Aneesh Kumar K V <aneesh.kumar@xxxxxxxxxxxxx> writes: > On 8/2/22 12:27 PM, Huang, Ying wrote: >> Dan Williams <dan.j.williams@xxxxxxxxx> writes: >> >>> Huang, Ying wrote: >>>> Dan Williams <dan.j.williams@xxxxxxxxx> writes: >>>> >>>>> Aneesh Kumar K.V wrote: >>>>>> 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 highest 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 implementation 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-backed memory-only node on a virtual machine) that >>>>>> 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 or GPU devices, the memory-only NUMA nodes mapping these devices >>>>>> should be in the top tier, and DRAM nodes with CPUs are better to be placed into >>>>>> the next lower tier. >>>>>> >>>>>> With current kernel higher tier node can only be demoted to nodes with shortest >>>>>> distance on the next lower tier as defined by the demotion path, not any other >>>>>> node from any lower tier. This strict, 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), 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. >>>>>> >>>>>> This patch series address the above by defining memory tiers explicitly. >>>>>> >>>>>> Linux kernel presents memory devices as NUMA nodes and each memory device is of >>>>>> a specific type. The memory type of a device is represented by its abstract >>>>>> distance. A memory tier corresponds to a range of abstract distance. This allows >>>>>> for classifying memory devices with a specific performance range into a memory >>>>>> tier. >>>>>> >>>>>> This patch configures the range/chunk size to be 128. The default DRAM >>>>>> abstract distance is 512. We can have 4 memory tiers below the default DRAM >>>>>> abstract distance which cover the range 0 - 127, 127 - 255, 256- 383, 384 - 511. >>>>>> Slower memory devices like persistent memory will have abstract distance below >>>>>> the default DRAM level and hence will be placed in these 4 lower tiers. >>>>>> >>>>>> A kernel parameter is provided to override the default memory tier. >>>>>> >>>>>> Link: https://lore.kernel.org/linux-mm/CAAPL-u9Wv+nH1VOZTj=9p9S70Y3Qz3+63EkqncRDdHfubsrjfw@xxxxxxxxxxxxxx >>>>>> Link: https://lore.kernel.org/linux-mm/7b72ccf4-f4ae-cb4e-f411-74d055482026@xxxxxxxxxxxxx >>>>>> >>>>>> Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx> >>>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx> >>>>>> --- >>>>>> include/linux/memory-tiers.h | 17 ++++++ >>>>>> mm/Makefile | 1 + >>>>>> mm/memory-tiers.c | 102 +++++++++++++++++++++++++++++++++++ >>>>>> 3 files changed, 120 insertions(+) >>>>>> create mode 100644 include/linux/memory-tiers.h >>>>>> create mode 100644 mm/memory-tiers.c >>>>>> >>>>>> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h >>>>>> new file mode 100644 >>>>>> index 000000000000..8d7884b7a3f0 >>>>>> --- /dev/null >>>>>> +++ b/include/linux/memory-tiers.h >>>>>> @@ -0,0 +1,17 @@ >>>>>> +/* SPDX-License-Identifier: GPL-2.0 */ >>>>>> +#ifndef _LINUX_MEMORY_TIERS_H >>>>>> +#define _LINUX_MEMORY_TIERS_H >>>>>> + >>>>>> +/* >>>>>> + * Each tier cover a abstrace distance chunk size of 128 >>>>>> + */ >>>>>> +#define MEMTIER_CHUNK_BITS 7 >>>>>> +#define MEMTIER_CHUNK_SIZE (1 << MEMTIER_CHUNK_BITS) >>>>>> +/* >>>>>> + * For now let's have 4 memory tier below default DRAM tier. >>>>>> + */ >>>>>> +#define MEMTIER_ADISTANCE_DRAM (1 << (MEMTIER_CHUNK_BITS + 2)) >>>>>> +/* leave one tier below this slow pmem */ >>>>>> +#define MEMTIER_ADISTANCE_PMEM (1 << MEMTIER_CHUNK_BITS) >>>>> >>>>> Why is memory type encoded in these values? There is no reason to >>>>> believe that PMEM is of a lower performance tier than DRAM. Consider >>>>> high performance energy backed DRAM that makes it "PMEM", consider CXL >>>>> attached DRAM over a switch topology and constrained links that makes it >>>>> a lower performance tier than locally attached DRAM. The names should be >>>>> associated with tiers that indicate their usage. Something like HOT, >>>>> GENERAL, and COLD. Where, for example, HOT is low capacity high >>>>> performance compared to the general purpose pool, and COLD is high >>>>> capacity low performance intended to offload the general purpose tier. >>>>> >>>>> It does not need to be exactly that ontology, but please try to not >>>>> encode policy meaning behind memory types. There has been explicit >>>>> effort to avoid that to date because types are fraught for declaring >>>>> relative performance characteristics, and the relative performance >>>>> changes based on what memory types are assembled in a given system. >>>> >>>> Yes. MEMTIER_ADISTANCE_PMEM is something over simplified. That is only >>>> used in this very first version to make it as simple as possible. >>> >>> I am failing to see the simplicity of using names that convey a >>> performance contract that are invalid depending on the system. >>> >>>> I think we can come up with something better in the later version. >>>> For example, identify the abstract distance of a PMEM device based on >>>> HMAT, etc. >>> >>> Memory tiering has nothing to do with persistence why is PMEM in the >>> name at all? >>> >>>> And even in this first version, we should put MEMTIER_ADISTANCE_PMEM >>>> in dax/kmem.c. Because it's just for that specific type of memory >>>> used now, not for all PMEM. >>> >>> dax/kmem.c also handles HBM and "soft reserved" memory in general. There >>> is also nothing PMEM specific about the device-dax subsystem. >> >> Ah... I see the issue here. For the systems in our hand, dax/kmem.c is >> used to online PMEM only. Even the "soft reserved" memory is used for >> PMEM or simulating PMEM too. So to make the code as simple as possible, >> we treat all memory devices onlined by dax/kmem as PMEM in the first >> version. And plan to support more memory types in the future versions. >> >> But from your above words, our assumption are wrong here. dax/kmem.c >> can online HBM and other memory devices already. If so, how do we >> distinguish between them and how to get the performance character of >> these devices? We can start with SLIT? >> > > We would let low level driver register memory_dev_types for the NUMA nodes > that will be mapped to these devices. ie, a papr_scm, ACPI NFIT or CXL > can register different memory_dev_type based on device tree, HMAT or CDAT. I didn't find ACPI NFIT can provide any performance information, just whether it's non-volatile. HMAT or CDAT should help here, but it's not available always. For now, what we have is just SLIT at least for quite some machines. I prefer to create memory_dev_type in high level driver like dax/kmem. And it may query low level driver like SLIT, HMAT, CDAT, etc for more information based on availability etc. Best Regards, Huang, Ying
