On 7/12/22 10:12 AM, Aneesh Kumar K V wrote: > On 7/12/22 6:46 AM, Huang, Ying wrote: >> Aneesh Kumar K V <aneesh.kumar@xxxxxxxxxxxxx> writes: >> >>> On 7/5/22 9:59 AM, Huang, Ying wrote: >>>> Hi, Aneesh, >>>> >>>> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> writes: >>>> >>>>> The current kernel has the basic memory tiering support: Inactive >>>>> pages on a higher tier NUMA node can be migrated (demoted) to a lower >>>>> tier NUMA node to make room for new allocations on the higher tier >>>>> NUMA node. Frequently accessed pages on a lower tier NUMA node can be >>>>> migrated (promoted) to a higher tier NUMA node to improve the >>>>> performance. >>>>> >>>>> In the current kernel, memory tiers are defined implicitly via a >>>>> demotion path relationship between NUMA nodes, which is created during >>>>> the kernel initialization and updated when a NUMA node is hot-added or >>>>> hot-removed. The current implementation puts all nodes with CPU into >>>>> the top tier, and builds the tier hierarchy tier-by-tier by establishing >>>>> the per-node demotion targets based on the distances between nodes. >>>>> >>>>> This current memory tier kernel interface needs to be improved for >>>>> several important use cases: >>>>> >>>>> * The current tier initialization code always initializes >>>>> each memory-only NUMA node into a lower tier. But a memory-only >>>>> NUMA node may have a high performance memory device (e.g. a DRAM >>>>> device attached via CXL.mem or a DRAM-backed memory-only node on >>>>> a virtual machine) and should be put into a higher tier. >>>>> >>>>> * The current tier hierarchy always puts CPU nodes into the top >>>>> tier. But on a system with HBM (e.g. GPU memory) devices, these >>>>> memory-only HBM NUMA nodes should be in the top tier, and DRAM nodes >>>>> with CPUs are better to be placed into the next lower tier. >>>>> >>>>> * Also because the current tier hierarchy always puts CPU nodes >>>>> into the top tier, when a CPU is hot-added (or hot-removed) and >>>>> triggers a memory node from CPU-less into a CPU node (or vice >>>>> versa), the memory tier hierarchy gets changed, even though no >>>>> memory node is added or removed. This can make the tier >>>>> hierarchy unstable and make it difficult to support tier-based >>>>> memory accounting. >>>>> >>>>> * A higher tier node can only be demoted to selected nodes on the >>>>> next lower tier as defined by the demotion path, not any other >>>>> node from any lower tier. This strict, hard-coded demotion order >>>>> does not work in all use cases (e.g. some use cases may want to >>>>> allow cross-socket demotion to another node in the same demotion >>>>> tier as a fallback when the preferred demotion node is out of >>>>> space), and has resulted in the feature request for an interface to >>>>> override the system-wide, per-node demotion order from the >>>>> userspace. This demotion order is also inconsistent with the page >>>>> allocation fallback order when all the nodes in a higher tier are >>>>> out of space: The page allocation can fall back to any node from >>>>> any lower tier, whereas the demotion order doesn't allow that. >>>>> >>>>> * There are no interfaces for the userspace to learn about the memory >>>>> tier hierarchy in order to optimize its memory allocations. >>>>> >>>>> This patch series make the creation of memory tiers explicit under >>>>> the control of userspace or device driver. >>>>> >>>>> Memory Tier Initialization >>>>> ========================== >>>>> >>>>> By default, all memory nodes are assigned to the default tier with >>>>> tier ID value 200. >>>>> >>>>> A device driver can move up or down its memory nodes from the default >>>>> tier. For example, PMEM can move down its memory nodes below the >>>>> default tier, whereas GPU can move up its memory nodes above the >>>>> default tier. >>>>> >>>>> The kernel initialization code makes the decision on which exact tier >>>>> a memory node should be assigned to based on the requests from the >>>>> device drivers as well as the memory device hardware information >>>>> provided by the firmware. >>>>> >>>>> Hot-adding/removing CPUs doesn't affect memory tier hierarchy. >>>>> >>>>> Memory Allocation for Demotion >>>>> ============================== >>>>> This patch series keep the demotion target page allocation logic same. >>>>> The demotion page allocation pick the closest NUMA node in the >>>>> next lower tier to the current NUMA node allocating pages from. >>>>> >>>>> This will be later improved to use the same page allocation strategy >>>>> using fallback