Re: RFC: Memory Tiering Kernel Interfaces

Alistair Popple
2022-05-06T00:19:57Z

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Wei Xu <weixugc@google.com> writes:

[...]

>> >
>> >
>> > 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 that in an ideal world the hierarchy should be built by firmware based
on something like the HMAT. But I also think being able to override this will be
useful in getting there. Therefore a way of overriding the generated hierarchy
would be good, either via sysfs or kernel boot parameter if we don't want to
commit to a particular user interface now.

However I'm less sure letting device-drivers override this is a good idea. How
for example would a GPU driver make sure it's node is in the top tier? By moving
every node that the driver does not know about out of N_TOPTIER_MEMORY? That
could get messy if say there were two drivers both of which wanted their node to
be in the top tier.

> 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.

At some point I think we will need to deal with 3 tiers but I'd be ok with
limiting it to 2 for now if it makes things simpler.

- Alistair

>> >
>> > 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