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

[Yang Shi <shy828301@gmail.com>]

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.

> +
>  config HUGETLB_PAGE_SIZE_VARIABLE
>         def_bool n
>         help
> diff --git a/mm/Makefile b/mm/Makefile
> index 6f9ffa968a1a..482557fbc9d1 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -92,6 +92,7 @@ obj-$(CONFIG_KFENCE) += kfence/
>  obj-$(CONFIG_FAILSLAB) += failslab.o
>  obj-$(CONFIG_MEMTEST)          += memtest.o
>  obj-$(CONFIG_MIGRATION) += migrate.o
> +obj-$(CONFIG_TIERED_MEMORY) += memory-tiers.o
>  obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o
>  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
>  obj-$(CONFIG_PAGE_COUNTER) += page_counter.o
> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> new file mode 100644
> index 000000000000..7de18d94a08d
> --- /dev/null
> +++ b/mm/memory-tiers.c
> @@ -0,0 +1,188 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/types.h>
> +#include <linux/device.h>
> +#include <linux/nodemask.h>
> +#include <linux/slab.h>
> +#include <linux/memory-tiers.h>
> +
> +struct memory_tier {
> +       struct list_head list;
> +       struct device dev;
> +       nodemask_t nodelist;
> +       int rank;
> +};
> +
> +#define to_memory_tier(device) container_of(device, struct memory_tier, dev)
> +
> +static struct bus_type memory_tier_subsys = {
> +       .name = "memtier",
> +       .dev_name = "memtier",
> +};
> +
> +static DEFINE_MUTEX(memory_tier_lock);
> +static LIST_HEAD(memory_tiers);
> +
> +
> +static ssize_t nodelist_show(struct device *dev,
> +                            struct device_attribute *attr, char *buf)
> +{
> +       struct memory_tier *memtier = to_memory_tier(dev);
> +
> +       return sysfs_emit(buf, "%*pbl\n",
> +                         nodemask_pr_args(&memtier->nodelist));
> +}
> +static DEVICE_ATTR_RO(nodelist);
> +
> +static ssize_t rank_show(struct device *dev,
> +                        struct device_attribute *attr, char *buf)
> +{
> +       struct memory_tier *memtier = to_memory_tier(dev);
> +
> +       return sysfs_emit(buf, "%d\n", memtier->rank);
> +}
> +static DEVICE_ATTR_RO(rank);
> +
> +static struct attribute *memory_tier_dev_attrs[] = {
> +       &dev_attr_nodelist.attr,
> +       &dev_attr_rank.attr,
> +       NULL
> +};
> +
> +static const struct attribute_group memory_tier_dev_group = {
> +       .attrs = memory_tier_dev_attrs,
> +};
> +
> +static const struct attribute_group *memory_tier_dev_groups[] = {
> +       &memory_tier_dev_group,
> +       NULL
> +};
> +
> +static void memory_tier_device_release(struct device *dev)
> +{
> +       struct memory_tier *tier = to_memory_tier(dev);
> +
> +       kfree(tier);
> +}
> +
> +/*
> + * Keep it simple by having  direct mapping between
> + * tier index and rank value.
> + */
> +static inline int get_rank_from_tier(unsigned int tier)
> +{
> +       switch (tier) {
> +       case MEMORY_TIER_HBM_GPU:
> +               return MEMORY_RANK_HBM_GPU;
> +       case MEMORY_TIER_DRAM:
> +               return MEMORY_RANK_DRAM;
> +       case MEMORY_TIER_PMEM:
> +               return MEMORY_RANK_PMEM;
> +       }
> +
> +       return 0;
> +}
> +
> +static void insert_memory_tier(struct memory_tier *memtier)
> +{
> +       struct list_head *ent;
> +       struct memory_tier *tmp_memtier;
> +
> +       list_for_each(ent, &memory_tiers) {
> +               tmp_memtier = list_entry(ent, struct memory_tier, list);
> +               if (tmp_memtier->rank < memtier->rank) {
> +                       list_add_tail(&memtier->list, ent);
> +                       return;
> +               }
> +       }
> +       list_add_tail(&memtier->list, &memory_tiers);
> +}
> +
> +static struct memory_tier *register_memory_tier(unsigned int tier)
> +{
> +       int error;
> +       struct memory_tier *memtier;
> +
> +       if (tier >= MAX_MEMORY_TIERS)
> +               return NULL;
> +
> +       memtier = kzalloc(sizeof(struct memory_tier), GFP_KERNEL);
> +       if (!memtier)
> +               return NULL;
> +
> +       memtier->dev.id = tier;
> +       memtier->rank = get_rank_from_tier(tier);
> +       memtier->dev.bus = &memory_tier_subsys;
> +       memtier->dev.release = memory_tier_device_release;
> +       memtier->dev.groups = memory_tier_dev_groups;
> +
> +       insert_memory_tier(memtier);
> +
> +       error = device_register(&memtier->dev);
> +       if (error) {
> +               list_del(&memtier->list);
> +               put_device(&memtier->dev);
> +               return NULL;
> +       }
> +       return memtier;
> +}
> +
> +__maybe_unused // temporay to prevent warnings during bisects
> +static void unregister_memory_tier(struct memory_tier *memtier)
> +{
> +       list_del(&memtier->list);
> +       device_unregister(&memtier->dev);
> +}
> +
> +static ssize_t
> +max_tier_show(struct device *dev, struct device_attribute *attr, char *buf)
> +{
> +       return sysfs_emit(buf, "%d\n", MAX_MEMORY_TIERS);
> +}
> +static DEVICE_ATTR_RO(max_tier);
> +
> +static ssize_t
> +default_tier_show(struct device *dev, struct device_attribute *attr, char *buf)
> +{
> +       return sysfs_emit(buf, "memtier%d\n", DEFAULT_MEMORY_TIER);
> +}
> +static DEVICE_ATTR_RO(default_tier);
> +
> +static struct attribute *memory_tier_attrs[] = {
> +       &dev_attr_max_tier.attr,
> +       &dev_attr_default_tier.attr,
> +       NULL
> +};
> +
> +static const struct attribute_group memory_tier_attr_group = {
> +       .attrs = memory_tier_attrs,
> +};
> +
> +static const struct attribute_group *memory_tier_attr_groups[] = {
> +       &memory_tier_attr_group,
> +       NULL,
> +};
> +
> +static int __init memory_tier_init(void)
> +{
> +       int ret;
> +       struct memory_tier *memtier;
> +
> +       ret = subsys_system_register(&memory_tier_subsys, memory_tier_attr_groups);
> +       if (ret)
> +               panic("%s() failed to register subsystem: %d\n", __func__, ret);
> +
> +       /*
> +        * Register only default memory tier to hide all empty
> +        * memory tier from sysfs.
> +        */
> +       memtier = register_memory_tier(DEFAULT_MEMORY_TIER);
> +       if (!memtier)
> +               panic("%s() failed to register memory tier: %d\n", __func__, ret);
> +
> +       /* CPU only nodes are not part of memory tiers. */
> +       memtier->nodelist = node_states[N_MEMORY];
> +
> +       return 0;
> +}
> +subsys_initcall(memory_tier_init);
> +
> --
> 2.36.1
>