Re: [RFC PATCH v4 1/7] mm/demotion: Add support for explicit memory tiers

Ying Huang
2022-06-02T06:08:12Z

On Fri, 2022-05-27 at 17:55 +0530, Aneesh Kumar K.V wrote:
> From: Jagdish Gediya <jvgediya@linux.ibm.com>
>
> 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 adds below sysfs interface which is read-only and
> can be used to read nodes available in specific tier.
>
> /sys/devices/system/memtier/memtierN/nodelist
>
> Tier 0 is the highest tier, while tier MAX_MEMORY_TIERS - 1 is the
> lowest tier. The absolute value of a tier id number has no specific
> meaning. what matters is the relative order of the tier id numbers.
>
> All the tiered memory code is guarded by CONFIG_TIERED_MEMORY.
> Default number of memory tiers are MAX_MEMORY_TIERS(3). All the
> nodes are by default assigned to DEFAULT_MEMORY_TIER(1).
>
> Default memory tier can be read from,
> /sys/devices/system/memtier/default_tier
>
> Max memory tier can be read from,
> /sys/devices/system/memtier/max_tiers
>
> This patch implements the RFC spec sent by Wei Xu <weixugc@google.com> at [1].
>
> [1] https://lore.kernel.org/linux-mm/CAAPL-u-DGLcKRVDnChN9ZhxPkfxQvz9Sb93kVoX_4J2oiJSkUw@mail.gmail.com/
>
> Signed-off-by: Jagdish Gediya <jvgediya@linux.ibm.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

IMHO, we should change the kernel internal implementation firstly, then
implement the kerne/user space interface. That is, make memory tier
explicit inside kernel, then expose it to user space.

Best Regards,
Huang, Ying

[snip]