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    SubjectRe: [PATCH 02/10] mm/numa: automatically generate node migration order
    On Thu, Mar 4, 2021 at 4:00 PM Dave Hansen <dave.hansen@linux.intel.com> wrote:
    >
    >
    > From: Dave Hansen <dave.hansen@linux.intel.com>
    >
    > When memory fills up on a node, memory contents can be
    > automatically migrated to another node. The biggest problems are
    > knowing when to migrate and to where the migration should be
    > targeted.
    >
    > The most straightforward way to generate the "to where" list
    > would be to follow the page allocator fallback lists. Those
    > lists already tell us if memory is full where to look next. It
    > would also be logical to move memory in that order.
    >
    > But, the allocator fallback lists have a fatal flaw: most nodes
    > appear in all the lists. This would potentially lead to
    > migration cycles (A->B, B->A, A->B, ...).
    >
    > Instead of using the allocator fallback lists directly, keep a
    > separate node migration ordering. But, reuse the same data used
    > to generate page allocator fallback in the first place:
    > find_next_best_node().
    >
    > This means that the firmware data used to populate node distances
    > essentially dictates the ordering for now. It should also be
    > architecture-neutral since all NUMA architectures have a working
    > find_next_best_node().
    >
    > The protocol for node_demotion[] access and writing is not
    > standard. It has no specific locking and is intended to be read
    > locklessly. Readers must take care to avoid observing changes
    > that appear incoherent. This was done so that node_demotion[]
    > locking has no chance of becoming a bottleneck on large systems
    > with lots of CPUs in direct reclaim.
    >
    > This code is unused for now. It will be called later in the
    > series.
    >
    > 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>
    >
    > --
    >
    > changes from 20200122:
    > * Add big node_demotion[] comment

