Re: RFC: Memory Tiering Kernel Interfaces

Wei Xu
2022-04-30T06:40:52Z

On Fri, Apr 29, 2022 at 8:59 PM Yang Shi <shy828301@gmail.com> wrote:
>
> Hi Wei,
>
> Thanks for the nice writing. Please see the below inline comments.

Thanks for the quick response and comments.

> On Fri, Apr 29, 2022 at 7:10 PM Wei Xu <weixugc@google.com> 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.
> >
> > 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.
>
> I'd prefer the firmware builds up tiers topology then passes it to
> kernel so that kernel knows what nodes are in what tiers. No matter
> what nodes are hot-removed/hot-added they always stay in their tiers
> defined by the firmware. I think this is important information like
> numa distances. NUMA distance alone can't satisfy all the usecases
> IMHO.

I agree that the firmware needs to play a bigger role in tier
topology, though it is not clear to me yet that we should require the
tier topology be fully defined by the firmware. If yes, a standard
needs to be established. Alternatively, with additional hardware
information provided by the firmware (e.g. HMAT), the kernel can be in
a much better position to initialize the proper tier topology by
itself.

It is important to keep tier topology stable, especially if we want to
account and limit memory usage based on tiers. So I agree that the
nodes should not change their tiers no matter what nodes are
hot-added/hot-removed.

Given that the additional tier-related information is not yet
available from the firmware and NUMA distance alone is not sufficient
for all the tiering use cases, and also that we want to keep tier
topology stable after the kernel boots, I suggest that we add a kernel
boot parameter to override the default tier topology (which nodes are
in which tiers). An example is: tier=2:0-1;2-3, which defines two
tiers: tier 0 includes node 0 & 1, and tier 1 includes node 2 & 3.

> >
> > * 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.
>
> TBH I still think it is too soon to define proper user visible
> interfaces for now, particularly for override.

I agree, but there are also needs to make use of tiering even as it
evolves. This is why only a minimal sysfs interface is proposed. We
can make it read-only and resort to a kernel boot parameter to
override tiers.

> >
> >
> > 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.
>
> I'd prefer nodemask_t node_states[MAX_TIERS][]. Tier 0 is always the
> top tier. The kernel could build this with the topology built by
> firmware.

node_states[N_TOPTIER_MEMORY] is for convenience and can be removed.

node_states is already an existing kernel array (defined as nodemask_t
node_states[NR_NODE_STATES]). We need an array for memory tiers, too,
which is why a new array, memory_tiers, is proposed.

Are you proposing that we make node_states a 2-dimensional array?
That can duplicate the information already in node_states, which is
not ideal.

> >
> > * 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).
>
> It seems unnecessary to define preferred and allowed IMHO. Why not
> just use something like the allocation fallback list? The first node
> in the list is the preferred one. When allocating memory for demotion,
> convert the list to a nodemask, then call __alloc_pages(gfp, order,
> first_node, nodemask). So the allocation could fallback to the allowed
> nodes automatically.

The nodemask "preferred" is an attempt to preserve a current feature
in node_demotion[]: load balancing among multiple equally-close target
nodes via random selection. We can remove it to keep things simple.

The idea of defining "preferred" and "allowed" is exactly to use
__alloc_pages(gfp, order, preferred_node, allowed_nodemask). Given
that the page allocator already computes the allocation fallback list,
it should be unnecessary to maintain an ordered demotion node list for
each node and convert such a list to a nodemask for demotion
allocation. This is why allowed is stored as a nodemask.

When demoting a page from node N, I think we can just call
__alloc_pages(gfp, order, N, memory_tiers[node_to_tier(N) + 1]). If
so, we can remove node_demotion[] entirely and add a tier field to
NODE_DATA for node_to_tier().

> >
> >
> > Tiering Hierarchy Initialization
> > ================================
> >
> > By default, all memory nodes are in the top tier (N_TOPTIER_MEMORY).
> >
> > A device driver can remove its memory nodes from the top tier, e.g.
> > a dax driver can remove PMEM nodes from the top tier.
>
> With the topology built by firmware we should not need this.

I agree. But before we have such a firmware, the kernel needs to do
its best to initialize memory tiers.

Given that we know PMEM is slower than DRAM, but a dax device might
not be PMEM, a better place to set the tier for PMEM nodes can be the
ACPI code, e.g. acpi_numa_memory_affinity_init() where we can examine
the ACPI_SRAT_MEM_NON_VOLATILE bit.

> >
> > The kernel builds the memory tiering hierarchy and per-node demotion
> > order tier-by-tier starting from N_TOPTIER_MEMORY. For a node N, the
> > best distance nodes in the next lower tier are assigned to
> > node_demotion[N].preferred and all the nodes in the next lower tier
> > are assigned to node_demotion[N].allowed.
>
> I'm not sure whether it should be allowed to demote to multiple lower
> tiers. But it is totally fine to *NOT* allow it at the moment. Once we
> figure out a good way to define demotion targets, it could be extended
> to support this easily.

