From: Oscar Salvador osalvador@suse.de Subject: mm: only re-generate demotion targets when a numa node changes its N_CPU state

Abhishek reported that after patch [1], hotplug operations are taking ~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.c... [2] https://lore.kernel.org/linux-mm/eb438ddd-2919-73d4-bd9f-b7eecdd9577a@linux....

[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 ---

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~mm-only-re-generate-demotion-targets-when-a-numa-node-changes-its-n_cpu-state +++ a/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~mm-only-re-generate-demotion-targets-when-a-numa-node-changes-its-n_cpu-state +++ a/mm/migrate.c @@ -3209,7 +3209,7 @@ 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(); @@ -3273,51 +3273,24 @@ 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~mm-only-re-generate-demotion-targets-when-a-numa-node-changes-its-n_cpu-state +++ a/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"

@@ -2049,7 +2050,12 @@ 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; }

@@ -2072,6 +2078,8 @@ static int vmstat_cpu_dead(unsigned int return 0;

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

@@ -2103,6 +2111,9 @@ 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); _