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

From: Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
To: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
	Huang Ying <ying.huang@intel.com>,
	Greg Thelen <gthelen@google.com>, Yang Shi <shy828301@gmail.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Tim C Chen <tim.c.chen@intel.com>,
	Brice Goglin <brice.goglin@gmail.com>,
	Michal Hocko <mhocko@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Hesham Almatary <hesham.almatary@huawei.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Alistair Popple <apopple@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Feng Tang <feng.tang@intel.com>,
	Jagdish Gediya <jvgediya@linux.ibm.com>,
	Baolin Wang <baolin.wang@linux.alibaba.com>,
	David Rientjes <rientjes@google.com>
Subject: Re: [RFC PATCH v4 2/7] mm/demotion: Expose per node memory tier to sysfs
Date: Fri, 3 Jun 2022 14:10:47 +0530	[thread overview]
Message-ID: <fbebbd2b-2ddb-bee6-5e12-67e3e18648ee@linux.ibm.com> (raw)
In-Reply-To: <20220527151531.00002a0c@Huawei.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