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

Aneesh Kumar K.V
2022-05-27T12:41:42Z

From: Jagdish Gediya <jvgediya@linux.ibm.com>

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@linux.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
---
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