[RFC PATCH v4 3/7] mm/demotion: Build demotion targets based on explicit memory tiers

[Aneesh Kumar K.V]

From: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>

This patch switch the demotion target building logic to use memory tiers
instead of NUMA distance. All N_MEMORY NUMA nodes will be placed in the
default tier 1 and additional memory tiers will be added by drivers like
dax kmem.

This patch builds the demotion target for a NUMA node by looking at all
memory tiers below the tier to which the NUMA node belongs. The closest node
in the immediately following memory tier is used as a demotion target.

Since we are now only building demotion target for N_MEMORY NUMA nodes
the CPU hotplug calls are removed in this patch.

Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
---
 include/linux/migrate.h |   8 -
 mm/migrate.c            | 460 +++++++++++++++-------------------------
 mm/vmstat.c             |   5 -
 3 files changed, 172 insertions(+), 301 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index d37d1d5dee82..cbef71a499c1 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -177,12 +177,6 @@ enum memory_tier_type {
 };
 
 int next_demotion_node(int node);
-extern void migrate_on_reclaim_init(void);
-#ifdef CONFIG_HOTPLUG_CPU
-extern void set_migration_target_nodes(void);
-#else
-static inline void set_migration_target_nodes(void) {}
-#endif
 int node_get_memory_tier(int node);
 int node_set_memory_tier(int node, int tier);
 int node_reset_memory_tier(int node, int tier);
@@ -193,8 +187,6 @@ static inline int next_demotion_node(int node)
 	return NUMA_NO_NODE;
 }
 
-static inline void set_migration_target_nodes(void) {}
-static inline void migrate_on_reclaim_init(void) {}
 #endif	/* CONFIG_TIERED_MEMORY */
 
 #endif /* _LINUX_MIGRATE_H */
diff --git a/mm/migrate.c b/mm/migrate.c
index 304559ba3372..d819a64db5b1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2125,6 +2125,10 @@ struct memory_tier {
 	nodemask_t nodelist;
 };
 
+struct demotion_nodes {
+	nodemask_t preferred;
+};
+
 #define to_memory_tier(device) container_of(device, struct memory_tier, dev)
 
 static struct bus_type memory_tier_subsys = {
@@ -2132,9 +2136,73 @@ static struct bus_type memory_tier_subsys = {
 	.dev_name = "memtier",
 };
 
+static void establish_migration_targets(void);
+
 DEFINE_MUTEX(memory_tier_lock);
 static struct memory_tier *memory_tiers[MAX_MEMORY_TIERS];
 
+/*
+ * node_demotion[] examples:
+ *
+ * Example 1:
+ *
+ * Node 0 & 1 are CPU + DRAM nodes, node 2 & 3 are PMEM nodes.
+ *
+ * node distances:
+ * node   0    1    2    3
+ *    0  10   20   30   40
+ *    1  20   10   40   30
+ *    2  30   40   10   40
+ *    3  40   30   40   10
+ *
+ * memory_tiers[0] = <empty>
+ * memory_tiers[1] = 0-1
+ * memory_tiers[2] = 2-3
+ *
+ * node_demotion[0].preferred = 2
+ * node_demotion[1].preferred = 3
+ * node_demotion[2].preferred = <empty>
+ * node_demotion[3].preferred = <empty>
+ *
+ * Example 2:
+ *
+ * Node 0 & 1 are CPU + DRAM nodes, node 2 is memory-only DRAM node.
+ *
+ * node distances:
+ * node   0    1    2
+ *    0  10   20   30
+ *    1  20   10   30
+ *    2  30   30   10
+ *
+ * memory_tiers[0] = <empty>
+ * memory_tiers[1] = 0-2
+ * memory_tiers[2] = <empty>
+ *
+ * node_demotion[0].preferred = <empty>
+ * node_demotion[1].preferred = <empty>
+ * node_demotion[2].preferred = <empty>
+ *
+ * Example 3:
+ *
+ * Node 0 is CPU + DRAM nodes, Node 1 is HBM node, node 2 is PMEM node.
+ *
+ * node distances:
+ * node   0    1    2
+ *    0  10   20   30
+ *    1  20   10   40
+ *    2  30   40   10
+ *
+ * memory_tiers[0] = 1
+ * memory_tiers[1] = 0
+ * memory_tiers[2] = 2
+ *
+ * node_demotion[0].preferred = 2
+ * node_demotion[1].preferred = 0
+ * node_demotion[2].preferred = <empty>
+ *
+ */
+static struct demotion_nodes *node_demotion __read_mostly;
+
 static ssize_t nodelist_show(struct device *dev,
 			     struct device_attribute *attr, char *buf)
 {
@@ -2238,6 +2306,28 @@ static int __node_get_memory_tier(int node)
 	return -1;
 }
 
