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From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To:     "ying.huang@intel.com" <ying.huang@intel.com>,
        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>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Greg Thelen <gthelen@google.com>,
        MichalHocko <mhocko@kernel.org>,
        Brice Goglin <brice.goglin@gmail.com>
Subject: Re: [PATCH v2 0/5] mm: demotion: Introduce new node state
 N_DEMOTION_TARGETS
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Date:   Mon, 25 Apr 2022 09:20:14 +0530
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"ying.huang@intel.com" <ying.huang@intel.com> writes:

> 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.

It is not just that the demotion can be disabled. We should be able to
use demotion on a system where we can find DRAM only NUMA nodes. That
cannot be achieved by /sys/kernel/mm/numa/demotion_enabled. It needs
something similar to to N_DEMOTION_TARGETS

>
> 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

This is achieved by 

[PATCH v2 1/5] mm: demotion: Set demotion list differently

>
> 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.


IMHO the above (2b and 3) can be done using per node demotion targets. Below is
what I think we could do with a single slow memory NUMA node 4.

/sys/devices/system/node# cat node[0-4]/demotion_targets
4
4
4
4

/sys/devices/system/node# echo 1 > node1/demotion_targets 
bash: echo: write error: Invalid argument
/sys/devices/system/node# cat node[0-4]/demotion_targets
4
4
4
4

/sys/devices/system/node# echo 0 > node1/demotion_targets 
/sys/devices/system/node# cat node[0-4]/demotion_targets
4
0
4
4

/sys/devices/system/node# echo 1 > node0/demotion_targets 
bash: echo: write error: Invalid argument
/sys/devices/system/node# cat node[0-4]/demotion_targets
4
0
4
4

Disable demotion for a specific node.
/sys/devices/system/node# echo > node1/demotion_targets 
/sys/devices/system/node# cat node[0-4]/demotion_targets
4

4
4

Reset demotion to default
/sys/devices/system/node# echo -1 > node1/demotion_targets 
/sys/devices/system/node# cat node[0-4]/demotion_targets
4
4
4
4

When a specific device/NUMA node is used for demotion target via the user interface, it is taken
out of other NUMA node targets.
root@ubuntu-guest:/sys/devices/system/node# cat node[0-4]/demotion_targets
4
4
4
4

/sys/devices/system/node# echo 4 > node1/demotion_targets 
/sys/devices/system/node# cat node[0-4]/demotion_targets

4


If more than one node requies the same demotion target
/sys/devices/system/node# echo 4 > node0/demotion_targets 
/sys/devices/system/node# cat node[0-4]/demotion_targets
4
4


-aneesh