    Thanks for adding the comment. Reviewed-by: Yang Shi <shy828301@gmail.com>

    > ---
    >
    > b/mm/internal.h | 5 +
    > b/mm/migrate.c | 174 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
    > b/mm/page_alloc.c | 4 -
    > 3 files changed, 180 insertions(+), 3 deletions(-)
    >
    > diff -puN mm/internal.h~auto-setup-default-migration-path-from-firmware mm/internal.h
    > --- a/mm/internal.h~auto-setup-default-migration-path-from-firmware 2021-03-04 15:35:52.407806439 -0800
    > +++ b/mm/internal.h 2021-03-04 15:35:52.426806439 -0800
    > @@ -520,12 +520,17 @@ static inline void mminit_validate_memmo
    >
    > #ifdef CONFIG_NUMA
    > extern int node_reclaim(struct pglist_data *, gfp_t, unsigned int);
    > +extern int find_next_best_node(int node, nodemask_t *used_node_mask);
    > #else
    > static inline int node_reclaim(struct pglist_data *pgdat, gfp_t mask,
    > unsigned int order)
    > {
    > return NODE_RECLAIM_NOSCAN;
    > }
    > +static inline int find_next_best_node(int node, nodemask_t *used_node_mask)
    > +{
    > + return NUMA_NO_NODE;
    > +}
    > #endif
    >
    > extern int hwpoison_filter(struct page *p);
    > diff -puN mm/migrate.c~auto-setup-default-migration-path-from-firmware mm/migrate.c
    > --- a/mm/migrate.c~auto-setup-default-migration-path-from-firmware 2021-03-04 15:35:52.409806439 -0800
    > +++ b/mm/migrate.c 2021-03-04 15:35:52.427806439 -0800
    > @@ -1157,6 +1157,44 @@ out:
    > 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:
    > + *
    > + * { 1, // Node 0 migrates to 1
    > + * 2, // Node 1 migrates to 2
    > + * -1, // Node 2 does not migrate
    > + * 4, // Node 3 migrates to 1
    > + * 5, // Node 4 migrates to 2
    > + * -1} // Node 5 does not migrate
    > + */
    > +
    > +/*
    > + * Writes to this array occur without locking. READ_ONCE()
    > + * is recommended for readers to ensure consistent reads.
    > + */
    > static int node_demotion[MAX_NUMNODES] __read_mostly =
    > {[0 ... MAX_NUMNODES - 1] = NUMA_NO_NODE};
    >
    > @@ -1171,7 +1209,13 @@ static int node_demotion[MAX_NUMNODES] _
    > */
    > int next_demotion_node(int node)
    > {
    > - return node_demotion[node];
    > + /*
    > + * node_demotion[] is updated without excluding
    > + * this function from running. READ_ONCE() avoids
    > + * reading multiple, inconsistent 'node' values
    > + * during an update.
    > + */
    > + return READ_ONCE(node_demotion[node]);
    > }
    >
    > /*
    > @@ -3175,3 +3219,131 @@ void migrate_vma_finalize(struct migrate
    > }
    > EXPORT_SYMBOL(migrate_vma_finalize);
    > #endif /* CONFIG_DEVICE_PRIVATE */
    > +
    > +/* Disable reclaim-based migration. */
    > +static void disable_all_migrate_targets(void)
    > +{
    > + int node;
    > +
    > + for_each_online_node(node)
    > + node_demotion[node] = NUMA_NO_NODE;
    > +}
    > +
    > +/*
    > + * Find an automatic demotion target for 'node'.
    > + * Failing here is OK. It might just indicate
    > + * being at the end of a chain.
    > + */
    > +static int establish_migrate_target(int node, nodemask_t *used)
    > +{
    > + int migration_target;
    > +
    > + /*
    > + * Can not set a migration target on a
    > + * node with it already set.
    > + *
    > + * No need for READ_ONCE() here since this
    > + * in the write path for node_demotion[].
    > + * This should be the only thread writing.
    > + */
    > + if (node_demotion[node] != NUMA_NO_NODE)
    > + return NUMA_NO_NODE;
    > +
    > + migration_target = find_next_best_node(node, used);
    > + if (migration_target == NUMA_NO_NODE)
    > + return NUMA_NO_NODE;
    > +
    > + node_demotion[node] = migration_target;
    > +
    > + return migration_target;
    > +}
    > +
    > +/*
    > + * When memory fills up on a node, memory contents can be
    > + * automatically migrated to another node instead of
    > + * discarded at reclaim.
    > + *
    > + * Establish a "migration path" which will start at nodes
    > + * with CPUs and will follow the priorities used to build the
    > + * page allocator zonelists.
    > + *
    > + * The difference here is that cycles must be avoided. If
    > + * node0 migrates to node1, then neither node1, nor anything
    > + * node1 migrates to can migrate to node0.
    > + *
    > + * This function can run simultaneously with readers of
    > + * node_demotion[]. However, it can not run simultaneously
    > + * with itself. Exclusion is provided by memory hotplug events
    > + * being single-threaded.
    > + */
    > +static void __set_migration_target_nodes(void)
    > +{
    > + nodemask_t next_pass = NODE_MASK_NONE;
    > + nodemask_t this_pass = NODE_MASK_NONE;
    > + nodemask_t used_targets = NODE_MASK_NONE;
    > + int node;
    > +
    > + /*
    > + * Avoid any oddities like cycles that could occur
    > + * from changes in the topology. This will leave
    > + * a momentary gap when migration is disabled.
    > + */
    > + 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.
    > + */
    > + next_pass = node_states[N_CPU];
    > +again:
    > + this_pass = next_pass;
    > + next_pass = NODE_MASK_NONE;
    > + /*
    > + * To avoid cycles in the migration "graph", ensure
    > + * that migration sources are not future targets by
    > + * setting them in 'used_targets'. Do this only
    > + * once per pass so that multiple source nodes can
    > + * share a target node.
    > + *
    > + * 'used_targets' will become unavailable in future
    > + * passes. This limits some opportunities for
    > + * multiple source nodes to share a destination.
    > + */
    > + nodes_or(used_targets, used_targets, this_pass);
    > + for_each_node_mask(node, this_pass) {
    > + int target_node = establish_migrate_target(node, &used_targets);
    > +
    > + if (target_node == NUMA_NO_NODE)
    > + continue;
    > +
    > + /* Visit targets from this pass in the next pass: */
    > + node_set(target_node, next_pass);
    > + }
    > + /* Is another pass necessary? */
    > + if (!nodes_empty(next_pass))
    > + goto again;
    > +}
    > +
    > +/*
    > + * For callers that do not hold get_online_mems() already.
    > + */
    > +static void set_migration_target_nodes(void)
    > +{
    > + get_online_mems();
    > + __set_migration_target_nodes();
    > + put_online_mems();
    > +}
    > diff -puN mm/page_alloc.c~auto-setup-default-migration-path-from-firmware mm/page_alloc.c
    > --- a/mm/page_alloc.c~auto-setup-default-migration-path-from-firmware 2021-03-04 15:35:52.422806439 -0800
    > +++ b/mm/page_alloc.c 2021-03-04 15:35:52.429806439 -0800
    > @@ -3916,7 +3916,7 @@ retry:
    > if (alloc_flags & ALLOC_NO_WATERMARKS)
    > goto try_this_zone;
    >
    > - if (!node_reclaim_enabled() ||
    > + if (node_reclaim_mode == 0 ||
    > !zone_allows_reclaim(ac->preferred_zoneref->zone, zone))
    > continue;
    >
    > @@ -5773,7 +5773,7 @@ static int node_load[MAX_NUMNODES];
    > *
    > * Return: node id of the found node or %NUMA_NO_NODE if no node is found.
    > */
    > -static int find_next_best_node(int node, nodemask_t *used_node_mask)
    > +int find_next_best_node(int node, nodemask_t *used_node_mask)
    > {
    > int n, val;
    > int min_val = INT_MAX;
    > _
    >

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