Re: [PATCH v6 01/13] mm/demotion: Add support for explicit memory tiers

Ying Huang
2022-06-13T03:22:44Z

Hi, Aneesh,

On Fri, 2022-06-10 at 19:22 +0530, 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 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
>
> "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 -> memtier1 -> memtier2, where memtier0 is the highest tier
> and memtier2 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.
>
> This patchset introduce 3 memory tiers (memtier0, memtier1 and memtier2)
> which are created by different kernel subsystems. The default memory
> tier created by the kernel is memtier1. Once created these memory tiers
> are not destroyed even if they don't have any NUMA nodes assigned to
> them.
>
> 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
>
> /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
>
> 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 | 3 ++
>  mm/Makefile | 1 +
>  mm/memory-tiers.c | 89 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 113 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..bb5aa585ab41 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -614,6 +614,9 @@ config ARCH_ENABLE_HUGEPAGE_MIGRATION
>  config ARCH_ENABLE_THP_MIGRATION
>   bool
>
>
> +config TIERED_MEMORY
> + def_bool NUMA
> +

As Yang pointed out, why not just use CONFIG_NUMA? I suspect the
added value of CONIFIG_TIRED_MEMORY.

>  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..d9fa955f208e
> --- /dev/null
> +++ b/mm/memory-tiers.c
> @@ -0,0 +1,89 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/types.h>
> +#include <linux/nodemask.h>
> +#include <linux/slab.h>
> +#include <linux/memory-tiers.h>
> +
> +struct memory_tier {
> + struct list_head list;
> + nodemask_t nodelist;
> + int id;
> + int rank;
> +};
> +
> +static DEFINE_MUTEX(memory_tier_lock);
> +static LIST_HEAD(memory_tiers);
> +
> +/*
> + * 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 -1;
> +}
> +
> +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);

list_for_each_entry() ?

> + if (tmp_memtier->rank < memtier->rank) {
> + list_add_tail(&memtier->list, ent);

> + return;
> + }
> + }
> + list_add_tail(&memtier->list, &memory_tiers);
> +}
> +

IMHO, the locking requirements are needed here as comments to avoid
confusing.

> +static struct memory_tier *register_memory_tier(unsigned int tier,
> + unsigned int rank)
> +{
> + struct memory_tier *memtier;
> +
> + if (tier >= MAX_MEMORY_TIERS)
> + return ERR_PTR(-EINVAL);
> +
> + memtier = kzalloc(sizeof(struct memory_tier), GFP_KERNEL);
> + if (!memtier)
> + return ERR_PTR(-ENOMEM);
> +
> + memtier->id = tier;
> + memtier->rank = rank;
> +
> + insert_memory_tier(memtier);
> +
> + return memtier;
> +}
> +
> +static int __init memory_tier_init(void)
> +{
> + struct memory_tier *memtier;
> +
> + /*
> + * Register only default memory tier to hide all empty
> + * memory tier from sysfs.
> + */
> + memtier = register_memory_tier(DEFAULT_MEMORY_TIER,
> + get_rank_from_tier(DEFAULT_MEMORY_TIER));
> +
> + if (IS_ERR(memtier))
> + panic("%s() failed to register memory tier: %ld\n",
> + __func__, PTR_ERR(memtier));
> +
> + /* CPU only nodes are not part of memory tiers. */
> + memtier->nodelist = node_states[N_MEMORY];
> +
> + return 0;
> +}
> +subsys_initcall(memory_tier_init);

Best Regards,
Huang, Ying