Re: RFC: Memory Tiering Kernel Interfaces

[Wei Xu <weixugc@google.com>]

On Tue, May 3, 2022 at 12:12 PM Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
> On Fri, 2022-04-29 at 19:10 -0700, Wei Xu wrote:
> > 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.
> >
> > A tiering relationship between NUMA nodes in the form of demotion path
> > 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 then builds the tiering hierarchy
> > tier-by-tier by establishing the per-node demotion targets based on
> > the distances between nodes.
>
> Thanks for making this proposal.  It has many of the elements needed
> for the tiering support.
>
> >
> > The current memory tiering interface needs to be improved to address
> > several important use cases:
> >
> > * The current tiering 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 the top tier.
> >
> > * The current tiering 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 tiering 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 tiering hierarchy gets changed, even though no
> >   memory node is added or removed.  This can make the tiering
> >   hierarchy much less stable.
> >
> > * A higher tier node can only be demoted to selected nodes on the
> >   next lower tier, not any other node from the next 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.
> >
> > * There are no interfaces for the userspace to learn about the memory
> >   tiering hierarchy in order to optimize its memory allocations.
> >
> > I'd like to propose revised memory tiering kernel interfaces based on
> > the discussions in the threads:
> >
> > - https://lore.kernel.org/lkml/20220425201728.5kzm4seu7rep7ndr@offworld/T/
> > - https://lore.kernel.org/linux-mm/20220426114300.00003ad8@Huawei.com/t/
> >
> >
> > Sysfs Interfaces
> > ================
> >
> > * /sys/devices/system/node/memory_tiers
> >
> >   Format: node list (one tier per line, in the tier order)
> >
> >   When read, list memory nodes by tiers.
> >
> >   When written (one tier per line), take the user-provided node-tier
> >   assignment as the new tiering hierarchy and rebuild the per-node
> >   demotion order.  It is allowed to only override the top tiers, in
> >   which cases, the kernel will establish the lower tiers automatically.
> >
> >
> > Kernel Representation
> > =====================
> >
> > * nodemask_t node_states[N_TOPTIER_MEMORY]
> >
> >   Store all top-tier memory nodes.
> >
> > * nodemask_t memory_tiers[MAX_TIERS]
> >
> >   Store memory nodes by tiers.
> >
> > * struct demotion_nodes node_demotion[]
> >
> >   where: struct demotion_nodes { nodemask_t preferred; nodemask_t allowed; }
> >
> >   For a node N:
> >
> >   node_demotion[N].preferred lists all preferred demotion targets;
> >
> >   node_demotion[N].allowed lists all allowed demotion targets
> >   (initialized to be all the nodes in the same demotion tier).
> >
>
> I assume that the preferred list is auto-configured/initialized based on
> NUMA distances.  Not sure why "allowed" list is only to the same demotion
> tier?  For example, I think the default should be tier 0 should
> is allowed to demote to tier 1 and tier 2, not just to tier 1.  So if we
> fail to demote to tier 1, we can demote to tier 2.

I agree that we can allow demotion to go to all the lower tiers, not
just the immediate next tier.  I have mentioned the same idea as well
when replying to Dan's comments.

> Do you also expose the demotion preferred node and allowed
> list via /sys/devices/system/node/memory_tiers, as you have done in the examples?

To keep the memory tier sysfs minimal for now, I didn't propose
exposing the demotion preferred/allowed list in
/sys/devices/system/node/memory_tiers.  But now I can see that in the
way that the examples were presented, N_TOPTIER_MEMORY and
node_demotion[] can be thought as part of the memory_tiers output,
which is not the intention.

> > Examples
> > ========
> >
> > * Example 2:
> >   Node 0 & 1 are DRAM nodes.
> >   Node 2 is a PMEM node and closer to node 0.
> >
> >   Node 0 has node 2 as the preferred and only demotion target.
> >
> >   Node 1 has no preferred demotion target, but can still demote
> >   to node 2.
> >
> >   Set mempolicy to prevent cross-socket demotion and memory access,
> >   e.g. cpuset.mems=0,2
>
> Do we expect to later allow configuration of the demotion list explicitly?
> Something like:
>
> echo "demotion 0 1 1-3" > /sys/devices/system/node/memory_tiers
>
> to set demotion list for node 0, where preferred demote node is 1,
> allowed demote node list is 1-3.

IMHO, we'd better follow the allocation fallback order for the
demotion node order in each tier and avoid userspace override of
per-node demotion list.

In general, I think we'd better keep the tier assignment of each node
stable.  If adding/changing one node can redefine the tiers of other
nodes, it can make tier-based memory accounting very difficult.
Overriding the per-node demotion list can have such undesirable side
effects (if the per-node demotion list is used to redefine tiers).

> Thanks.
>
> Tim
>
>