[RFC][PATCH 1/8] mm/numa: node demotion data structure and lookup

[Dave Hansen <dave.hansen@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

Prepare for the kernel to auto-migrate pages to other memory nodes
with a user defined node migration table. This allows creating single
migration target for each NUMA node to enable the kernel to do NUMA
page migrations instead of simply reclaiming colder pages. A node
with no target is a "terminal node", so reclaim acts normally there.
The migration target does not fundamentally _need_ to be a single node,
but this implementation starts there to limit complexity.

If you consider the migration path as a graph, cycles (loops) in the
graph are disallowed.  This avoids wasting resources by constantly
migrating (A->B, B->A, A->B ...).  The expectation is that cycles will
never be allowed.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
---

 b/mm/migrate.c |   23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff -puN mm/migrate.c~0006-node-Define-and-export-memory-migration-path mm/migrate.c
--- a/mm/migrate.c~0006-node-Define-and-export-memory-migration-path	2020-06-29 16:34:36.849312609 -0700
+++ b/mm/migrate.c	2020-06-29 16:34:36.853312609 -0700
@@ -1159,6 +1159,29 @@ out:
 	return rc;
 }
 
+static int node_demotion[MAX_NUMNODES] = {[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
+
+/**
+ * next_demotion_node() - Get the next node in the demotion path
+ * @node: The starting node to lookup the next node
+ *
+ * @returns: node id for next memory node in the demotion path hierarchy
+ * from @node; -1 if @node is terminal
+ */
+int next_demotion_node(int node)
+{
+	get_online_mems();
+	while (true) {
+		node = node_demotion[node];
+		if (node == NUMA_NO_NODE)
+			break;
+		if (node_online(node))
+			break;
+	}
+	put_online_mems();
+	return node;
+}
+
 /*
  * gcc 4.7 and 4.8 on arm get an ICEs when inlining unmap_and_move().  Work
  * around it.
_