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 Oyb5IZlx 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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 12/11/2024 12:23 AM, SeongJae Park wrote: > Hello Raghavendra, > > > Thank you for posting this nice patch series. I gave you some feedback > offline. Adding those here again for transparency on this grateful public > discussion. > > On Sun, 1 Dec 2024 15:38:08 +0000 Raghavendra K T wrote: > >> Introduction: >> ============= >> This patchset is an outcome of an ongoing collaboration between AMD and Meta. >> Meta wanted to explore an alternative page promotion technique as they >> observe high latency spikes in their workloads that access CXL memory. >> >> In the current hot page promotion, all the activities including the >> process address space scanning, NUMA hint fault handling and page >> migration is performed in the process context. i.e., scanning overhead is >> borne by applications. > > Yet another approach is using DAMON. DAMON does access monitoring, and further > allows users to request access pattern-driven system operations in name of > DAMOS (Data Access Monitoring-based Operation Schemes). Using it, users can > request DAMON to find hot pages and promote, while finding cold pages and > demote. SK hynix has made their CXL-based memory capacity expansion solution > in the way (https://github.com/skhynix/hmsdk/wiki/Capacity-Expansion). We > collaboratively developed new DAMON features for that, and those are all > in the mainline since Linux v6.11. > > I also proposed an idea for advancing it using DAMOS auto-tuning on more > general (>2 tiers) setup > (https:lore.kernel.org/20231112195602.61525-1-sj@kernel.org). I haven't had a > time to further implement and test the idea so far, though. > >> >> This is an early RFC patch series to do (slow tier) CXL page promotion. >> The approach in this patchset assists/addresses the issue by adding PTE >> Accessed bit scanning. >> >> Scanning is done by a global kernel thread which routinely scans all >> the processes' address spaces and checks for accesses by reading the >> PTE A bit. It then migrates/promotes the pages to the toptier node >> (node 0 in the current approach). >> >> Thus, the approach pushes overhead of scanning, NUMA hint faults and >> migrations off from process context. > > DAMON also uses PTE A bit as major source of the access information. And DAMON > does both access monitoring and promotion/demotion in a global kernel thread, > namely kdamond. Hence the DAMON-based approach would also offload the > overheads from process context. So I feel your approach has a sort of > similarity with DAMON-based one in a way, and we might have a chance to avoid > unnecessary duplicates. > > [...] >> >> Limitations: >> =========== >> PTE A bit scanning approach lacks information about exact destination >> node to migrate to. > > This is same for DAMON-based approach, since DAMON also uses PTE A bit as the > major source of the information. We aim to extend DAMON to aware of the access > source CPU, and use it for solving this problem, though. Utilizing page faults > or AMD IBS-like h/w features are on the table of the ideas. > >> >> Notes/Observations on design/Implementations/Alternatives/TODOs... >> ================================ >> 1. Fine-tuning scan throttling > > DAMON allows users set the upper-limit of monitoring overhead, using > max_nr_regions parameter. Then it provides its best-effort accuracy. We also > have ongoing projects for making it more accurate and easier to tune. > >> >> 2. Use migrate_balanced_pgdat() to balance toptier node before migration >> OR Use migrate_misplaced_folio_prepare() directly. >> But it may need some optimizations (for e.g., invoke occasionaly so >> that overhead is not there for every migration). >> >> 3. Explore if a separate PAGE_EXT flag is needed instead of reusing >> PAGE_IDLE flag (cons: complicates PTE A bit handling in the system), >> But practically does not look good idea. >> >> 4. Use timestamp information-based migration (Similar to numab mode=2). >> instead of migrating immediately when PTE A bit set. >> (cons: >> - It will not be accurate since it is done outside of process >> context. >> - Performance benefit may be lost.) > > DAMON provides a sort of time-based aggregated monitoring results. And DAMOS > provides prioritization of pages based on the access temperature. Hence, > DAMON-based apparoach can also be used for a similar purpose (promoting not > every accessed pages but pages that more frequently used for longer time). > >> >> 5. Explore if we need to use PFN information + hash list instead of >> simple migration list. Here scanning is directly done with PFN belonging >> to CXL node. > > DAMON supports physical address space monitoring, and maintains the access > monitoring results in its own data structure called damon_region. So I think > similar benefit can be achieved using DAMON? > > [...] >> 8. Using DAMON APIs OR Reusing part of DAMON which already tracks range of >> physical addresses accessed. > > My biased humble opinion is that it would be very nice to explore this > opportunity, since I show some similarities and opportunities to solve some of > challenges on your approach in an easier way. Even if it turns out that DAMON > cannot be used for your use case, failing earlier is a good thing, I'd say :) > >> >> 9. Gregory has nicely mentioned some details/ideas on different approaches in >> [1] : development notes, in the context of promoting unmapped page cache folios. > > DAMON supports monitoring accesses to unmapped page cache folios, so hopefully > DAMON-based approaches can also solve this issue. > Hello SJ, Thank you for detailed explanation again. (Sorry for late acknowledgement as I was looking forward to MM alignment discussion when this message came). I think once the direction is fixed, we could surely use / Reuse lot source code from DAMON, MGLRU. Amazing design of DAMON should surely help. Will keep in mind all the points raised here. Thanks and Regards - Raghu