Re: [PATCH] mm/migrate: move node demotion code to near its user

[Wei Xu <weixugc@google.com>]

On Sun, Dec 5, 2021 at 7:12 PM Huang Ying <ying.huang@intel.com> wrote:
>
> Now, node_demotion and next_demtion_node() is placed between
> __unmap_and_move() and unmap_and_move().  This hurts the code
> readability.  So, move it to near its user in the file.  There's no
> any functionality change in this patch.
>
> 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: Oscar Salvador <osalvador@suse.de>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Wei Xu <weixugc@google.com>
> 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>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>

Reviewed-by: Wei Xu <weixugc@google.com>

> ---
>  mm/migrate.c | 265 +++++++++++++++++++++++++--------------------------
>  1 file changed, 132 insertions(+), 133 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c503ef1f4360..d487a399253b 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1083,139 +1083,6 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>         return rc;
>  }
>
> -
> -/*
> - * node_demotion[] example:
> - *
> - * Consider a system with two sockets.  Each socket has
> - * three classes of memory attached: fast, medium and slow.
> - * Each memory class is placed in its own NUMA node.  The
> - * CPUs are placed in the node with the "fast" memory.  The
> - * 6 NUMA nodes (0-5) might be split among the sockets like
> - * this:
> - *
> - *     Socket A: 0, 1, 2
> - *     Socket B: 3, 4, 5
> - *
> - * When Node 0 fills up, its memory should be migrated to
> - * Node 1.  When Node 1 fills up, it should be migrated to
> - * Node 2.  The migration path start on the nodes with the
> - * processors (since allocations default to this node) and
> - * fast memory, progress through medium and end with the
> - * slow memory:
> - *
> - *     0 -> 1 -> 2 -> stop
> - *     3 -> 4 -> 5 -> stop
> - *
> - * This is represented in the node_demotion[] like this:
> - *
> - *     {  nr=1, nodes[0]=1 }, // Node 0 migrates to 1
> - *     {  nr=1, nodes[0]=2 }, // Node 1 migrates to 2
> - *     {  nr=0, nodes[0]=-1 }, // Node 2 does not migrate
> - *     {  nr=1, nodes[0]=4 }, // Node 3 migrates to 4
> - *     {  nr=1, nodes[0]=5 }, // Node 4 migrates to 5
> - *     {  nr=0, nodes[0]=-1 }, // Node 5 does not migrate
> - *
> - * Moreover some systems may have multiple slow memory nodes.
> - * Suppose a system has one socket with 3 memory nodes, node 0
> - * is fast memory type, and node 1/2 both are slow memory
> - * type, and the distance between fast memory node and slow
> - * memory node is same. So the migration path should be:
> - *
> - *     0 -> 1/2 -> stop
> - *
> - * This is represented in the node_demotion[] like this:
> - *     { nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
> - *     { nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
> - *     { nr=0, nodes[0]=-1, }, // Node 2 does not migrate
> - */
> -
> -/*
> - * 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.
> - */
> -#define DEFAULT_DEMOTION_TARGET_NODES 15
> -
> -#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
> -#define DEMOTION_TARGET_NODES  (MAX_NUMNODES - 1)
> -#else
> -#define DEMOTION_TARGET_NODES  DEFAULT_DEMOTION_TARGET_NODES
> -#endif
> -
> -struct demotion_nodes {
> -       unsigned short nr;
> -       short nodes[DEMOTION_TARGET_NODES];
> -};
> -
> -static struct demotion_nodes *node_demotion __read_mostly;
> -
> -/**
> - * next_demotion_node() - Get the next node in the demotion path
> - * @node: The starting node to lookup the next node
> - *
> - * Return: node id for next memory node in the demotion path hierarchy
> - * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
> - * @node online or guarantee that it *continues* to be the next demotion
> - * target.
> - */
> -int next_demotion_node(int node)
> -{
> -       struct demotion_nodes *nd;
> -       unsigned short target_nr, index;
> -       int target;
> -
> -       if (!node_demotion)
> -               return NUMA_NO_NODE;
> -
> -       nd = &node_demotion[node];
> -
> -       /*
> -        * 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.
> -        */
> -       rcu_read_lock();
> -       target_nr = READ_ONCE(nd->nr);
> -
> -       switch (target_nr) {
> -       case 0:
> -               target = NUMA_NO_NODE;
> -               goto out;
> -       case 1:
> -               index = 0;
> -               break;
> -       default:
> -               /*
> -                * If there are multiple target nodes, just select one
> -                * target node randomly.
> -                *
> -                * In addition, we can also use round-robin to select
> -                * target node, but we should introduce another variable
> -                * for node_demotion[] to record last selected target node,
> -                * that may cause cache ping-pong due to the changing of
> -                * last target node. Or introducing per-cpu data to avoid
> -                * caching issue, which seems more complicated. So selecting
