Re: [RFC][PATCH 06/13] mm/migrate: update migration order during on hotplug events

[Yang Shi <shy828301@gmail.com>]

On Mon, Jan 25, 2021 at 4:41 PM Dave Hansen <dave.hansen@linux.intel.com> wrote:
>
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> Reclaim-based migration is attempting to optimize data placement in
> memory based on the system topology.  If the system changes, so must
> the migration ordering.
>
> The implementation here is pretty simple and entirely unoptimized.  On
> any memory or CPU hotplug events, assume that a node was added or
> removed and recalculate all migration targets.  This ensures that the
> node_demotion[] array is always ready to be used in case the new
> reclaim mode is enabled.
>
> This recalculation is far from optimal, most glaringly that it does
> not even attempt to figure out if nodes are actually coming or going.
> But, given the expected paucity of hotplug events, this should be
> fine.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: osalvador <osalvador@suse.de>
> ---
>
>  b/mm/migrate.c |   97 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 95 insertions(+), 2 deletions(-)
>
> diff -puN mm/migrate.c~enable-numa-demotion mm/migrate.c
> --- a/mm/migrate.c~enable-numa-demotion 2021-01-25 16:23:11.850866703 -0800
> +++ b/mm/migrate.c      2021-01-25 16:23:11.855866703 -0800
> @@ -49,6 +49,7 @@
>  #include <linux/sched/mm.h>
>  #include <linux/ptrace.h>
>  #include <linux/oom.h>
> +#include <linux/memory.h>
>
>  #include <asm/tlbflush.h>
>
> @@ -3135,6 +3136,7 @@ void migrate_vma_finalize(struct migrate
>  EXPORT_SYMBOL(migrate_vma_finalize);
>  #endif /* CONFIG_DEVICE_PRIVATE */
>
> +#if defined(CONFIG_MEMORY_HOTPLUG)
>  /* Disable reclaim-based migration. */
>  static void disable_all_migrate_targets(void)
>  {
> @@ -3191,7 +3193,7 @@ static int establish_migrate_target(int
>   * with itself.  Exclusion is provided by memory hotplug events
>   * being single-threaded.
>   */
> -void __set_migration_target_nodes(void)
> +static void __set_migration_target_nodes(void)
>  {
>         nodemask_t next_pass    = NODE_MASK_NONE;
>         nodemask_t this_pass    = NODE_MASK_NONE;
> @@ -3253,9 +3255,100 @@ again:
>                 goto again;
>  }
>
> -void set_migration_target_nodes(void)
> +/*
> + * For callers that do not hold get_online_mems() already.
> + */
> +static void set_migration_target_nodes(void)

Aha, it is changed to static here. I think this hunk should be folded
into the previous patch.

>  {
>         get_online_mems();
>         __set_migration_target_nodes();
>         put_online_mems();
>  }
> +
> +/*
> + * React to hotplug events that might affect the migration targes

s/targes/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;
> +}
> +
> +/*
> + * This leaves migrate-on-reclaim transiently disabled
> + * between the MEM_GOING_OFFLINE and MEM_OFFLINE events.
> + * This runs reclaim-based micgration is enabled or not.

s/micgration/migration

> + * This ensures that the user can turn reclaim-based
> + * migration at any time without needing to recalcuate

s/reclcuate/recalculate

> + * migration targets.
> + *
> + * These callbacks already hold get_online_mems().  That
> + * is why __set_migration_target_nodes() can be used as
> + * opposed to set_migration_target_nodes().
> + */
> +static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
> +                                                unsigned long action, void *arg)
> +{
> +       switch (action) {
> +       case MEM_GOING_OFFLINE:
> +               /*
> +                * Make sure there are not transient states where
> +                * an offline node is a migration target.  This
> +                * will leave migration disabled until the offline
> +                * completes and the MEM_OFFLINE case below runs.
> +                */
> +               disable_all_migrate_targets();

Don't we need smp_wmb() here? In the previous patch the comment says
write memory barrier is needed to guarantee readers see the consistent
values. The the smp_wmb() is called by __set_migration_target_nodes().
So, it seems it'd better to move smp_wmb() into
disable_all_migrate_targets().

> +               break;
> +       case MEM_OFFLINE:
> +       case MEM_ONLINE:
> +               /*
> +                * Recalculate the target nodes once the node
> +                * reaches its final state (online or offline).
> +                */
> +               __set_migration_target_nodes();
> +               break;
> +       case MEM_CANCEL_OFFLINE:
> +               /*
> +                * MEM_GOING_OFFLINE disabled all the migration
> +                * targets.  Reenable them.
> +                */
> +               __set_migration_target_nodes();
> +               break;
> +       case MEM_GOING_ONLINE:
> +       case MEM_CANCEL_ONLINE:
> +               break;
> +       }
> +
> +       return notifier_from_errno(0);
> +}
> +
> +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);
> +       /*
> +        * 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);
> +
> +       hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
> +       return 0;
> +}
> +late_initcall(migrate_on_reclaim_init);
> +#endif /* CONFIG_MEMORY_HOTPLUG */
> _
>