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SubjectRe: [RFC][PATCH 06/13] mm/migrate: update migration order during on hotplug events
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 */
> _
>

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