On 18 Jun 2021, at 2:15, Huang Ying wrote: > From: Dave Hansen > > 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 > Signed-off-by: "Huang, Ying" > Reviewed-by: Yang Shi > Cc: Michal Hocko > Cc: Wei Xu > Cc: David Rientjes > Cc: Dan Williams > Cc: David Hildenbrand > Cc: osalvador > > -- > > Changes from 20200122: > * Add big node_demotion[] comment > Changes from 20210302: > * Fix typo in node_demotion[] comment > --- > mm/internal.h | 5 ++ > mm/migrate.c | 175 +++++++++++++++++++++++++++++++++++++++++++++++- > mm/page_alloc.c | 2 +- > 3 files changed, 180 insertions(+), 2 deletions(-) > > diff --git a/mm/internal.h b/mm/internal.h > index 2f1182948aa6..0344cd78e170 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -522,12 +522,17 @@ static inline void mminit_validate_memmodel_limits(unsigned long *start_pfn, > > #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 --git a/mm/migrate.c b/mm/migrate.c > index 6cab668132f9..111f8565f75d 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1136,6 +1136,44 @@ 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: > + * > + * { 1, // Node 0 migrates to 1 > + * 2, // Node 1 migrates to 2 > + * -1, // Node 2 does not migrate > + * 4, // Node 3 migrates to 4 > + * 5, // Node 4 migrates to 5 > + * -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}; > > @@ -1150,7 +1188,13 @@ static int node_demotion[MAX_NUMNODES] __read_mostly = > */ > 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]); > } Is it necessary to have two separate patches to add node_demotion and next_demotion_node() then modify it immediately? Maybe merge Patch 1 into 2? Hmm, I just checked Patch 3 and it changes node_demotion again and uses RCU. I guess it might be much simpler to just introduce node_demotion with RCU in this patch and Patch 3 only takes care of hotplug events. > > /* > @@ -3144,3 +3188,132 @@ void migrate_vma_finalize(struct migrate_vma *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]; Is there a plan of allowing user to change where the migration path starts? Or maybe one step further providing an interface to allow user to specify the demotion path. Something like /sys/devices/system/node/node*/node_demotion. > +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. > + */ > +__maybe_unused // <- temporay to prevent warnings during bisects > +static void set_migration_target_nodes(void) > +{ > + get_online_mems(); > + __set_migration_target_nodes(); > + put_online_mems(); > +} > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index d1f5de1c1283..e033ae2e8bce 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -5973,7 +5973,7 @@ 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; > -- > 2.30.2 — Best Regards, Yan, Zi