You mean to only support MAX_TIERS=2 for now. I am fine with that.
There can be systems with 3 tiers, e.g. GPU -> DRAM -> PMEM, but it is
not clear yet whether we want to enable transparent memory tiering to
all the 3 tiers on such systems.

> >
> > node_demotion[N].preferred can be empty if no preferred demotion node
> > is available for node N.
> >
> > If the userspace overrides the tiers via the memory_tiers sysfs
> > interface, the kernel then only rebuilds the per-node demotion order
> > accordingly.
> >
> > Memory tiering hierarchy is rebuilt upon hot-add or hot-remove of a
> > memory node, but is NOT rebuilt upon hot-add or hot-remove of a CPU
> > node.
> >
> >
> > Memory Allocation for Demotion
> > ==============================
> >
> > When allocating a new demotion target page, both a preferred node
> > and the allowed nodemask are provided to the allocation function.
> > The default kernel allocation fallback order is used to allocate the
> > page from the specified node and nodemask.
> >
> > The memopolicy of cpuset, vma and owner task of the source page can
> > be set to refine the demotion nodemask, e.g. to prevent demotion or
> > select a particular allowed node as the demotion target.
> >
> >
> > Examples
> > ========
> >
> > * Example 1:
> > Node 0 & 1 are DRAM nodes, node 2 & 3 are PMEM nodes.
> >
> > Node 0 has node 2 as the preferred demotion target and can also
> > fallback demotion to node 3.
> >
> > Node 1 has node 3 as the preferred demotion target and can also
> > fallback demotion to node 2.
> >
> > Set mempolicy to prevent cross-socket demotion and memory access,
> > e.g. cpuset.mems=0,2
> >
> > node distances:
> > node 0 1 2 3
> > 0 10 20 30 40
> > 1 20 10 40 30
> > 2 30 40 10 40
> > 3 40 30 40 10
> >
> > /sys/devices/system/node/memory_tiers
> > 0-1
> > 2-3
> >
> > N_TOPTIER_MEMORY: 0-1
> >
> > node_demotion[]:
> > 0: [2], [2-3]
> > 1: [3], [2-3]
> > 2: [], []
> > 3: [], []
> >
> > * 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
> >
> > node distances:
> > node 0 1 2
> > 0 10 20 30
> > 1 20 10 40
> > 2 30 40 10
> >
> > /sys/devices/system/node/memory_tiers
> > 0-1
> > 2
> >
> > N_TOPTIER_MEMORY: 0-1
> >
> > node_demotion[]:
> > 0: [2], [2]
> > 1: [], [2]
> > 2: [], []
> >
> >
> > * Example 3:
> > Node 0 & 1 are DRAM nodes.
> > Node 2 is a PMEM node and has the same distance to node 0 & 1.
> >
> > Node 0 has node 2 as the preferred and only demotion target.
> >
> > Node 1 has node 2 as the preferred and only demotion target.
> >
> > node distances:
> > node 0 1 2
> > 0 10 20 30
> > 1 20 10 30
> > 2 30 30 10
> >
> > /sys/devices/system/node/memory_tiers
> > 0-1
> > 2
> >
> > N_TOPTIER_MEMORY: 0-1
> >
> > node_demotion[]:
> > 0: [2], [2]
> > 1: [2], [2]
> > 2: [], []
> >
> >
> > * Example 4:
> > Node 0 & 1 are DRAM nodes, Node 2 is a memory-only DRAM node.
> >
> > All nodes are top-tier.
> >
> > node distances:
> > node 0 1 2
> > 0 10 20 30
> > 1 20 10 30
> > 2 30 30 10
> >
> > /sys/devices/system/node/memory_tiers
> > 0-2
> >
> > N_TOPTIER_MEMORY: 0-2
> >
> > node_demotion[]:
> > 0: [], []
> > 1: [], []
> > 2: [], []
> >
> >
> > * Example 5:
> > Node 0 is a DRAM node with CPU.
> > Node 1 is a HBM node.
> > Node 2 is a PMEM node.
> >
> > With userspace override, node 1 is the top tier and has node 0 as
> > the preferred and only demotion target.
> >
> > Node 0 is in the second tier, tier 1, and has node 2 as the
> > preferred and only demotion target.
> >
> > Node 2 is in the lowest tier, tier 2, and has no demotion targets.
> >
> > node distances:
> > node 0 1 2
> > 0 10 21 30
> > 1 21 10 40
> > 2 30 40 10
> >
> > /sys/devices/system/node/memory_tiers (userspace override)
> > 1
> > 0
> > 2
> >
> > N_TOPTIER_MEMORY: 1
> >
> > node_demotion[]:
> > 0: [2], [2]
> > 1: [0], [0]
> > 2: [], []
> >
> > -- Wei