> -                * target node randomly seems better until now.
> -                */
> -               index = get_random_int() % target_nr;
> -               break;
> -       }
> -
> -       target = READ_ONCE(nd->nodes[index]);
> -
> -out:
> -       rcu_read_unlock();
> -       return target;
> -}
> -
>  /*
>   * Obtain the lock on page, remove all ptes and migrate the page
>   * to the newly allocated page in newpage.
> @@ -3035,6 +2902,138 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
>  EXPORT_SYMBOL(migrate_vma_finalize);
>  #endif /* CONFIG_DEVICE_PRIVATE */
>
> +/*
> + * node_demotion[] example:
> + *
> + * Consider a system with two sockets.  Each socket has
> + * three classes of memory attached: fast, medium and slow.
> + * Each memory class is placed in its own NUMA node.  The
> + * CPUs are placed in the node with the "fast" memory.  The
> + * 6 NUMA nodes (0-5) might be split among the sockets like
> + * this:
> + *
> + *     Socket A: 0, 1, 2
> + *     Socket B: 3, 4, 5
> + *
> + * When Node 0 fills up, its memory should be migrated to
> + * Node 1.  When Node 1 fills up, it should be migrated to
> + * Node 2.  The migration path start on the nodes with the
> + * processors (since allocations default to this node) and
> + * fast memory, progress through medium and end with the
> + * slow memory:
> + *
> + *     0 -> 1 -> 2 -> stop
> + *     3 -> 4 -> 5 -> stop
> + *
> + * This is represented in the node_demotion[] like this:
> + *
> + *     {  nr=1, nodes[0]=1 }, // Node 0 migrates to 1
> + *     {  nr=1, nodes[0]=2 }, // Node 1 migrates to 2
> + *     {  nr=0, nodes[0]=-1 }, // Node 2 does not migrate
> + *     {  nr=1, nodes[0]=4 }, // Node 3 migrates to 4
> + *     {  nr=1, nodes[0]=5 }, // Node 4 migrates to 5
> + *     {  nr=0, nodes[0]=-1 }, // Node 5 does not migrate
> + *
> + * Moreover some systems may have multiple slow memory nodes.
> + * Suppose a system has one socket with 3 memory nodes, node 0
> + * is fast memory type, and node 1/2 both are slow memory
> + * type, and the distance between fast memory node and slow
> + * memory node is same. So the migration path should be:
> + *
> + *     0 -> 1/2 -> stop
> + *
> + * This is represented in the node_demotion[] like this:
> + *     { nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
> + *     { nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
> + *     { nr=0, nodes[0]=-1, }, // Node 2 does not migrate
> + */
> +
> +/*
> + * 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.
> + */
> +#define DEFAULT_DEMOTION_TARGET_NODES 15
> +
> +#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
> +#define DEMOTION_TARGET_NODES  (MAX_NUMNODES - 1)
> +#else
> +#define DEMOTION_TARGET_NODES  DEFAULT_DEMOTION_TARGET_NODES
> +#endif
> +
> +struct demotion_nodes {
> +       unsigned short nr;
> +       short nodes[DEMOTION_TARGET_NODES];
> +};
> +
> +static struct demotion_nodes *node_demotion __read_mostly;
> +
> +/**
> + * next_demotion_node() - Get the next node in the demotion path
> + * @node: The starting node to lookup the next node
> + *
> + * Return: node id for next memory node in the demotion path hierarchy
> + * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
> + * @node online or guarantee that it *continues* to be the next demotion
> + * target.
> + */
> +int next_demotion_node(int node)
> +{
> +       struct demotion_nodes *nd;
> +       unsigned short target_nr, index;
> +       int target;
> +
> +       if (!node_demotion)
> +               return NUMA_NO_NODE;
> +
> +       nd = &node_demotion[node];
> +
> +       /*
> +        * 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.
> +        */
> +       rcu_read_lock();
> +       target_nr = READ_ONCE(nd->nr);
> +
> +       switch (target_nr) {
> +       case 0:
> +               target = NUMA_NO_NODE;
> +               goto out;
> +       case 1:
> +               index = 0;
> +               break;
> +       default:
> +               /*
> +                * If there are multiple target nodes, just select one
> +                * target node randomly.
> +                *
> +                * In addition, we can also use round-robin to select
> +                * target node, but we should introduce another variable
> +                * for node_demotion[] to record last selected target node,
> +                * that may cause cache ping-pong due to the changing of
> +                * last target node. Or introducing per-cpu data to avoid
> +                * caching issue, which seems more complicated. So selecting
> +                * target node randomly seems better until now.
> +                */
> +               index = get_random_int() % target_nr;
> +               break;
> +       }
> +
> +       target = READ_ONCE(nd->nodes[index]);
> +
> +out:
> +       rcu_read_unlock();
> +       return target;
> +}
> +
>  #if defined(CONFIG_HOTPLUG_CPU)
>  /* Disable reclaim-based migration. */
>  static void __disable_all_migrate_targets(void)
> --
> 2.30.2
>