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[RFC PATCH v2 0/8] mm: Hot page tracking and promotion infrastructure To: Wei Xu , David Rientjes , Bharata B Rao Cc: Gregory Price , Matthew Wilcox , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Jonathan.Cameron@huawei.com, dave.hansen@intel.com, hannes@cmpxchg.org, mgorman@techsingularity.net, mingo@redhat.com, peterz@infradead.org, raghavendra.kt@amd.com, riel@surriel.com, sj@kernel.org, ying.huang@linux.alibaba.com, ziy@nvidia.com, dave@stgolabs.net, nifan.cxl@gmail.com, xuezhengchu@huawei.com, yiannis@zptcorp.com, akpm@linux-foundation.org, david@redhat.com, byungchul@sk.com, kinseyho@google.com, joshua.hahnjy@gmail.com, yuanchu@google.com, alok.rathore@samsung.com References: <20250910144653.212066-1-bharata@amd.com> <7e3e7327-9402-bb04-982e-0fb9419d1146@google.com> Content-Language: en-US From: Balbir Singh In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SJ0PR05CA0140.namprd05.prod.outlook.com (2603:10b6:a03:33d::25) 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 8nuQsPYK 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at 04:39:16PM +0100, Matthew Wilcox wrote: >>>> On Wed, Sep 10, 2025 at 08:16:45PM +0530, Bharata B Rao wrote: >>>>> This patchset introduces a new subsystem for hot page tracking >>>>> and promotion (pghot) that consolidates memory access information >>>>> from various sources and enables centralized promotion of hot >>>>> pages across memory tiers. >>>> >>>> Just to be clear, I continue to believe this is a terrible idea and we >>>> should not do this. If systems will be built with CXL (and given the >>>> horrendous performance, I cannot see why they would be), the kernel >>>> should not be migrating memory around like this. >>> >>> I've been considered this problem from the opposite approach since LSFMM. >>> >>> Rather than decide how to move stuff around, what if instead we just >>> decide not to ever put certain classes of memory on CXL. Right now, so >>> long as CXL is in the page allocator, it's the wild west - any page can >>> end up anywhere. >>> >>> I have enough data now from ZONE_MOVABLE-only CXL deployments on real >>> workloads to show local CXL expansion is valuable and performant enough >>> to be worth deploying - but the key piece for me is that ZONE_MOVABLE >>> disallows GFP_KERNEL. For example: this keeps SLAB meta-data out of >>> CXL, but allows any given user-driven page allocation (including page >>> cache, file, and anon mappings) to land there. >>> >> >> This is similar to our use case, although the direct allocation can be >> controlled by cpusets or mempolicies as needed depending on the memory >> access latency required for the workload; nothing new there, though, it's >> the same argument as NUMA in general and the abstraction of these far >> memory nodes as separate NUMA nodes makes this very straightforward. >> >>> I'm hoping to share some of this data in the coming months. >>> >>> I've yet to see any strong indication that a complex hotness/movement >>> system is warranted (yet) - but that may simply be because we have >>> local cards with no switching involved. So far LRU-based promotion and >>> demotion has been sufficient. >>> >> >> To me, this is a key point. As we've discussed in meetings, we're in the >> early days here. The CHMU does provide a lot of flexibility, both to >> create very good and very bad hotness trackers. But I think the key point >> is that we have multiple sources of hotness information depending on the >> platform and some of these sources only make sense for the kernel (or a >> BPF offload) to maintain as the source of truth. Some of these sources >> will be clear-on-read so only one entity would be possible to have as the >> source of truth of page hotness. >> >> I've been pretty focused on the promotion story here rather than demotion >> because of how responsive it needs to be. Harvesting the page table >> accessed bits or waiting on a sliding window through NUMA Balancing (even >> NUMAB=2) is not as responsive as needed for very fast promotion to top >> tier memory, hence things like the CHMU (or PEBS or IBS etc). >> >> A few things that I think we need to discuss and align on: >> >> - the kernel as the source of truth for all memory hotness information, >> which can then be abstracted and used for multiple downstream purposes, >> memory tiering only being one of them >> >> - the long-term plan for NUMAB=2 and memory tiering support in the kernel >> in general, are we planning on supporting this through NUMA hint faults >> forever despite their drawbacks (too slow, too much overhead for KVM) >> >> - the role of the kernel vs userspace in driving the memory migration; >> lots of discussion on hardware assists that can be leveraged for memory >> migration but today the balancing is driven in process context. The >> kthread as the driver of migration is yet to be a sold argument, but >> are where a number of companies are currently looking >> >> There's also some feature support that is possible with these CXL memory >> expansion devices that have started to pop up in labs that can also >> drastically reduce overall TCO. Perhaps Wei Xu, cc'd, will be able to >> chime in as well. >> >> This topic seems due for an alignment session as well, so will look to get >> that scheduled in the coming weeks if people are up for it. > > Our experience is that workloads in hyper-scalar data centers such as > Google often have significant cold memory. Offloading this to CXL memory > devices, backed by cheaper, lower-performance media (e.g. DRAM with > hardware compression), can be a practical approach to reduce overall > TCO. Page promotion and demotion are then critical for such a tiered > memory system. > > A kernel thread to drive hot page collection and promotion seems > logical, especially since hot page data from new sources (e.g. CHMU) > are collected outside the process execution context and in the form of > physical addresses. > > I do agree that we need to balance the complexity and benefits of any > new data structures for hotness tracking. I think there is a mismatch in the tiering structure and the patches. If you see the example in memory tiering /* * ... * Example 3: * * Node 0 is CPU + DRAM nodes, Node 1 is HBM node, node 2 is PMEM node. * * node distances: * node 0 1 2 * 0 10 20 30 * 1 20 10 40 * 2 30 40 10 * * memory_tiers0 = 1 * memory_tiers1 = 0 * memory_tiers2 = 2 *.. */ The topmost tier need not be DRAM, patch 3 states " [..] * kpromoted is a kernel thread that runs on each toptier node and * promotes pages from max_heap. " Also, there is no data in the cover letter to indicate what workloads benefit from migration to top-tier and by how much? Balbir