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Sat, 26 Sep 2026 14:43:25 -0700 (PDT) Received: from [2a00:79e0:2eb4:9:125b:13f7:c69d:3887] ([2a00:79e0:2eb4:9:125b:13f7:c69d:3887]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3a0b898cabesm11736892a91.0.2026.09.26.14.43.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 26 Sep 2026 14:43:24 -0700 (PDT) Date: Sat, 26 Sep 2026 14:43:23 -0700 (PDT) From: David Rientjes To: Davidlohr Bueso , Fan Ni , Frank van der Linden , Gregory Price , Jonathan Cameron , Joshua Hahn , Raghavendra K T , "Rao, Bharata Bhasker" , SeongJae Park , Wei Xu , Xuezheng Chu , Yiannis Nikolakopoulos , Zi Yan cc: linux-mm@kvack.org Subject: [Linux Memory Hotness and Promotion] Notes from September 24, 2026 Message-ID: <0c8a7227-c7eb-bef1-7326-c7b04cde78a8@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Stat-Signature: tyb3zwcjow6ukzg761sf4ux3gt39cowb X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 0E08540007 X-HE-Tag: 1790459008-604615 X-HE-Meta: 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 +0c85Zzy 5gg8BWBHeL3AISPNa5YcFVkhWDXbykYx+vT2hM4Wqyfuks+mbl+l+yK3ye6JgdcJR+TWU5UNgf4STjE55RBmBbezFeQpLVwd1sJOmc9eAVUkjiIeLmpJJV2iMw1UFqkyzIZZ0zuQ3BjTSJ5ledc0Exd2lZMBWqLJf/fj1uxSgD2vjUp857lyyfP8tTJ6UijKZ4U1ziPAdbsuE8njB5+7iPLtT73LnoyLeZGyYa5zgqVpQX4fkrn8DoHJ9UQdxwedF85HWKxdU3OkWcvCxN7tJsaRQval5IHkd80K2vZ+jEV6vKt+llT0L9GAJKb3KC21UWAOWnrTInes3PTFMwhOq0oMc9/GHrTZEfDZNlnjWQHfpoMJqSwdAumDQhxmKYXaTVPTaE9NNkfkxYyQmWwaJl97ff5Rq8UHj6JDWB8UvENzQTjO9iOTgkkZpI320FytmBqscKjGqfXaymPhkQNvbpXT86f41/G4fscSJvxhzvViROcg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi everybody, Here are the notes from the last Linux Memory Hotness and Promotion call that happened on Thursday, September 24. Thanks to everybody who was involved! These notes are intended to bring people up to speed who could not attend the call as well as keep the conversation going in between meetings. ----->o----- Bharata updated on the status of his work. He posted pghot-hwhints for the IBS Memory Profiler as a separate but dependent patch series as it is x86 specific and the pghot patch series is becoming too big with multiple sources of hotness information posted together. Bharata noted that the benchmark numbers for pghot-hwhints so far looks promising and that he is looking forward to discussing this topic at LPC coming up. ----->o----- Huan updated on the status of his work with Yiannis for non-temporal stores in migrate_pages(), he prepared a patch series to send for feedback to the group. He looked into Shivank's feedback and work being done for batching with 2MB pages. Today, we need to do a loop for each 4KB page, Shivank's work speeds this up but Huan has not yet integrated this since Shivank's work is not yet upstream. There are also mixed results posted upstream. Thus, for now this is being considered orthogonally until Shivank's changes land upstream. I asked if Meta was also going to be looking into these optimizations; Gregory noted that they'll be waiting for it to land upstream. They've been looking at basic correctness of NUMA Balancing rather than optimizations for now. Per his discussion upstream, it turns out that NUMA Balancing, at least for tiering, has been broken for the last four years. Shivank updated on his patches[1] for non-temporal stores without overloading migrate_mode. Feedback on this would be useful. He will also be presenting at LPC including additional experiments that have been run. ----->o----- Davidlohr expressed concern about the direction of pghot overall. He noted the upstream discussion has focused on not having any regressions, but there isn't an obvious gain either. It looks like Joshua had the same conclusion although PSI may have improved. Davidlohr has been working on integrating CHMU into pghot and found the experience to be complicated so he ended up bypassing most of it. Whenever he got an interrupt from the device, he would try to create as many contiguous HPA ranges as he could to feed to pghot; once he got that, he decided to promote instead of doing decay and got a 3x improvement over NUMA Balancing that he was expecting. He suggested we may want to go back to the drawing board with regard to the complexity and interfaces of pghot. Davidlohr suggested we may want to rely more on user-driven proactive reclaim and can then tune CHMU as desired so there is some control over freeing and promotion. Another option would be to integrate this into DAMON as well. One concern is that if this is integrated into pghot then we lose locality information. Gregory said that pghot provides value in doing this asynchronously, which NUMA Balancing does not do, and that avoids lengthy stalled faults. In this case, Davidlohr suggested we may just punt it to a kthread that would recheck if it's hot and promote. Gregory thinks we're proposing significant complexity in pghot that doesn't yet carry all its weight but if we were to focus on the async part then this could be a start and we could build incrementally on top. Davidlohr said we're still losing locality information; Gregory suggested this is a general limitation of tiering in general. Davidlohr said we could feed the locality information to the kthread although this would include overhead for tracking. Davidlohr agreed with the value of the asynchronous promotion handling, including for NUMA Balancing, but not with catering to IBS necessarily. Gregory is a proponent of splitting out kpromoted from the rest of the system. Wei agreed with Gregory's assessment although the data structures are up for debate. We could iterate on the data structures for this. DAMON has a ton of additional support and we need to discuss what all comes along with it if this is a general tiering solution. Gregory suggested the asynchronous promotion work could also be used by DAMON. Joshua broke down the components of pghot: device drivers, kscand, kmigrated, and pghot interface. If they get separated, it would also be useful for code reviews. He was concerned that pghot may be heading in the same direction as DAMON. Davidlohr agreed with this. Bharata, have you thought about the possibility of only introducing the asynchronous promotion support first, getting alignment from stakeholders on that, and then extending pghot with additional enhancements later? ----->o----- NOTE!!! The next meeting will be canceled due to LPC 2026. Next meeting will be on Thursday, October 22 at 8:30am PDT (UTC-7), everybody is welcome: https://meet.google.com/jak-ytdx-hnm Topics for the next meeting: - debrief discussions at LPC - directional discussion for pghot and how to build it incrementally: focus on async promotion mechanisms first and then add enhancements on top of this separately - update on combined patch series for supporting non-temporal stores in migrate_pages() with memory error handling (series from Yiannis + Huan) for 4KB, extensions for 2MB later from Shivank - v6 of Shivank's series for enlightening migrate_pages() for hardware assists and his rmap batch series as well as non-temporal stores without overloading migrate_mode - Teja's update on SDXI page migration based on AMD patches and hardware issues being encountered that do not result in page migration - update on tier-aware memcg limits status and production testing based on the latest major overhaul - first class support for virtualization based memory tier support, how to leverge memory tiers in the guest - discuss generalized subsystem for providing bandwidth information independent of the underlying platform, ideally through resctrl + preferably this bandwidth monitoring is not per NUMA node but rather slow and fast Please let me know if you'd like to propose additional topics for discussion, thank you! [1] https://lore.kernel.org/linux-mm/20260902-migrate-refactor-shivank-v1-9-9dcca87669c4@amd.com/