Re: [PATCH] mm: Only re-generate demotion targets when a numa node changes its N_CPU state

[Huang, Ying]

Oscar Salvador <osalvador@xxxxxxx> writes:

> 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@xxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/eb438ddd-2919-73d4-bd9f-b7eecdd9577a@xxxxxxxxxxxxxxxxxx/
>
> Reported-by: Abhishek Goel <huntbag@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> ---
> I think there is further room for improvement like should we call in to
> set_migration_target_nodes() when demotion is disabled via sysctl?
> I will have a look into that, but let us go with this quick fix for now.
> Also, I am not really strong about the Fixes tag, but it can be added
> if you think it makes sense.
> ---
>  include/linux/migrate.h |  5 +++++
>  mm/migrate.c            | 36 +-----------------------------------
>  mm/vmstat.c             | 10 +++++++++-
>  3 files changed, 15 insertions(+), 36 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index db96e10eb8da..c64fe2923fb0 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -48,6 +48,11 @@ int folio_migrate_mapping(struct address_space *mapping,
>  		struct folio *newfolio, struct folio *folio, int extra_count);
>  
>  extern bool numa_demotion_enabled;
> +#ifdef CONFIG_HOTPLUG_CPU
> +extern void set_migration_target_nodes(void);
> +#else
> +static inline void set_migration_target_nodes() {}
> +#endif
>  #else
>  
>  static inline void putback_movable_pages(struct list_head *l) {}
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c7da064b4781..7847e4de01d7 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -3190,7 +3190,7 @@ static void __set_migration_target_nodes(void)
>  /*
>   * 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();
> @@ -3254,47 +3254,13 @@ static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
>  	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)
>  {
> -	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);
> -	/*
> -	 * 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.
> -	 */
> -	WARN_ON(ret < 0);
> -	ret = cpuhp_setup_state(CPUHP_AP_MM_DEMOTION_ONLINE, "mm/demotion:online",
> -				migration_online_cpu, NULL);
> -	WARN_ON(ret < 0);
> -

We need to call set_migration_target_nodes() during system boot
somewhere, either here or in init_mm_internals().

Best Regards,
Huang, Ying


>  	hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
>  	return 0;
>  }
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4057372745d0..0529a83c8f89 100644
> --- 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(void)
>  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 cpu)
>  		return 0;
>  
>  	node_clear_state(node, N_CPU);
> +	set_migration_target_nodes();
> +
>  	return 0;
>  }