+static void node_remove_from_memory_tier(int node)
+{
+	int tier;
+
+	mutex_lock(&memory_tier_lock);
+
+	tier = __node_get_memory_tier(node);
+
+	/*
+	 * Remove node from tier, if tier becomes
+	 * empty then unregister it to make it invisible
+	 * in sysfs.
+	 */
+	node_clear(node, memory_tiers[tier]->nodelist);
+	if (nodes_empty(memory_tiers[tier]->nodelist))
+		unregister_memory_tier(tier);
+
+	establish_migration_targets();
+
+	mutex_unlock(&memory_tier_lock);
+}
+
 int node_get_memory_tier(int node)
 {
 	int tier;
@@ -2271,6 +2361,7 @@ int __node_set_memory_tier(int node, int tier)
 	}
 
 	node_set(node, memory_tiers[tier]->nodelist);
+	establish_migration_targets();
 
 out:
 	return ret;
@@ -2328,75 +2419,6 @@ int node_set_memory_tier(int node, int tier)
 	return ret;
 }
 
-/*
- * node_demotion[] example:
- *
- * Consider a system with two sockets.  Each socket has
- * three classes of memory attached: fast, medium and slow.
- * Each memory class is placed in its own NUMA node.  The
- * CPUs are placed in the node with the "fast" memory.  The
- * 6 NUMA nodes (0-5) might be split among the sockets like
- * this:
- *
- *	Socket A: 0, 1, 2
- *	Socket B: 3, 4, 5
- *
- * When Node 0 fills up, its memory should be migrated to
- * Node 1.  When Node 1 fills up, it should be migrated to
- * Node 2.  The migration path start on the nodes with the
- * processors (since allocations default to this node) 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:
- *
- *	{  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 slow memory nodes.
- * 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
- */
-
-/*
- * Writes to this array occur without locking.  Cycles are
- * not allowed: Node X demotes to Y which demotes to X...
- *
- * If multiple reads are performed, a single rcu_read_lock()
- * must be held over all reads to ensure that no cycles are
- * observed.
- */
-#define DEFAULT_DEMOTION_TARGET_NODES 15
-
-#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
-#define DEMOTION_TARGET_NODES	(MAX_NUMNODES - 1)
-#else
-#define DEMOTION_TARGET_NODES	DEFAULT_DEMOTION_TARGET_NODES
-#endif
-
-struct demotion_nodes {
-	unsigned short nr;
-	short nodes[DEMOTION_TARGET_NODES];
-};
-
-static struct demotion_nodes *node_demotion __read_mostly;
-
 /**
  * next_demotion_node() - Get the next node in the demotion path
  * @node: The starting node to lookup the next node
@@ -2409,8 +2431,7 @@ static struct demotion_nodes *node_demotion __read_mostly;
 int next_demotion_node(int node)
 {
 	struct demotion_nodes *nd;
-	unsigned short target_nr, index;
-	int target;
+	int target, nnodes, i;
 
 	if (!node_demotion)
 		return NUMA_NO_NODE;
@@ -2419,61 +2440,46 @@ int next_demotion_node(int node)
 
 	/*
 	 * node_demotion[] is updated without excluding this
-	 * function from running.  RCU doesn't provide any
-	 * compiler barriers, so the READ_ONCE() is required
-	 * to avoid compiler reordering or read merging.
+	 * function from running.
 	 *
 	 * Make sure to use RCU over entire code blocks if
 	 * node_demotion[] reads need to be consistent.
 	 */
 	rcu_read_lock();
-	target_nr = READ_ONCE(nd->nr);
 
