On 8/4/22 6:26 AM, Huang, Ying wrote: > 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. > The lower level driver that is creating the nvdimm regions can assign a memory type to the numa node which it associates with the region. For now, drivers like papr_scm do that on ppc64. When it associates a numa node to nvdimm regions, it can query every detail available (device tree in case of papr_scm, can be HMAT/SLIT or CDAT) to associate the NUMA node to a memory type. > 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
