Date:   Mon, 8 Mar 2021 16:03:47 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>,
        Huang Ying <ying.huang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH 03/10] mm/migrate: update node demotion order during on
 hotplug events

On Thu, Mar 4, 2021 at 4:00 PM Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>
>
> From: Dave Hansen <dave.hansen@...ux.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 is conceptually 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 the hotplug event would have some
> *actual* effect on the demotion order.  But, given the expected
> paucity of hotplug events, this should be fine.
>
> === What does RCU provide? ===
>
> Imaginge a simple loop which walks down the demotion path looking
> for the last node:
>
>         terminal_node = start_node;
>         while (node_demotion[terminal_node] != NUMA_NO_NODE) {
>                 terminal_node = node_demotion[terminal_node];
>         }
>
> The initial values are:
>
>         node_demotion[0] = 1;
>         node_demotion[1] = NUMA_NO_NODE;
>
> and are updated to:
>
>         node_demotion[0] = NUMA_NO_NODE;
>         node_demotion[1] = 0;
>
> What guarantees that the loop did not observe:
>
>         node_demotion[0] = 1;
>         node_demotion[1] = 0;
>
> and would loop forever?
>
> With RCU, a rcu_read_lock/unlock() can be placed around the
> loop.  Since the write side does a synchronize_rcu(), the loop
> that observed the old contents is known to be complete after the
> synchronize_rcu() has completed.
>
> RCU, combined with disable_all_migrate_targets(), ensures that
> the old migration state is not visible by the time
> __set_migration_target_nodes() is called.
>
> === What does READ_ONCE() provide? ===
>
> READ_ONCE() forbids the compiler from merging or reordering
> successive reads of node_demotion[].  This ensures that any
> updates are *eventually* observed.
>
> Consider the above loop again.  The compiler could theoretically
> read the entirety of node_demotion[] into local storage
> (registers) and never go back to memory, and *permanently*
> observe bad values for node_demotion[].
>
> Note: RCU does not provide any universal compiler-ordering
> guarantees:
>
>         https://lore.kernel.org/lkml/20150921204327.GH4029@linux.vnet.ibm.com/
>
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Yang Shi <yang.shi@...ux.alibaba.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: osalvador <osalvador@...e.de>
> ---
>
>  b/mm/migrate.c |  159 +++++++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 137 insertions(+), 22 deletions(-)
>
> diff -puN mm/migrate.c~enable-numa-demotion mm/migrate.c
> --- a/mm/migrate.c~enable-numa-demotion 2021-03-04 15:35:53.670806436 -0800
> +++ b/mm/migrate.c      2021-03-04 15:35:53.677806436 -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>
>
> @@ -1192,8 +1193,12 @@ out:
>   */
>
>  /*
> - * Writes to this array occur without locking.  READ_ONCE()
> - * is recommended for readers to ensure consistent reads.
> + * Writes to this array occur without locking.  Cycles are
> + * not allowed: Node X demotes to Y which demotes to X...
> + *
> + * If multiple reads are performed, a single rcu_read_lock()
> + * must be held over all reads to ensure that no cycles are
> + * observed.
>   */
>  static int node_demotion[MAX_NUMNODES] __read_mostly =
>         {[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
> @@ -1209,13 +1214,22 @@ static int node_demotion[MAX_NUMNODES] _
>   */
>  int next_demotion_node(int node)
>  {
> +       int target;
> +
>         /*
> -        * node_demotion[] is updated without excluding
> -        * this function from running.  READ_ONCE() avoids
> -        * reading multiple, inconsistent 'node' values
> -        * during an update.
> +        * node_demotion[] is updated without excluding this
> +        * function from running.  RCU doesn't provide any
> +        * compiler barriers, so the READ_ONCE() is required
> +        * to avoid compiler reordering or read merging.
> +        *
> +        * Make sure to use RCU over entire code blocks if
> +        * node_demotion[] reads need to be consistent.
>          */
> -       return READ_ONCE(node_demotion[node]);
> +       rcu_read_lock();
> +       target = READ_ONCE(node_demotion[node]);
> +       rcu_read_unlock();
> +
> +       return target;
>  }
>
>  /*
> @@ -3220,8 +3234,9 @@ 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)
> +static void __disable_all_migrate_targets(void)
>  {
>         int node;
>
> @@ -3229,6 +3244,25 @@ static void disable_all_migrate_targets(
>                 node_demotion[node] = NUMA_NO_NODE;
>  }
>
> +static void disable_all_migrate_targets(void)
> +{
> +       __disable_all_migrate_targets();
> +
> +       /*
> +        * Ensure that the "disable" is visible across the system.
> +        * Readers will see either a combination of before+disable
> +        * state or disable+after.  They will never see before and
> +        * after state together.
> +        *
> +        * The before+after state together might have cycles and
> +        * could cause readers to do things like loop until this
> +        * function finishes.  This ensures they can only see a
> +        * single "bad" read and would, for instance, only loop
> +        * once.
> +        */
> +       synchronize_rcu();
> +}
> +
>  /*
>   * Find an automatic demotion target for 'node'.
>   * Failing here is OK.  It might just indicate
> @@ -3291,20 +3325,6 @@ static void __set_migration_target_nodes
>         disable_all_migrate_targets();
>
>         /*
> -        * Ensure that the "disable" is visible across the system.
> -        * Readers will see either a combination of before+disable
> -        * state or disable+after.  They will never see before and
> -        * after state together.
> -        *
> -        * The before+after state together might have cycles and
> -        * could cause readers to do things like loop until this
> -        * function finishes.  This ensures they can only see a
> -        * single "bad" read and would, for instance, only loop
> -        * once.
> -        */
> -       smp_wmb();
> -
> -       /*
>          * Allocations go close to CPUs, first.  Assume that
>          * the migration path starts at the nodes with CPUs.
>          */
> @@ -3347,3 +3367,98 @@ static void set_migration_target_nodes(v
>         __set_migration_target_nodes();
>         put_online_mems();
>  }
> +
> +/*
> + * 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;
> +}
> +
> +/*
> + * This leaves migrate-on-reclaim transiently disabled between
> + * the MEM_GOING_OFFLINE and MEM_OFFLINE events.  This runs
> + * whether reclaim-based migration is enabled or not, which
> + * ensures that the user can turn reclaim-based migration at
> + * any time without needing to 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();
> +
> +               /*
> +                * Ensure the disable operation is globally visible.
> +                * This avoids readers ever being able to
> +                * simultaneously observe the old (pre-hotplug) and
> +                * new (post-hotplug) migration targets.
> +                */
> +               synchronize_rcu();

It seems disable_all_migrate_targets() already has synchronize_rcu()
called. We don't need to call it twice. Otherwise, it looks good to
me. Reviewed-by: Yang Shi <shy828301@...il.com>

> +               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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