On Tue, Jul 12, 2022 at 8:42 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Yang Shi <shy828301@xxxxxxxxx> writes: > > > On Mon, Jul 11, 2022 at 10:10 PM Aneesh Kumar K V > > <aneesh.kumar@xxxxxxxxxxxxx> wrote: > >> > >> 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. > > > > I think the most controversial part is the user visible interfaces so > > far. And IIUC the series could be split roughly into two parts, patch > > 1 - 5 and others. The patch 1 -5 added the explicit memory tier > > support and fixed the issue reported by Jagdish. I think we are on the > > same page for this part. But I haven't seen any thorough review on > > those patches yet since we got distracted by spending most time > > discussing about the user visible interfaces. > > > > So would it help to move things forward to submit patch 1 - 5 as a > > standalone series to get thorough review then get merged? > > Yes. I think this is a good idea. We can discuss the in kernel > implementation (without user space interface) in details and try to make > it merged. > > And we can continue our discussion of user space interface in a separate > thread. > > Best Regards, > Huang, Ying > I also agree that it is a good idea to split this patch series into the kernel and userspace parts. The current sysfs interface provides more dynamic memtiers than what I have expected. Let's have more discussions on that after the kernel space changes are finalized. Wei
