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From: "Huang, Ying" <ying.huang@intel.com>
To: Li Zhijian <lizhijian@fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,  Greg Kroah-Hartman
 <gregkh@linuxfoundation.org>,  rafael@kernel.org,  linux-mm@kvack.org,
  y-goto@fujitsu.com,  linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 1/4] drivers/base/node: Add demotion_nodes sys
 infterface
In-Reply-To: <d1d5add8-8f4a-4578-8bf0-2cbe79b09989@fujitsu.com> (Li Zhijian's
	message of "Tue, 30 Jan 2024 16:53:10 +0800")
References: <20231102025648.1285477-1-lizhijian@fujitsu.com>
	<20231102025648.1285477-2-lizhijian@fujitsu.com>
	<878r7g3ktj.fsf@yhuang6-desk2.ccr.corp.intel.com>
	<d1d5add8-8f4a-4578-8bf0-2cbe79b09989@fujitsu.com>
Date: Wed, 31 Jan 2024 09:13:35 +0800
Message-ID: <87fryegv9c.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Li Zhijian <lizhijian@fujitsu.com> writes:

> Hi Ying
>
>
> I need to pick up this thread/patch again.
>
>> We have /sys/devices/virtual/memory_tiering/memory_tier*/nodelist
>> already.  A node in a higher tier can demote to any node in the lower
>> tiers.  What's more need to be displayed in nodeX/demotion_nodes?
>> 
>
> Yes, it's believed that /sys/devices/virtual/memory_tiering/memory_tierN/nodelist
> are intended to show nodes in memory_tierN. But IMHO, it's not enough, especially
> for the preferred demotion node(s).
>
> Currently, when a demotion occurs, it will prioritize selecting a node
> from the preferred nodes as the destination node for the demotion. If
> the preferred nodes does not meet the requirements, it will try from all
> the lower memory tier nodes until it finds a suitable demotion destination
> node or ultimately fails.
>                                                                                 However,
> currently it only lists the nodes of each tier. If the
> administrators want to know all the possible demotion destinations for a
> given node, they need to calculate it themselves:
> Step 1, find the memory tier where the given node is located
> Step 2, list all nodes under all its lower tiers
>                                                                                    It
> will be even more difficult to know the preferred nodes which depend
> on
> more factors, distance etc. For the following example, we may have 6 nodes
> splitting into three memory tiers.
>                                                                                 For
> emulated hmat numa topology example:
>> $ numactl -H
>> available: 6 nodes (0-5)
>> node 0 cpus: 0
>> node 0 size: 1974 MB
>> node 0 free: 1767 MB
>> node 1 cpus: 1
>> node 1 size: 1694 MB
>> node 1 free: 1454 MB
>> node 2 cpus:
>> node 2 size: 896 MB
>> node 2 free: 896 MB
>> node 3 cpus:
>> node 3 size: 896 MB
>> node 3 free: 896 MB
>> node 4 cpus:
>> node 4 size: 896 MB
>> node 4 free: 896 MB
>> node 5 cpus:
>> node 5 size: 896 MB
>> node 5 free: 896 MB
>> node distances:
>> node   0   1   2   3   4   5
>> 0:  10  31  21  41  21  41
>> 1:  31  10  41  21  41  21
>> 2:  21  41  10  51  21  51
>> 3:  31  21  51  10  51  21
>> 4:  21  41  21  51  10  51
>> 5:  31  21  51  21  51  10
>> $ cat memory_tier4/nodelist
>> 0-1
>> $ cat memory_tier12/nodelist
>> 2,5
>> $ cat memory_tier54/nodelist
>> 3-4                                                                           
>                                                                                 For
> above topology, memory-tier will build the demotion path for each node
> like this:
> node[0].preferred = 2
> node[0].demotion_targets = 2-5
> node[1].preferred = 5
> node[1].demotion_targets = 2-5
> node[2].preferred = 4
> node[2].demotion_targets = 3-4
> node[3].preferred = <empty>
> node[3].demotion_targets = <empty>
> node[4].preferred = <empty>
> node[4].demotion_targets = <empty>
> node[5].preferred = 3
> node[5].demotion_targets = 3-4
>                                                                          But
> this demotion path is not explicitly known to administrator. And with
> the
> feedback from our customers, they also think it is helpful to know demotion
> path built by kernel to understand the demotion behaviors.
>
> So i think we should have 2 new interfaces for each node:
>
> /sys/devices/system/node/nodeN/demotion_allowed_nodes
> /sys/devices/system/node/nodeN/demotion_preferred_nodes
>
> I value your opinion, and I'd like to know what you think about...

Per my understanding, we will not expose everything inside kernel to
user space.  For page placement in a tiered memory system, demotion is
just a part of the story.  For example, if the DRAM of a system becomes
full, new page allocation will fall back to the CXL memory.  Have we
exposed the default page allocation fallback order to user space?

All in all, in my opinion, we only expose as little as possible to user
space because we need to maintain the ABI for ever.