list. >>>>> >>>>> Sysfs Interface: >>>>> ------------- >>>>> Listing current list of memory tiers details: >>>>> >>>>> :/sys/devices/system/memtier$ ls >>>>> default_tier max_tier memtier1 power uevent >>>>> :/sys/devices/system/memtier$ cat default_tier >>>>> memtier200 >>>>> :/sys/devices/system/memtier$ cat max_tier >>>>> 400 >>>>> :/sys/devices/system/memtier$ >>>>> >>>>> Per node memory tier details: >>>>> >>>>> For a cpu only NUMA node: >>>>> >>>>> :/sys/devices/system/node# cat node0/memtier >>>>> :/sys/devices/system/node# echo 1 > node0/memtier >>>>> :/sys/devices/system/node# cat node0/memtier >>>>> :/sys/devices/system/node# >>>>> >>>>> For a NUMA node with memory: >>>>> :/sys/devices/system/node# cat node1/memtier >>>>> 1 >>>>> :/sys/devices/system/node# ls ../memtier/ >>>>> default_tier max_tier memtier1 power uevent >>>>> :/sys/devices/system/node# echo 2 > node1/memtier >>>>> :/sys/devices/system/node# >>>>> :/sys/devices/system/node# ls ../memtier/ >>>>> default_tier max_tier memtier1 memtier2 power uevent >>>>> :/sys/devices/system/node# cat node1/memtier >>>>> 2 >>>>> :/sys/devices/system/node# >>>>> >>>>> Removing a memory tier >>>>> :/sys/devices/system/node# cat node1/memtier >>>>> 2 >>>>> :/sys/devices/system/node# echo 1 > node1/memtier >>>> >>>> Thanks a lot for your patchset. >>>> >>>> Per my understanding, we haven't reach consensus on >>>> >>>> - how to create the default memory tiers in kernel (via abstract >>>> distance provided by drivers? Or use SLIT as the first step?) >>>> >>>> - how to override the default memory tiers from user space >>>> >>>> As in the following thread and email, >>>> >>>> https://lore.kernel.org/lkml/YqjZyP11O0yCMmiO@xxxxxxxxxxx/ >>>> >>>> I think that we need to finalized on that firstly? >>> >>> I did list the proposal here >>> >>> https://lore.kernel.org/linux-mm/7b72ccf4-f4ae-cb4e-f411-74d055482026@xxxxxxxxxxxxx >>> >>> So both the kernel default and driver-specific default tiers now become kernel parameters that can be updated >>> if the user wants a different tier topology. >>> >>> All memory that is not managed by a driver gets added to default_memory_tier which got a default value of 200 >>> >>> For now, the only driver that is updated is dax kmem, which adds the memory it manages to memory tier 100. >>> Later as we learn more about the device attributes (HMAT or something similar) that we might want to use >>> to control the tier assignment this can be a range of memory tiers. >>> >>> Based on the above, I guess we can merge what is posted in this series and later fine-tune/update >>> the memory tier assignment based on device attributes. >> >> Sorry for late reply. >> >> As the first step, it may be better to skip the parts that we haven't >> reached consensus yet, for example, the user space interface to override >> the default memory tiers. And we can use 0, 1, 2 as the default memory >> tier IDs. We can refine/revise the in-kernel implementation, but we >> cannot change the user space ABI. >> > > Can you help list the use case that will be broken by using tierID as outlined in this series? > One of the details that were mentioned earlier was the need to track top-tier memory usage in a > memcg and IIUC the patchset posted https://lore.kernel.org/linux-mm/cover.1655242024.git.tim.c.chen@xxxxxxxxxxxxxxx > can work with tier IDs too. Let me know if you think otherwise. So at this point > I am not sure which area we are still debating w.r.t the userspace interface. > > I will still keep the default tier IDs with a large range between them. That will allow > us to go back to tierID based demotion order if we can. That is much simpler than using tierID and rank > together. If we still want to go back to rank based approach the tierID value won't have much > meaning anyway. > > Any feedback on patches 1 - 5, so that I can request Andrew to merge them? > Looking at this again, I guess we just need to drop patch 7 mm/demotion: Add per node memory tier attribute to sysfs ? We do agree to use the device model to expose memory tiers to userspace so patch 6 can still be included. It also exposes max_tier, default_tier, and node list of a memory tier. All these are useful and agreed upon. Hence patch 6 can be merged? patch 8 - 10 -> are done based on the request from others and is independent of how memory tiers are exposed/created from userspace. Hence that can be merged? If you agree I can rebase the series moving patch 7,11,12 as the last patches in the series so that we can skip merging them based on what we conclude w.r.t usage of rank. -aneesh
