[PATCH 5.17 0109/1126] mm: only re-generate demotion targets when a numa node changes its N_CPU state

Greg Kroah-Hartman
2022-04-05T07:47:02Z

From: Oscar Salvador <osalvador@suse.de>

commit 734c15700cdf9062ae98d8b131c6fe873dfad26d upstream.

Abhishek reported that after patch [1], hotplug operations are taking
roughly double the expected time. [2]

The reason behind is that the CPU callbacks that
migrate_on_reclaim_init() sets always call set_migration_target_nodes()
whenever a CPU is brought up/down.

But we only care about numa nodes going from having cpus to become
cpuless, and vice versa, as that influences the demotion_target order.

We do already have two CPU callbacks (vmstat_cpu_online() and
vmstat_cpu_dead()) that check exactly that, so get rid of the CPU
callbacks in migrate_on_reclaim_init() and only call
set_migration_target_nodes() from vmstat_cpu_{dead,online}() whenever a
numa node change its N_CPU state.

[1] https://lore.kernel.org/linux-mm/20210721063926.3024591-2-ying.huang@intel.com/
[2] https://lore.kernel.org/linux-mm/eb438ddd-2919-73d4-bd9f-b7eecdd9577a@linux.vnet.ibm.com/

[osalvador@suse.de: add feedback from Huang Ying]
Link: https://lkml.kernel.org/r/20220314150945.12694-1-osalvador@suse.de

Link: https://lkml.kernel.org/r/20220310120749.23077-1-osalvador@suse.de
Fixes: 884a6e5d1f93b ("mm/migrate: update node demotion order on hotplug events")
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reported-by: Abhishek Goel <huntbag@linux.vnet.ibm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Abhishek Goel <huntbag@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/migrate.h | 8 ++++++++
mm/migrate.c | 47 ++++++++++-------------------------------------
mm/vmstat.c | 13 ++++++++++++-
3 files changed, 30 insertions(+), 38 deletions(-)

--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -48,7 +48,15 @@ int folio_migrate_mapping(struct address
struct folio *newfolio, struct folio *folio, int extra_count);

extern bool numa_demotion_enabled;
+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
+#else
+
+static inline void set_migration_target_nodes(void) {}

static inline void putback_movable_pages(struct list_head *l) {}
static inline int migrate_pages(struct list_head *l, new_page_t new,
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -3190,7 +3190,7 @@ again:
/*
* For callers that do not hold get_online_mems() already.
*/
-static void set_migration_target_nodes(void)
+void set_migration_target_nodes(void)
{
get_online_mems();
__set_migration_target_nodes();
@@ -3254,51 +3254,24 @@ static int __meminit migrate_on_reclaim_
return notifier_from_errno(0);
}

-/*
- * React to hotplug events that might affect the migration targets
- * like events that online or offline NUMA nodes.
- *
- * The ordering is also currently dependent on which nodes have
- * CPUs. That means we need CPU on/offline notification too.
- */
-static int migration_online_cpu(unsigned int cpu)
-{
- set_migration_target_nodes();
- return 0;
-}
-
-static int migration_offline_cpu(unsigned int cpu)
-{
- set_migration_target_nodes();
- return 0;
-}
-
-static int __init migrate_on_reclaim_init(void)
+void __init migrate_on_reclaim_init(void)
{
- int ret;
-
node_demotion = kmalloc_array(nr_node_ids,
sizeof(struct demotion_nodes),
GFP_KERNEL);
WARN_ON(!node_demotion);

- ret = cpuhp_setup_state_nocalls(CPUHP_MM_DEMOTION_DEAD, "mm/demotion:offline",
- NULL, migration_offline_cpu);
+ hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
/*
- * In the unlikely case that this fails, the automatic
- * migration targets may become suboptimal for nodes
- * where N_CPU changes. With such a small impact in a
- * rare case, do not bother trying to do anything special.
+ * 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.
*/
- WARN_ON(ret < 0);
- ret = cpuhp_setup_state(CPUHP_AP_MM_DEMOTION_ONLINE, "mm/demotion:online",
- migration_online_cpu, NULL);
- WARN_ON(ret < 0);
-
- hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
- return 0;
+ cpus_read_lock();
+ set_migration_target_nodes();
+ cpus_read_unlock();
}
-late_initcall(migrate_on_reclaim_init);
#endif /* CONFIG_HOTPLUG_CPU */

bool numa_demotion_enabled = false;
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -28,6 +28,7 @@
#include <linux/mm_inline.h>
#include <linux/page_ext.h>
#include <linux/page_owner.h>
+#include <linux/migrate.h>

#include "internal.h"

@@ -2043,7 +2044,12 @@ static void __init init_cpu_node_state(v
static int vmstat_cpu_online(unsigned int cpu)
{
refresh_zone_stat_thresholds();
- node_set_state(cpu_to_node(cpu), N_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;
}

@@ -2066,6 +2072,8 @@ static int vmstat_cpu_dead(unsigned int
return 0;

node_clear_state(node, N_CPU);
+ set_migration_target_nodes();
+
return 0;
}

@@ -2097,6 +2105,9 @@ 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);