-	switch (target_nr) {
-	case 0:
-		target = NUMA_NO_NODE;
-		goto out;
-	case 1:
-		index = 0;
-		break;
-	default:
-		/*
-		 * If there are multiple target nodes, just select one
-		 * target node randomly.
-		 *
-		 * In addition, we can also use round-robin to select
-		 * target node, but we should introduce another variable
-		 * for node_demotion[] to record last selected target node,
-		 * that may cause cache ping-pong due to the changing of
-		 * last target node. Or introducing per-cpu data to avoid
-		 * caching issue, which seems more complicated. So selecting
-		 * target node randomly seems better until now.
-		 */
-		index = get_random_int() % target_nr;
-		break;
-	}
+	nnodes = nodes_weight(nd->preferred);
+	if (!nnodes)
+		return NUMA_NO_NODE;
 
-	target = READ_ONCE(nd->nodes[index]);
+	/*
+	 * If there are multiple target nodes, just select one
+	 * target node randomly.
+	 *
+	 * In addition, we can also use round-robin to select
+	 * target node, but we should introduce another variable
+	 * for node_demotion[] to record last selected target node,
+	 * that may cause cache ping-pong due to the changing of
+	 * last target node. Or introducing per-cpu data to avoid
+	 * caching issue, which seems more complicated. So selecting
+	 * target node randomly seems better until now.
+	 */
+	nnodes = get_random_int() % nnodes;
+	target = first_node(nd->preferred);
+	for (i = 0; i < nnodes; i++)
+		target = next_node(target, nd->preferred);
 
-out:
 	rcu_read_unlock();
+
 	return target;
 }
 
-#if defined(CONFIG_HOTPLUG_CPU)
 /* Disable reclaim-based migration. */
 static void __disable_all_migrate_targets(void)
 {
-	int node, i;
+	int node;
 
-	if (!node_demotion)
-		return;
-
-	for_each_online_node(node) {
-		node_demotion[node].nr = 0;
-		for (i = 0; i < DEMOTION_TARGET_NODES; i++)
-			node_demotion[node].nodes[i] = NUMA_NO_NODE;
-	}
+	for_each_node_mask(node, node_states[N_MEMORY])
+		node_demotion[node].preferred = NODE_MASK_NONE;
 }
 
 static void disable_all_migrate_targets(void)
@@ -2485,173 +2491,70 @@ static void disable_all_migrate_targets(void)
 	 * Readers will see either a combination of before+disable
 	 * state or disable+after.  They will never see before and
 	 * after state together.
-	 *
-	 * The before+after state together might have cycles and
-	 * could cause readers to do things like loop until this
-	 * function finishes.  This ensures they can only see a
-	 * single "bad" read and would, for instance, only loop
-	 * once.
 	 */
 	synchronize_rcu();
 }
 
 /*
- * Find an automatic demotion target for 'node'.
- * Failing here is OK.  It might just indicate
- * being at the end of a chain.
- */
-static int establish_migrate_target(int node, nodemask_t *used,
-				    int best_distance)
+* Find an automatic demotion target for all memory
+* nodes. Failing here is OK.  It might just indicate
+* being at the end of a chain.
+*/
+static void establish_migration_targets(void)
 {
-	int migration_target, index, val;
 	struct demotion_nodes *nd;
+	int tier, target = NUMA_NO_NODE, node;
+	int distance, best_distance;
+	nodemask_t used;
 
 	if (!node_demotion)
-		return NUMA_NO_NODE;
-
-	nd = &node_demotion[node];
-
-	migration_target = find_next_best_node(node, used);
-	if (migration_target == NUMA_NO_NODE)
-		return NUMA_NO_NODE;
-
-	/*
-	 * If the node has been set a migration target node before,
-	 * which means it's the best distance between them. Still
-	 * check if this node can be demoted to other target nodes
-	 * if they have a same best distance.
-	 */
-	if (best_distance != -1) {
-		val = node_distance(node, migration_target);
-		if (val > best_distance)
-			goto out_clear;
-	}
-
-	index = nd->nr;
-	if (WARN_ONCE(index >= DEMOTION_TARGET_NODES,
-		      "Exceeds maximum demotion target nodes\n"))
-		goto out_clear;
-
-	nd->nodes[index] = migration_target;
-	nd->nr++;
+		return;
 
