On Mon, Jan 25, 2021 at 4:41 PM Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote: > > > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxxxx> > Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> > Cc: David Rientjes <rientjes@xxxxxxxxxx> > Cc: Huang Ying <ying.huang@xxxxxxxxx> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx> > Cc: David Hildenbrand <david@xxxxxxxxxx> > Cc: osalvador <osalvador@xxxxxxx> > --- > > b/mm/internal.h | 5 + > b/mm/migrate.c | 137 +++++++++++++++++++++++++++++++++++++++++++++++++++++- > b/mm/page_alloc.c | 2 > 3 files changed, 142 insertions(+), 2 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 2021-01-25 16:23:10.607866706 -0800 > +++ b/mm/internal.h 2021-01-25 16:23:10.616866706 -0800 > @@ -515,12 +515,17 @@ 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 2021-01-25 16:23:10.609866706 -0800 > +++ b/mm/migrate.c 2021-01-25 16:23:10.617866706 -0800 > @@ -1161,6 +1161,10 @@ out: > return rc; > } > > +/* > + * Writes to this array occur without locking. READ_ONCE() > + * is recommended for readers to ensure consistent reads. > + */ > static int node_demotion[MAX_NUMNODES] = {[0 ... MAX_NUMNODES - 1] = NUMA_NO_NODE}; > > /** > @@ -1174,7 +1178,13 @@ 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. > + */ Don't we need a smp_rmb() here? The single write barrier might be not enough in migration target set. Typically a write barrier should be used in pairs with a read barrier. > + return READ_ONCE(node_demotion[node]); Why not consolidate the patch #4 in this patch? The patch #4 just add the definition of node_demotion and the function, then the function is changed in this patch, and the function is not used by anyone between the adding and changing. > } > > /* > @@ -3124,3 +3134,128 @@ 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. Maybe an example diagram for the physical topology and how the migration target is generated in the comment seems helpful to understand the code. > + */ > +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 desintation. s/desination/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; > +} > + > +void set_migration_target_nodes(void) It seems this function is not called outside migrate.c, so it should be static. > +{ > + 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 2021-01-25 16:23:10.612866706 -0800 > +++ b/mm/page_alloc.c 2021-01-25 16:23:10.619866706 -0800 > @@ -5704,7 +5704,7 @@ 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; > _ >