Re: [PATCH v3 0/7] mm: demotion: Introduce new node state N_DEMOTION_TARGETS

From: Aneesh Kumar K V
Date: Mon Apr 25 2022 - 10:45:30 EST

On 4/25/22 7:27 PM, Jonathan Cameron wrote:
On Mon, 25 Apr 2022 16:45:38 +0530
Jagdish Gediya <jvgediya@xxxxxxxxxxxxx> wrote:

On Sun, Apr 24, 2022 at 11:19:53AM +0800, ying.huang@xxxxxxxxx wrote:
On Sat, 2022-04-23 at 01:25 +0530, Jagdish Gediya wrote:
Some systems(e.g. PowerVM) can have both DRAM(fast memory) only
NUMA node which are N_MEMORY and slow memory(persistent memory)
only NUMA node which are also N_MEMORY. As the current demotion
target finding algorithm works based on N_MEMORY and best distance,
it will choose DRAM only NUMA node as demotion target instead of
persistent memory node on such systems. If DRAM only NUMA node is
filled with demoted pages then at some point new allocations can
start falling to persistent memory, so basically cold pages are in
fast memor (due to demotion) and new pages are in slow memory, this
is why persistent memory nodes should be utilized for demotion and
dram node should be avoided for demotion so that they can be used
for new allocations.

Current implementation can work fine on the system where the memory
only numa nodes are possible only for persistent/slow memory but it
is not suitable for the like of systems mentioned above.

Can you share the NUMA topology information of your machine? And the
demotion order before and after your change?

Whether it's good to use the PMEM nodes as the demotion targets of the
DRAM-only node too?

$ numactl -H
available: 2 nodes (0-1)
node 0 cpus: 0 1 2 3 4 5 6 7
node 0 size: 14272 MB
node 0 free: 13392 MB
node 1 cpus:
node 1 size: 2028 MB
node 1 free: 1971 MB
node distances:
node 0 1
0: 10 40
1: 40 10

1) without N_DEMOTION_TARGETS patch series, 1 is demotion target
for 0 even when 1 is DRAM node and there is no demotion targets for 1.

I'm not convinced the distinction between DRAM and persistent memory is
valid. There will definitely be systems with a large pool
of remote DRAM (and potentially no NV memory) where the right choice
is to demote to that DRAM pool.

Basing the decision on whether the memory is from kmem or
normal DRAM doesn't provide sufficient information to make the decision.

Hence the suggestion for the ability to override this from userspace. Now, for example, we could build a system with memory from the remote machine (memory inception in case of power which will mostly be plugged in as regular hotpluggable memory ) and a slow CXL memory or OpenCAPI memory.

In the former case, we won't consider that for demotion with this series because that is not instantiated via dax kmem. So yes definitely we would need the ability to override this from userspace so that we could put these remote memory NUMA nodes as demotion targets if we want.

$ cat /sys/bus/nd/devices/dax0.0/target_node
# cd /sys/bus/dax/drivers/
:/sys/bus/dax/drivers# ls
device_dax kmem
:/sys/bus/dax/drivers# cd device_dax/
:/sys/bus/dax/drivers/device_dax# echo dax0.0 > unbind
:/sys/bus/dax/drivers/device_dax# echo dax0.0 > ../kmem/new_id
:/sys/bus/dax/drivers/device_dax# numactl -H
available: 3 nodes (0-2)
node 0 cpus: 0 1 2 3 4 5 6 7
node 0 size: 14272 MB
node 0 free: 13380 MB
node 1 cpus:
node 1 size: 2028 MB
node 1 free: 1961 MB
node 2 cpus:
node 2 size: 0 MB
node 2 free: 0 MB
node distances:
node 0 1 2
0: 10 40 80
1: 40 10 80
2: 80 80 10

2) Once this new node brought online, without N_DEMOTION_TARGETS
patch series, 1 is demotion target for 0 and 2 is demotion target
for 1.

With this patch series applied,
1) No demotion target for either 0 or 1 before dax device is online

I'd argue that is wrong. At this state you have a tiered memory system
be it one with just DRAM. Using it as such is correct behavior that
we should not be preventing. Sure some usecases wouldn't want that
arrangement but some do want it.

For your case we could add a heuristic along the lines of the demotion
target should be at least as big as the starting point but that would
be a bit hacky.

Hence the proposal to do a per node demotion target override with the semantics that i explained here

Let me know if that interface would be good to handle all the possible demotion target configs we would want to have.