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Subject[RFC][PATCH 04/13] mm/numa: node demotion data structure and lookup
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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>
Cc: David Hildenbrand <david@redhat.com>
Cc: osalvador <osalvador@suse.de>

--

changes in July 2020:
- Remove loop from next_demotion_node() and get_online_mems().
This means that the node returned by next_demotion_node()
might now be offline, but the worst case is that the
allocation fails. That's fine since it is transient.
---

b/mm/migrate.c | 16 ++++++++++++++++
1 file changed, 16 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 2021-01-25 16:23:09.553866709 -0800
+++ b/mm/migrate.c 2021-01-25 16:23:09.558866709 -0800
@@ -1161,6 +1161,22 @@ 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; NUMA_NO_NODE if @node is terminal. This does not keep
+ * @node online or guarantee that it *continues* to be the next demotion
+ * target.
+ */
+int next_demotion_node(int node)
+{
+ return node_demotion[node];
+}
+
/*
* Obtain the lock on page, remove all ptes and migrate the page
* to the newly allocated page in newpage.
_
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