Re: [PATCH v11 1/8] mm/demotion: Add support for explicit memory tiers

Aneesh Kumar K V
2022-08-04T04:50:12Z

On 8/4/22 6:26 AM, Huang, Ying wrote:
> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>
>> On 8/2/22 12:27 PM, Huang, Ying wrote:
>>> Dan Williams <dan.j.williams@intel.com> writes:
>>>
>>>> Huang, Ying wrote:
>>>>> Dan Williams <dan.j.williams@intel.com> 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@mail.gmail.com
>>>>>>> Link: https://lore.kernel.org/linux-mm/7b72ccf4-f4ae-cb4e-f411-74d055482026@linux.ibm.com
>>>>>>>
>>>>>>> Signed-off-by: Jagdish Gediya <jvgediya@linux.ibm.com>
>>>>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>>>>>> ---
>>>>>>> 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