Re: [RFC PATCH v4 2/7] mm/demotion: Expose per node memory tier to sysfs

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 5/27/22 7:45 PM, Jonathan Cameron wrote:
> On Fri, 27 May 2022 17:55:23 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:
> 
>> From: Jagdish Gediya <jvgediya@linux.ibm.com>
>>
>> Add support to read/write the memory tierindex for a NUMA node.
>>
>> /sys/devices/system/node/nodeN/memtier
>>
>> where N = node id
>>
>> When read, It list the memory tier that the node belongs to.
>>
>> When written, the kernel moves the node into the specified
>> memory tier, the tier assignment of all other nodes are not
>> affected.
>>
>> If the memory tier does not exist, writing to the above file
>> create the tier and assign the NUMA node to that tier.
> creates
> 
> There was some discussion in v2 of Wei Xu's RFC that what matter
> for creation is the rank, not the tier number.
> 
> My suggestion is move to an explicit creation file such as
> memtier/create_tier_from_rank
> to which writing the rank gives results in a new tier
> with the next device ID and requested rank.

I think the below workflow is much simpler.

:/sys/devices/system# cat memtier/memtier1/nodelist
1-3
:/sys/devices/system# cat node/node1/memtier
1
:/sys/devices/system# ls memtier/memtier*
nodelist  power  rank  subsystem  uevent
/sys/devices/system# ls memtier/
default_rank  max_tier  memtier1  power  uevent
:/sys/devices/system# echo 2 > node/node1/memtier
:/sys/devices/system#

:/sys/devices/system# ls memtier/
default_rank  max_tier  memtier1  memtier2  power  uevent
:/sys/devices/system# cat memtier/memtier1/nodelist
2-3
:/sys/devices/system# cat memtier/memtier2/nodelist
1
:/sys/devices/system#

ie, to create a tier we just write the tier id/tier index to 
node/nodeN/memtier file. That will create a new memory tier if needed 
and add the node to that specific memory tier. Since for now we are 
having 1:1 mapping between tier index to rank value, we can derive the 
rank value from the memory tier index.

For dynamic memory tier support, we can assign a rank value such that 
new memory tiers are always created such that it comes last in the 
demotion order.

-aneesh