.node1.SReclaimable >>>>>> 559381 -37.3% 350757 stress-ng.bigheap.realloc_calls_per_sec >>>>>> 11468 +1.2% 11603 stress-ng.time.system_time >>>>>> 296.25 +4.5% 309.70 stress-ng.time.user_time >>>>>> 0.81 ą187% -100.0% 0.00 perf-sched.sch_delay.avg.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0 >>>>>> 9.36 ą165% -100.0% 0.00 perf-sched.sch_delay.max.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0 >>>>>> 0.81 ą187% -100.0% 0.00 perf-sched.wait_time.avg.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0 >>>>>> 9.36 ą165% -100.0% 0.00 perf-sched.wait_time.max.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0 >>>>>> 5.50 ą 17% +390.9% 27.00 ą 56% perf-c2c.DRAM.local >>>>>> 388.50 ą 10% +114.7% 834.17 ą 33% perf-c2c.DRAM.remote >>>>>> 1214 ą 13% +107.3% 2517 ą 31% perf-c2c.HITM.local >>>>>> 135.00 ą 19% +130.9% 311.67 ą 32% perf-c2c.HITM.remote >>>>>> 1349 ą 13% +109.6% 2829 ą 31% perf-c2c.HITM.total >>>>> >>>>> Yeah this also looks pretty consistent too... >>>> >>>> FWIW, HITM hat different meanings depending on exactly which >>>> microarchitecture that test happened on; the message says it is from >>>> Sapphire Rapids, which is a successor of Ice Lake, so HITM is less >>>> meaningful than if it came from a pre-IceLake system (see >>>> https://lore.kernel.org/all/CAG48ez3RmV6SsVw9oyTXxQXHp3rqtKDk2qwJWo9TGvXCq7Xr-w@mail.gmail.com/). >>>> >>>> To me those numbers mainly look like you're accessing a lot more >>>> cache-cold data. (On pre-IceLake they would indicate cacheline >>>> bouncing, but I guess here they probably don't.) And that makes sense, >>>> since before the patch, this path was just moving PTEs around without >>>> looking at the associated pages/folios; basically more or less like a >>>> memcpy() on x86-64. But after the patch, for every 8 bytes that you >>>> copy, you have to load a cacheline from the vmemmap to get the page. >>> >>> Yup this is representative of what my investigation is showing. >>> >>> I've narrowed it down but want to wait to report until I'm sure... >>> >>> But yeah we're doing a _lot_ more work. >>> >>> I'm leaning towards disabling except for arm64 atm tbh, seems mremap is >>> especially sensitive to this (I found issues with this with my abortive mremap >>> anon merging stuff too, but really expected it there...) >> >> Another approach would be to always read and write PTEs in >> contpte-sized chunks here, without caring whether they're actually >> contiguous or whatever, or something along those lines. > > Not sure I love that, you'd have to figure out offset without cont pte batch and > can it vary? And we're doing this on non-arm64 arches for what reason? > > And would it solve anything really? We'd still be looking at folio, yes less > than now, but uselessly for arches that don't benefit? > > The basis of this series was (and I did explicitly ask) that it wouldn't harm > other arches. We'd need some hint to detect "this is either small" or "this is unbatchable". Sure, we could use pte_batch_hint(), but I'm curious if x86 would also benefit with larger folios (e.g., 64K, 128K) with this patch. -- Cheers, David / dhildenb