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Subject: Re: [PATCH v2 0/5] mm: demotion: Introduce new node state
 N_DEMOTION_TARGETS
From:   "ying.huang@intel.com" <ying.huang@intel.com>
To:     Jagdish Gediya <jvgediya@linux.ibm.com>,
        Wei Xu <weixugc@google.com>, Yang Shi <shy828301@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Cc:     Linux MM <linux-mm@kvack.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Greg Thelen <gthelen@google.com>,
        MichalHocko <mhocko@kernel.org>,
        Brice Goglin <brice.goglin@gmail.com>
Date:   Sun, 24 Apr 2022 11:02:47 +0800
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Hi, All,

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

[snip]

> 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
default.

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,

a)
node	demotion targets
0	1
2	1

b)
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

https://lore.kernel.org/lkml/39cbe02a-d309-443d-54c9-678a0799342d@gmail.com/

> [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