Re: [RFC PATCH v2] mm: migrate: Support multiple target nodes demotion

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> · Wed, 10 Nov 2021 09:30:11 +0800

On 2021/11/10 8:47, Huang, Ying writes:
Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

On 2021/11/9 15:53, Huang, Ying writes:
Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

We have some machines with multiple memory types like below, which
have one fast (DRAM) memory node and two slow (persistent memory) memory
nodes. According to current node demotion, if node 0 fills up,
its memory should be migrated to node 1, when node 1 fills up, its
memory will be migrated to node 2: node 0 -> node 1 -> node 2 ->stop.

But this is not efficient and suitbale memory migration route
for our machine with multiple slow memory nodes. Since the distance
between node 0 to node 1 and node 0 to node 2 is equal, and memory
migration between slow memory nodes will increase persistent memory
bandwidth greatly, which will hurt the whole system's performance.

Thus for this case, we can treat the slow memory node 1 and node 2
as a whole slow memory region, and we should migrate memory from
node 0 to node 1 and node 2 if node 0 fills up.

This patch changes the node_demotion data structure to support multiple
target nodes, and establishes the migration path to support multiple
target nodes with validating if the node distance is the best or not.

available: 3 nodes (0-2)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
node 0 size: 62153 MB
node 0 free: 55135 MB
node 1 cpus:
node 1 size: 127007 MB
node 1 free: 126930 MB
node 2 cpus:
node 2 size: 126968 MB
node 2 free: 126878 MB
node distances:
node   0   1   2
    0:  10  20  20
    1:  20  10  20
    2:  20  20  10

Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
Changes from RFC v1:
   - Re-define the node_demotion structure.
   - Set up multiple target nodes by validating the node distance.
   - Add more comments.
---
   mm/migrate.c | 132 +++++++++++++++++++++++++++++++++++++++++++----------------
   1 file changed, 96 insertions(+), 36 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index cf25b00..95f170d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1119,12 +1119,25 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
    *
    * This is represented in the node_demotion[] like this:
    *
- *	{  1, // Node 0 migrates to 1
- *	   2, // Node 1 migrates to 2
- *	  -1, // Node 2 does not migrate
- *	   4, // Node 3 migrates to 4
- *	   5, // Node 4 migrates to 5
- *	  -1} // Node 5 does not migrate
+ *	{  nr=1, nodes[0]=1 }, // Node 0 migrates to 1
+ *	{  nr=1, nodes[0]=2 }, // Node 1 migrates to 2
+ *	{  nr=0, nodes[0]=-1 }, // Node 2 does not migrate
+ *	{  nr=1, nodes[0]=4 }, // Node 3 migrates to 4
+ *	{  nr=1, nodes[0]=5 }, // Node 4 migrates to 5
+ *	{  nr=0, nodes[0]=-1} // Node 5 does not migrate
+ *
+ * Moreover some systems may have multiple same class memory
+ * types. Suppose a system has one socket with 3 memory nodes,
+ * node 0 is fast memory type, and node 1/2 both are slow memory
+ * type, and the distance between fast memory node and slow
+ * memory node is same. So the migration path should be:
+ *
+ *	0 -> 1/2 -> stop
+ *
+ * This is represented in the node_demotion[] like this:
+ *	{ nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
+ *	{ nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
+ *	{ nr=0, nodes[0]=-1, }, // Node 2 does not migrate
    */
     /*
@@ -1135,8 +1148,13 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
    * must be held over all reads to ensure that no cycles are
    * observed.
    */
-static int node_demotion[MAX_NUMNODES] __read_mostly =
-	{[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
+#define DEMOTION_TARGET_NODES 15
+struct demotion_nodes {
+	unsigned short nr;
+	int nodes[DEMOTION_TARGET_NODES];
Why we cannot use "unsigned short" for nodes[]?

I think the default value of target node should be NUMA_NO_NODE(-1),
so a signed type is more suitable. I can change to 'short' type.

Yes.  'short' is better.


+};
+
+static struct demotion_nodes node_demotion[MAX_NUMNODES] __read_mostly;
     /**
    * next_demotion_node() - Get the next node in the demotion path
@@ -1149,7 +1167,9 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
    */
   int next_demotion_node(int node)
   {
-	int target;
+	struct demotion_nodes *current_node_demotion = &node_demotion[node];
+	int target, i;
+	nodemask_t target_nodes = NODE_MASK_NONE;
     	/*
   	 * node_demotion[] is updated without excluding this
@@ -1161,9 +1181,21 @@ int next_demotion_node(int node)
   	 * node_demotion[] reads need to be consistent.
   	 */
   	rcu_read_lock();
-	target = READ_ONCE(node_demotion[node]);
+	for (i = 0; i < DEMOTION_TARGET_NODES; i++) {
+		target = READ_ONCE(current_node_demotion->nodes[i]);
+		if (target == NUMA_NO_NODE)
+			break;
+
+		node_set(target, target_nodes);
Why do we need a nodemask?  Why not just find a target node from
current_node_demotion->nodes[] randomly and directly?

I think nodemask is scalable in future if we want to add more
requirements to select the target node if necessary. Anyway now I have
no strong preference with the nodemask, and can change to select the
target node randomly and directly, which are something like below.

+       target_nr = READ_ONCE(current_node_demotion->nr);
+
+       if (target_nr == 0) {
+               target = NUMA_NO_NODE;
+               goto out;
+       } else if (target_nr == 1) {
+               index = 0;
+       } else {
+               /*
+                * If there are multiple target nodes, just select one
+                * target node randomly.
+                */
+               index = get_random_int() % target_nr;
+       }
+
+       target = READ_ONCE(current_node_demotion->nodes[index]);

This looks generally OK.  You may consider about memory order.

Yes. Sorry, I just cut out a piece of sample code. They are must under 
RCU lock, and using READ_ONCE() to avoid compiler reordering or read 
merging like the comments said.







