Re: [PATCH v2] 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>
> ---
> v1 -> v2: - Fix prototype function declaration
>           - Fix build error on RISC
>           - Adressed feedback from Baolin and Huang
> ---
>  include/linux/migrate.h |  8 ++++++++
>  mm/migrate.c            | 41 +++++------------------------------------
>  mm/vmstat.c             | 13 ++++++++++++-
>  3 files changed, 25 insertions(+), 37 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index db96e10eb8da..90e75d5a54d6 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -48,7 +48,15 @@ int folio_migrate_mapping(struct address_space *mapping,
>  		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,
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c7da064b4781..f9d5b6092a42 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();
> @@ -3254,51 +3254,20 @@ 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)
> +void __init migrate_on_reclaim_init(void)
>  {
> -	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.
> +	 * At this point, all numa nodes with memory/CPus have their state
> +	 * properly set, so we can build the demotion order now.
>  	 */
> -	WARN_ON(ret < 0);
> -	ret = cpuhp_setup_state(CPUHP_AP_MM_DEMOTION_ONLINE, "mm/demotion:online",
> -				migration_online_cpu, NULL);
> -	WARN_ON(ret < 0);
> -
> +	set_migration_target_nodes();

If my understanding were correct, we should enclose
set_migration_target_nodes() here with cpus_read_lock().  And add some
comment before set_migration_target_nodes() for this.  I don't know
whether the locking order is right.

>  	hotplug_memory_notifier(migrate_on_reclaim_callback, 100);

And we should register the notifier before calling set_migration_target_nodes()?

Best Regards,
Huang, Ying

> -	return 0;
>  }
> -late_initcall(migrate_on_reclaim_init);
>  #endif /* CONFIG_HOTPLUG_CPU */
>  

[snip]