Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 6/8/22 3:02 AM, Yang Shi wrote:
> On Fri, Jun 3, 2022 at 6:43 AM Aneesh Kumar K.V
> <aneesh.kumar@linux.ibm.com> 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 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 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 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), 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.
>>
>> The current kernel also don't provide any interfaces for the
>> userspace to learn about the memory tier hierarchy in order to
>> optimize its memory allocations.
>>
>> This patch series address the above by defining memory tiers explicitly.
>>
>> This patch introduce explicity memory tiers with ranks. The rank
>> value of a memory tier is used to derive the demotion order between
>> NUMA nodes. The memory tiers present in a system can be found at
>>
>> /sys/devices/system/memtier/memtierN/
>>
>> The nodes which are part of a specific memory tier can be listed
>> via
>> /sys/devices/system/memtier/memtierN/nodelist
>>
>> "Rank" is an opaque value. Its absolute value doesn't have any
>> special meaning. But the rank values of different memtiers can be
>> compared with each other to determine the memory tier order.
>>
>> For example, if we have 3 memtiers: memtier0, memtier1, memiter2, and
>> their rank values are 300, 200, 100, then the memory tier order is:
>> memtier0 -> memtier2 -> memtier1, where memtier0 is the highest tier
>> and memtier1 is the lowest tier.
>>
>> The rank value of each memtier should be unique.
>>
>> A higher rank memory tier will appear first in the demotion order
>> than a lower rank memory tier. ie. while reclaim we choose a node
>> in higher rank memory tier to demote pages to as compared to a node
>> in a lower rank memory tier.
>>
>> For now we are not adding the dynamic number of memory tiers.
>> But a future series supporting that is possible. Currently
>> number of tiers supported is limitted to MAX_MEMORY_TIERS(3).
>> When doing memory hotplug, if not added to a memory tier, the NUMA
>> node gets added to DEFAULT_MEMORY_TIER(1).
>>
>> This patch is based on the proposal sent by Wei Xu <weixugc@google.com> at [1].
>>
>> [1] https://lore.kernel.org/linux-mm/CAAPL-u9Wv+nH1VOZTj=9p9S70Y3Qz3+63EkqncRDdHfubsrjfw@mail.gmail.com
>>
>> Suggested-by: Wei Xu <weixugc@google.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 |  20 ++++
>>   mm/Kconfig                   |  11 ++
>>   mm/Makefile                  |   1 +
>>   mm/memory-tiers.c            | 188 +++++++++++++++++++++++++++++++++++
>>   4 files changed, 220 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..e17f6b4ee177
>> --- /dev/null
>> +++ b/include/linux/memory-tiers.h
>> @@ -0,0 +1,20 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef _LINUX_MEMORY_TIERS_H
>> +#define _LINUX_MEMORY_TIERS_H
>> +
>> +#ifdef CONFIG_TIERED_MEMORY
>> +
>> +#define MEMORY_TIER_HBM_GPU    0
>> +#define MEMORY_TIER_DRAM       1
>> +#define MEMORY_TIER_PMEM       2
>> +
>> +#define MEMORY_RANK_HBM_GPU    300
>> +#define MEMORY_RANK_DRAM       200
>> +#define MEMORY_RANK_PMEM       100
>> +
>> +#define DEFAULT_MEMORY_TIER    MEMORY_TIER_DRAM
>> +#define MAX_MEMORY_TIERS  3
>> +
>> +#endif /* CONFIG_TIERED_MEMORY */
>> +
>> +#endif
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index 169e64192e48..08a3d330740b 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -614,6 +614,17 @@ config ARCH_ENABLE_HUGEPAGE_MIGRATION
>>   config ARCH_ENABLE_THP_MIGRATION
>>          bool
>>
>> +config TIERED_MEMORY
>> +       bool "Support for explicit memory tiers"
>> +       def_bool n
>> +       depends on MIGRATION && NUMA
>> +       help
>> +         Support to split nodes into memory tiers explicitly and
>> +         to demote pages on reclaim to lower tiers. This option
>> +         also exposes sysfs interface to read nodes available in
>> +         specific tier and to move specific node among different
>> +         possible tiers.
> 
> IMHO we should not need a new kernel config. If tiering is not present
> then there is just one tier on the system. And tiering is a kind of
> hardware configuration, the information could be shown regardless of
> whether demotion/promotion is supported/enabled or not.
> 

This was added so that we could avoid doing multiple

#if defined(CONFIG_MIGRATION) && defined(CONFIG_NUMA)

Initially I had that as def_bool y and depends on MIGRATION && NUMA. But 
it was later suggested that def_bool is not recommended for newer config.

How about

  config TIERED_MEMORY
  	bool "Support for explicit memory tiers"
-	def_bool n
-	depends on MIGRATION && NUMA
-	help
-	  Support to split nodes into memory tiers explicitly and
-	  to demote pages on reclaim to lower tiers. This option
-	  also exposes sysfs interface to read nodes available in
-	  specific tier and to move specific node among different
-	  possible tiers.
+	def_bool MIGRATION && NUMA

  config HUGETLB_PAGE_SIZE_VARIABLE
  	def_bool n

ie, we just make it a Kconfig variable without exposing it to the user?

-aneesh