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Date:   Mon, 25 Apr 2022 14:24:15 +0530
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Subject: Re: [PATCH v2 0/5] mm: demotion: Introduce new node state
 N_DEMOTION_TARGETS
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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>
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On 4/25/22 1:39 PM, Aneesh Kumar K V wrote:
> On 4/25/22 11:40 AM, ying.huang@intel.com wrote:
>> On Mon, 2022-04-25 at 09:20 +0530, Aneesh Kumar K.V wrote:
>>> "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
>>>
>>
>> Can you show NUMA information of your machines with DRAM-only nodes and
>> PMEM nodes?  We can try to find the proper demotion order for the
>> system.  If you can not show it, we can defer N_DEMOTION_TARGETS until
>> the machine is available.
> 
> 
> Sure will find one such config. As you might have noticed this is very 
> easy to have in a virtualization setup because the hypervisor can assign 
> memory to a guest VM from a numa node that doesn't have CPU assigned to 
> the same guest. This depends on the other guest VM instance config 
> running on the system. So on any virtualization config that has got 
> persistent memory attached, this can become an easy config to end up with.
> 
> 

something like this

$ 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
$ cat /sys/bus/nd/devices/dax0.0/target_node
2
$
# 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
:/sys/bus/dax/drivers/device_dax#