RE: [PATCH RFC 1/4] drivers/base/node: Add demotion_nodes sys infterface

[Yasunori Gotou (Fujitsu)]

Hello,

> Li Zhijian <lizhijian@xxxxxxxxxxx> 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?

In extreme terms, users want to analyze all the memory behaviors of memory management
while executing their workload, and want to trace ALL of them if possible.
Of course, it is impossible due to the heavy load, then users want to have other ways as
a compromise. Our request, the demotion target information, is just one of them.

In my impression, users worry about the impact of the CXL memory device on their workload, 
and want to have a way to understand the impact.
If they know there is no information to remove their anxious, they may avoid to buy CXL memory.

In addition, our support team also needs to have clues to solve users' performance problems. 
Even if new page allocation will fall back to the CXL memory, we need to explain why it would
happen as accountability.

> 
> 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.

I can understand there is a compatibility problem by our propose, and kernel may
change its logic in future. This is a tug-of-war situation between kernel developers
and users or support engineers. I suppose It often occurs in many place...

Hmm... I hope there is a new idea to solve this situation even if our proposal is rejected..
Anyone?

Thanks,
----
Yasunori Goto

> 
> --
> Best Regards,
> Huang, Ying
> 
> >
> > On 02/11/2023 11:17, Huang, Ying wrote:
> >> Li Zhijian <lizhijian@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> >>> ---
> >>>   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