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SubjectRe: [PATCH v2 0/5] mm: demotion: Introduce new node state N_DEMOTION_TARGETS
Hi, All,

On Fri, 2022-04-22 at 16:30 +0530, Jagdish Gediya wrote:


> I think it is necessary to either have per node demotion targets
> configuration or the user space interface supported by this patch
> series. As we don't have clear consensus on how the user interface
> should look like, we can defer the per node demotion target set
> interface to future until the real need arises.
> Current patch series sets N_DEMOTION_TARGET from dax device kmem
> driver, it may be possible that some memory node desired as demotion
> target is not detected in the system from dax-device kmem probe path.
> It is also possible that some of the dax-devices are not preferred as
> demotion target e.g. HBM, for such devices, node shouldn't be set to
> N_DEMOTION_TARGETS. In future, Support should be added to distinguish
> such dax-devices and not mark them as N_DEMOTION_TARGETS from the
> kernel, but for now this user space interface will be useful to avoid
> such devices as demotion targets.
> We can add read only interface to view per node demotion targets
> from /sys/devices/system/node/nodeX/demotion_targets, remove
> duplicated /sys/kernel/mm/numa/demotion_target interface and instead
> make /sys/devices/system/node/demotion_targets writable.
> Huang, Wei, Yang,
> What do you suggest?

We cannot remove a kernel ABI in practice. So we need to make it right
at the first time. Let's try to collect some information for the kernel
ABI definitation.

The below is just a starting point, please add your requirements.

1. Jagdish has some machines with DRAM only NUMA nodes, but they don't
want to use that as the demotion targets. But I don't think this is a
issue in practice for now, because demote-in-reclaim is disabled by

2. For machines with PMEM installed in only 1 of 2 sockets, for example,

Node 0 & 2 are cpu + dram nodes and node 1 are slow
memory node near node 0,

available: 3 nodes (0-2)
node 0 cpus: 0 1
node 0 size: n MB
node 0 free: n MB
node 1 cpus:
node 1 size: n MB
node 1 free: n MB
node 2 cpus: 2 3
node 2 size: n MB
node 2 free: n MB
node distances:
node 0 1 2
0: 10 40 20
1: 40 10 80
2: 20 80 10

We have 2 choices,

node demotion targets
0 1
2 1

node demotion targets
0 1
2 X

a) is good to take advantage of PMEM. b) is good to reduce cross-socket
traffic. Both are OK as defualt configuration. But some users may
prefer the other one. So we need a user space ABI to override the
default configuration.

3. For machines with HBM (High Bandwidth Memory), as in

> [1] local DDR = 10, remote DDR = 20, local HBM = 31, remote HBM = 41

Although HBM has better performance than DDR, in ACPI SLIT, their
distance to CPU is longer. We need to provide a way to fix this. The
user space ABI is one way. The desired result will be to use local DDR
as demotion targets of local HBM.

Best Regards,
Huang, Ying

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