-	return migration_target;
-out_clear:
-	node_clear(migration_target, *used);
-	return NUMA_NO_NODE;
-}
+	disable_all_migrate_targets();
 
-/*
- * 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. Also one node can
- * be migrated to multiple nodes if the target nodes all have
- * a same best-distance against the source node.
- *
- * 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.
- */
-static void __set_migration_target_nodes(void)
-{
-	nodemask_t next_pass	= NODE_MASK_NONE;
-	nodemask_t this_pass	= NODE_MASK_NONE;
-	nodemask_t used_targets = NODE_MASK_NONE;
-	int node, best_distance;
+	for_each_node_mask(node, node_states[N_MEMORY]) {
+		best_distance = -1;
+		nd = &node_demotion[node];
 
-	/*
-	 * Avoid any oddities like cycles that could occur
-	 * from changes in the topology.  This will leave
-	 * a momentary gap when migration is disabled.
-	 */
-	disable_all_migrate_targets();
+		tier = __node_get_memory_tier(node);
+		/*
+		 * Find next tier to demote.
+		 */
+		while (++tier < MAX_MEMORY_TIERS) {
+			if (memory_tiers[tier])
+				break;
+		}
 
-	/*
-	 * Allocations go close to CPUs, first.  Assume that
-	 * the migration path starts at the nodes with CPUs.
-	 */
-	next_pass = node_states[N_CPU];
-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 destination.
-	 */
-	nodes_or(used_targets, used_targets, this_pass);
+		if (tier >= MAX_MEMORY_TIERS)
+			continue;
 
-	for_each_node_mask(node, this_pass) {
-		best_distance = -1;
+		nodes_andnot(used, node_states[N_MEMORY], memory_tiers[tier]->nodelist);
 
 		/*
-		 * Try to set up the migration path for the node, and the target
-		 * migration nodes can be multiple, so doing a loop to find all
-		 * the target nodes if they all have a best node distance.
+		 * Find all the nodes in the memory tier node list of same best distance.
+		 * add add them to the preferred mask. We randomly select between nodes
+		 * in the preferred mask when allocating pages during demotion.
 		 */
 		do {
-			int target_node =
-				establish_migrate_target(node, &used_targets,
-							 best_distance);
-
-			if (target_node == NUMA_NO_NODE)
+			target = find_next_best_node(node, &used);
+			if (target == NUMA_NO_NODE)
 				break;
 
-			if (best_distance == -1)
-				best_distance = node_distance(node, target_node);
-
-			/*
-			 * Visit targets from this pass in the next pass.
-			 * Eventually, every node will have been part of
-			 * a pass, and will become set in 'used_targets'.
-			 */
-			node_set(target_node, next_pass);
+			distance = node_distance(node, target);
+			if (distance == best_distance || best_distance == -1) {
+				best_distance = distance;
+				node_set(target, nd->preferred);
+			} else {
+				break;
+			}
 		} while (1);
 	}
-	/*
-	 * 'next_pass' contains nodes which became migration
-	 * targets in this pass.  Make additional passes until
-	 * no more migrations targets are available.
-	 */
-	if (!nodes_empty(next_pass))
-		goto again;
 }
 
 /*
- * For callers that do not hold get_online_mems() already.
- */
-void set_migration_target_nodes(void)
-{
-	get_online_mems();
-	__set_migration_target_nodes();
-	put_online_mems();
-}
-
-/*
- * This leaves migrate-on-reclaim transiently disabled between
- * the MEM_GOING_OFFLINE and MEM_OFFLINE events.  This runs
- * whether reclaim-based migration is enabled or not, which
- * ensures that the user can turn reclaim-based migration at
- * any time without needing to recalculate migration targets.
- *
- * These callbacks already hold get_online_mems().  That is why
- * __set_migration_target_nodes() can be used as opposed to
- * set_migration_target_nodes().
+ * This runs whether reclaim-based migration is enabled or not,
+ * which ensures that the user can turn reclaim-based migration
+ * at any time without needing to recalculate migration targets.
  */
 static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
 						 unsigned long action, void *_arg)
@@ -2660,64 +2563,44 @@ static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
 
