Re: RFC: Memory Tiering Kernel Interfaces

[Wei Xu <weixugc@google.com>]

On Thu, May 5, 2022 at 5:19 PM Alistair Popple <apopple@nvidia.com> wrote:
>
> 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.

The suggestion is to allow a device driver to opt out its memory
devices from the top-tier, not the other way around.

I agree that the kernel should still be responsible for the final
node-tier assignment by taking into account all factors: the firmware
tables, device driver requests, and user-overrides (kernel argument or
sysfs).

> > 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