Re: [PATCH] mm/migrate: fix CPUHP state to update node demotion order

["Mika Penttilä" <mika.penttila@nextfour.com>]

Hi!

On 18.9.2021 5.58, Huang Ying wrote:
> The node demotion order needs to be updated during CPU hotplug.
> Because whether a NUMA node has CPU may influence the demotion order.
> The update function should be called during CPU online/offline after
> the node_states[N_CPU] has been updated.  That is done in
> CPUHP_AP_ONLINE_DYN during CPU online and in CPUHP_MM_VMSTAT_DEAD
> during CPU offline.  But in commit 884a6e5d1f93 ("mm/migrate: update
> node demotion order on hotplug events"), the function to update node
> demotion order is called in CPUHP_AP_ONLINE_DYN during CPU
> online/offline.  This doesn't satisfy the order requirement.  So in
> this patch, we added CPUHP_AP_MM_DEMOTION_ONLINE and
> CPUHP_MM_DEMOTION_OFFLINE to be called after CPUHP_AP_ONLINE_DYN and
> CPUHP_MM_VMSTAT_DEAD during CPU online/offline, and register the
> update function on them.
>
> Fixes: 884a6e5d1f93 ("mm/migrate: update node demotion order on hotplug events")
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Wei Xu <weixugc@google.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Greg Thelen <gthelen@google.com>
> Cc: Keith Busch <kbusch@kernel.org>
> ---
>   include/linux/cpuhotplug.h | 2 ++
>   mm/migrate.c               | 8 +++++---
>   2 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index 832d8a74fa59..5a92ea56f21b 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -72,6 +72,7 @@ enum cpuhp_state {
>   	CPUHP_SLUB_DEAD,
>   	CPUHP_DEBUG_OBJ_DEAD,
>   	CPUHP_MM_WRITEBACK_DEAD,
> +	CPUHP_MM_DEMOTION_OFFLINE,
>   	CPUHP_MM_VMSTAT_DEAD,
>   	CPUHP_SOFTIRQ_DEAD,
>   	CPUHP_NET_MVNETA_DEAD,
> @@ -240,6 +241,7 @@ enum cpuhp_state {
>   	CPUHP_AP_BASE_CACHEINFO_ONLINE,
>   	CPUHP_AP_ONLINE_DYN,
>   	CPUHP_AP_ONLINE_DYN_END		= CPUHP_AP_ONLINE_DYN + 30,
> +	CPUHP_AP_MM_DEMOTION_ONLINE,
>   	CPUHP_AP_X86_HPET_ONLINE,
>   	CPUHP_AP_X86_KVM_CLK_ONLINE,
>   	CPUHP_AP_DTPM_CPU_ONLINE,
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a6a7743ee98f..77d107a4577f 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -3278,9 +3278,8 @@ static int __init migrate_on_reclaim_init(void)
>   {
>   	int ret;
>   
> -	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "migrate on reclaim",
> -				migration_online_cpu,
> -				migration_offline_cpu);
> +	ret = cpuhp_setup_state_nocalls(CPUHP_MM_DEMOTION_OFFLINE, "mm/demotion:offline",
> +					NULL, migration_offline_cpu);
>   	/*
>   	 * In the unlikely case that this fails, the automatic
>   	 * migration targets may become suboptimal for nodes
> @@ -3288,6 +3287,9 @@ static int __init migrate_on_reclaim_init(void)
>   	 * rare case, do not bother trying to do anything special.
>   	 */
>   	WARN_ON(ret < 0);
> +	ret = cpuhp_setup_state_nocalls(CPUHP_AP_MM_DEMOTION_ONLINE, "mm/demotion:online",
> +					migration_online_cpu, NULL);
>

You changed to _nocalls variant, how does this handle initialization for 
cpus present at boot?

Thanks,
Mika