Re: [PATCH v11 0/8] mm/demotion: Memory tiers and demotion

Huang, Ying
2022-07-29T05:30:53Z

"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:

> The current kernel has the basic memory tiering support: Inactive pages on a
> higher tier NUMA node can be migrated (demoted) to a lower tier NUMA node to
> make room for new allocations on the higher tier NUMA node. Frequently accessed
> pages on a lower tier NUMA node can be migrated (promoted) to a higher tier NUMA
> node to improve the performance.
>
> 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 (e.g. GPU memory) devices, these memory-only HBM NUMA nodes
> should be in the top tier, and DRAM nodes with CPUs are better to be placed
> into the next lower tier.
>
> * Also because the current tier hierarchy always puts CPU nodes into the top
> tier, when a CPU is hot-added (or hot-removed) and triggers a memory node from
> CPU-less into a CPU node (or vice versa), the memory tier hierarchy gets
> changed, even though no memory node is added or removed. This can make the
> tier hierarchy unstable and make it difficult to support tier-based memory
> accounting.
>
> * A 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), and has resulted in the feature request for an interface to override
> the system-wide, per-node demotion order from the userspace. 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 make the creation of memory tiers explicit under
> the control of device driver.
>
> Memory Tier Initialization
> ==========================
>
> 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.
>
> By default, all memory nodes are assigned to the default tier with
> abstract distance 512.
>
> A device driver can move its memory nodes from the default tier. For example,
> PMEM can move its memory nodes below the default tier, whereas GPU can move its
> memory nodes above the default tier.
>
> The kernel initialization code makes the decision on which exact tier a memory
> node should be assigned to based on the requests from the device drivers as well
> as the memory device hardware information provided by the firmware.
>
> Hot-adding/removing CPUs doesn't affect memory tier hierarchy.

Some patch description of [0/8] is same as that of [1/8] originally. It
appears that you revised [1/8], but forget to revise [0/8] too. Please
do that.

Best Regards,
Huang, Ying

> Changes from v10:
> * rename performance level to abstract distance
> * Thanks to all the good feedback from Huang, Ying <ying.huang@intel.com>.
> Updated the patchset to cover most of the review feedback.
>
> Changes from v9:
> * Use performance level for initializing memory tiers.
>
> Changes from v8:
> * Drop the sysfs interface patches and related documentation changes.
>
> Changes from v7:
> * Fix kernel crash with demotion.
> * Improve documentation.
>
> Changes from v6:
> * Drop the usage of rank.
> * Address other review feedback.
>
> Changes from v5:
> * Remove patch supporting N_MEMORY node removal from memory tiers. memory tiers
> are going to be used for features other than demotion. Hence keep all N_MEMORY
> nodes in memory tiers irrespective of whether they want to participate in promotion or demotion.
> * Add NODE_DATA->memtier
> * Rearrage patches to add sysfs files later.
> * Add support to create memory tiers from userspace.
> * Address other review feedback.
>
>
> Changes from v4:
> * Address review feedback.
> * Reverse the meaning of "rank": higher rank value means higher tier.
> * Add "/sys/devices/system/memtier/default_tier".
> * Add node_is_toptier
>
> v4:
> Add support for explicit memory tiers and ranks.
>
> v3:
> - Modify patch 1 subject to make it more specific
> - Remove /sys/kernel/mm/numa/demotion_targets interface, use
> /sys/devices/system/node/demotion_targets instead and make
> it writable to override node_states[N_DEMOTION_TARGETS].
> - Add support to view per node demotion targets via sysfs
>
> v2:
> In v1, only 1st patch of this patch series was sent, which was
> implemented to avoid some of the limitations on the demotion
> target sharing, however for certain numa topology, the demotion
> targets found by that patch was not most optimal, so 1st patch
> in this series is modified according to suggestions from Huang
> and Baolin. Different examples of demotion list comparasion
> between existing implementation and changed implementation can
> be found in the commit message of 1st patch.
>
>
>
> Aneesh Kumar K.V (7):
> mm/demotion: Add support for explicit memory tiers
> mm/demotion: Move memory demotion related code
> mm/demotion: Add hotplug callbacks to handle new numa node onlined
> mm/demotion/dax/kmem: Set node's abstract distance to
> MEMTIER_ADISTANCE_PMEM
> mm/demotion: Build demotion targets based on explicit memory tiers
> mm/demotion: Add pg_data_t member to track node memory tier details
> mm/demotion: Update node_is_toptier to work with memory tiers
>
> Jagdish Gediya (1):
> mm/demotion: Demote pages according to allocation fallback order
>
> drivers/dax/kmem.c | 9 +
> include/linux/memory-tiers.h | 79 +++++
> include/linux/migrate.h | 15 -
> include/linux/mmzone.h | 3 +
> include/linux/node.h | 5 -
> mm/Makefile | 1 +
> mm/huge_memory.c | 1 +
> mm/memory-tiers.c | 586 +++++++++++++++++++++++++++++++++++
> mm/migrate.c | 453 +--------------------------
> mm/mprotect.c | 1 +
> mm/vmscan.c | 59 +++-
> mm/vmstat.c | 4 -
> 12 files changed, 725 insertions(+), 491 deletions(-)
> create mode 100644 include/linux/memory-tiers.h
> create mode 100644 mm/memory-tiers.c