 	/*
 	 * Only update the node migration order when a node is
-	 * changing status, like online->offline.  This avoids
-	 * the overhead of synchronize_rcu() in most cases.
+	 * changing status, like online->offline.
 	 */
 	if (arg->status_change_nid < 0)
 		return notifier_from_errno(0);
 
 	switch (action) {
-	case MEM_GOING_OFFLINE:
-		/*
-		 * Make sure there are not transient states where
-		 * an offline node is a migration target.  This
-		 * will leave migration disabled until the offline
-		 * completes and the MEM_OFFLINE case below runs.
-		 */
-		disable_all_migrate_targets();
-		break;
 	case MEM_OFFLINE:
-	case MEM_ONLINE:
 		/*
-		 * Recalculate the target nodes once the node
-		 * reaches its final state (online or offline).
+		 * In case we are moving out of N_MEMORY. Keep the node
+		 * in the memory tier so that when we bring memory online,
+		 * they appear in the right memory tier. We still need
+		 * to rebuild the demotion order.
 		 */
-		__set_migration_target_nodes();
+		mutex_lock(&memory_tier_lock);
+		establish_migration_targets();
+		mutex_unlock(&memory_tier_lock);
 		break;
-	case MEM_CANCEL_OFFLINE:
+	case MEM_ONLINE:
 		/*
-		 * MEM_GOING_OFFLINE disabled all the migration
-		 * targets.  Reenable them.
+		 * We ignore the error here, if the node already have the tier
+		 * registered, we will continue to use that for the new memory
+		 * we are adding here.
 		 */
-		__set_migration_target_nodes();
-		break;
-	case MEM_GOING_ONLINE:
-	case MEM_CANCEL_ONLINE:
+		node_set_memory_tier(arg->status_change_nid, DEFAULT_MEMORY_TIER);
 		break;
 	}
 
 	return notifier_from_errno(0);
 }
 
-void __init migrate_on_reclaim_init(void)
+static void __init migrate_on_reclaim_init(void)
 {
-	node_demotion = kmalloc_array(nr_node_ids,
-				      sizeof(struct demotion_nodes),
-				      GFP_KERNEL);
+	node_demotion = kcalloc(MAX_NUMNODES, sizeof(struct demotion_nodes),
+				GFP_KERNEL);
 	WARN_ON(!node_demotion);
 
 	hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
-	/*
-	 * At this point, all numa nodes with memory/CPus have their state
-	 * properly set, so we can build the demotion order now.
-	 * Let us hold the cpu_hotplug lock just, as we could possibily have
-	 * CPU hotplug events during boot.
-	 */
-	cpus_read_lock();
-	set_migration_target_nodes();
-	cpus_read_unlock();
 }
-#endif /* CONFIG_HOTPLUG_CPU */
 
 bool numa_demotion_enabled = false;
 
@@ -2800,6 +2683,7 @@ static int __init memory_tier_init(void)
 	 * CPU only nodes are not part of memoty tiers.
 	 */
 	memory_tiers[DEFAULT_MEMORY_TIER]->nodelist = node_states[N_MEMORY];
+	migrate_on_reclaim_init();
 
 	return 0;
 }
diff --git a/mm/vmstat.c b/mm/vmstat.c
index b75b1a64b54c..7815d21345a4 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -2053,7 +2053,6 @@ static int vmstat_cpu_online(unsigned int cpu)
 
 	if (!node_state(cpu_to_node(cpu), N_CPU)) {
 		node_set_state(cpu_to_node(cpu), N_CPU);
-		set_migration_target_nodes();
 	}
 
 	return 0;
@@ -2078,7 +2077,6 @@ static int vmstat_cpu_dead(unsigned int cpu)
 		return 0;
 
 	node_clear_state(node, N_CPU);
-	set_migration_target_nodes();
 
 	return 0;
 }
@@ -2111,9 +2109,6 @@ void __init init_mm_internals(void)
 
 	start_shepherd_timer();
 #endif
-#if defined(CONFIG_MIGRATION) && defined(CONFIG_HOTPLUG_CPU)
-	migrate_on_reclaim_init();
-#endif
 #ifdef CONFIG_PROC_FS
 	proc_create_seq("buddyinfo", 0444, NULL, &fragmentation_op);
 	proc_create_seq("pagetypeinfo", 0400, NULL, &pagetypeinfo_op);
-- 
2.36.1