On 1/29/21 12:46 PM, Yang Shi wrote:
...

quoted

 int next_demotion_node(int node)
 {
-       return node_demotion[node];
+       /*
+        * node_demotion[] is updated without excluding
+        * this function from running.  READ_ONCE() avoids
+        * reading multiple, inconsistent 'node' values
+        * during an update.
+        */

Don't we need a smp_rmb() here? The single write barrier might be not
enough in migration target set. Typically a write barrier should be
used in pairs with a read barrier.

I don't think we need one, practically.

Since there is no locking against node_demotion[] updates, although a
smp_rmb() would ensure that this read is up-to-date, it could change
freely after the smp_rmb().

In other words, smp_rmb() would shrink the window where a "stale" read
could occur but would not eliminate it.

quoted

+       return READ_ONCE(node_demotion[node]);

Why not consolidate the patch #4 in this patch? The patch #4 just add
the definition of node_demotion and the function, then the function is
changed in this patch, and the function is not used by anyone between
the adding and changing.

I really wanted to highlight that the locking scheme and the READ_ONCE()
(or lack thereof) was specifically due to how node_demotion[] was being
updated.

The READ_ONCE() is not, for instance, inherent to the data structure.

...

quoted

+/*
+ * When memory fills up on a node, memory contents can be
+ * automatically migrated to another node instead of
+ * discarded at reclaim.
+ *
+ * Establish a "migration path" which will start at nodes
+ * with CPUs and will follow the priorities used to build the
+ * page allocator zonelists.
+ *
+ * The difference here is that cycles must be avoided.  If
+ * node0 migrates to node1, then neither node1, nor anything
+ * node1 migrates to can migrate to node0.
+ *
+ * This function can run simultaneously with readers of
+ * node_demotion[].  However, it can not run simultaneously
+ * with itself.  Exclusion is provided by memory hotplug events
+ * being single-threaded.

Maybe an example diagram for the physical topology and how the
migration target is generated in the comment seems helpful to
understand the code.

Sure.  Were you thinking of a code comment, or enhanced changelog?

Let's say there's a system with two sockets each with the same three
classes of memory: fast, medium and slow.  Each memory class is placed
in its own NUMA node and the CPUs are attached to the fast memory.  That
leaves 6 NUMA nodes (0-5):

	Socket A: 0, 1, 2
	Socket B: 3, 4, 5

The migration path for this configuration path would start on the nodes
with the processors and fast memory, progress through medium and end
with the slow memory:

	0 -> 1 -> 2 -> stop
	3 -> 4 -> 5 -> stop

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 1
	   5, // Node 4 migrates to 2
	  -1} // Node 5 does not migrate

Is that what you were thinking of?

...

quoted

+again:
+       this_pass = next_pass;
+       next_pass = NODE_MASK_NONE;
+       /*
+        * To avoid cycles in the migration "graph", ensure
+        * that migration sources are not future targets by
+        * setting them in 'used_targets'.  Do this only
+        * once per pass so that multiple source nodes can
+        * share a target node.
+        *
+        * 'used_targets' will become unavailable in future
+        * passes.  This limits some opportunities for
+        * multiple source nodes to share a desintation.

s/desination/destination

Fixed, thanks.

quoted

+        */
+       nodes_or(used_targets, used_targets, this_pass);
+       for_each_node_mask(node, this_pass) {
+               int target_node = establish_migrate_target(node, &used_targets);
+
+               if (target_node == NUMA_NO_NODE)
+                       continue;
+
+               /* Visit targets from this pass in the next pass: */
+               node_set(target_node, next_pass);
+       }
+       /* Is another pass necessary? */
+       if (!nodes_empty(next_pass))
+               goto again;
+}
+
+void set_migration_target_nodes(void)

It seems this function is not called outside migrate.c, so it should be static.

Fixed, thanks.