--
Best Regards,
Huang, Ying

>
> On 02/11/2023 11:17, Huang, Ying wrote:
>> Li Zhijian <lizhijian@fujitsu.com> writes:
>> 
>>> It shows the demotion target nodes of a node. Export this information to
>>> user directly.
>>>
>>> Below is an example where node0 node1 are DRAM, node3 is a PMEM node.
>>> - Before PMEM is online, no demotion_nodes for node0 and node1.
>>> $ cat /sys/devices/system/node/node0/demotion_nodes
>>>   <show nothing>
>>> - After node3 is online as kmem
>>> $ daxctl reconfigure-device --mode=system-ram --no-online dax0.0 && daxctl online-memory dax0.0
>>> [
>>>    {
>>>      "chardev":"dax0.0",
>>>      "size":1054867456,
>>>      "target_node":3,
>>>      "align":2097152,
>>>      "mode":"system-ram",
>>>      "online_memblocks":0,
>>>      "total_memblocks":7
>>>    }
>>> ]
>>> $ cat /sys/devices/system/node/node0/demotion_nodes
>>> 3
>>> $ cat /sys/devices/system/node/node1/demotion_nodes
>>> 3
>>> $ cat /sys/devices/system/node/node3/demotion_nodes
>>>   <show nothing>
>> We have /sys/devices/virtual/memory_tiering/memory_tier*/nodelist
>> already.  A node in a higher tier can demote to any node in the lower
>> tiers.  What's more need to be displayed in nodeX/demotion_nodes?
>> --
>> Best Regards,
>> Huang, Ying
>> 
>>> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
>>> ---
>>>   drivers/base/node.c          | 13 +++++++++++++
>>>   include/linux/memory-tiers.h |  6 ++++++
>>>   mm/memory-tiers.c            |  8 ++++++++
>>>   3 files changed, 27 insertions(+)
>>>
>>> diff --git a/drivers/base/node.c b/drivers/base/node.c
>>> index 493d533f8375..27e8502548a7 100644
>>> --- a/drivers/base/node.c
>>> +++ b/drivers/base/node.c
>>> @@ -7,6 +7,7 @@
>>>   #include <linux/init.h>
>>>   #include <linux/mm.h>
>>>   #include <linux/memory.h>
>>> +#include <linux/memory-tiers.h>
>>>   #include <linux/vmstat.h>
>>>   #include <linux/notifier.h>
>>>   #include <linux/node.h>
>>> @@ -569,11 +570,23 @@ static ssize_t node_read_distance(struct device *dev,
>>>   }
>>>   static DEVICE_ATTR(distance, 0444, node_read_distance, NULL);
>>>   +static ssize_t demotion_nodes_show(struct device *dev,
>>> +			     struct device_attribute *attr, char *buf)
>>> +{
>>> +	int ret;
>>> +	nodemask_t nmask = next_demotion_nodes(dev->id);
>>> +
>>> +	ret = sysfs_emit(buf, "%*pbl\n", nodemask_pr_args(&nmask));
>>> +	return ret;
>>> +}
>>> +static DEVICE_ATTR_RO(demotion_nodes);
>>> +
>>>   static struct attribute *node_dev_attrs[] = {
>>>   	&dev_attr_meminfo.attr,
>>>   	&dev_attr_numastat.attr,
>>>   	&dev_attr_distance.attr,
>>>   	&dev_attr_vmstat.attr,
>>> +	&dev_attr_demotion_nodes.attr,
>>>   	NULL
>>>   };
>>>   diff --git a/include/linux/memory-tiers.h
>>> b/include/linux/memory-tiers.h
>>> index 437441cdf78f..8eb04923f965 100644
>>> --- a/include/linux/memory-tiers.h
>>> +++ b/include/linux/memory-tiers.h
>>> @@ -38,6 +38,7 @@ void init_node_memory_type(int node, struct memory_dev_type *default_type);
>>>   void clear_node_memory_type(int node, struct memory_dev_type *memtype);
>>>   #ifdef CONFIG_MIGRATION
>>>   int next_demotion_node(int node);
>>> +nodemask_t next_demotion_nodes(int node);
>>>   void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets);
>>>   bool node_is_toptier(int node);
>>>   #else
>>> @@ -46,6 +47,11 @@ static inline int next_demotion_node(int node)
>>>   	return NUMA_NO_NODE;
>>>   }
>>>   +static inline next_demotion_nodes next_demotion_nodes(int node)
>>> +{
>>> +	return NODE_MASK_NONE;
>>> +}
>>> +
>>>   static inline void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets)
>>>   {
>>>   	*targets = NODE_MASK_NONE;
>>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>>> index 37a4f59d9585..90047f37d98a 100644
>>> --- a/mm/memory-tiers.c
>>> +++ b/mm/memory-tiers.c
>>> @@ -282,6 +282,14 @@ void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets)
>>>   	rcu_read_unlock();
>>>   }
>>>   +nodemask_t next_demotion_nodes(int node)
>>> +{
>>> +	if (!node_demotion)
>>> +		return NODE_MASK_NONE;
>>> +
>>> +	return node_demotion[node].preferred;
>>> +}
>>> +
>>>   /**
>>>    * next_demotion_node() - Get the next node in the demotion path
>>>    * @node: The starting node to lookup the next node